diff --git a/compiler/jni/jni_cfi_test_expected.inc b/compiler/jni/jni_cfi_test_expected.inc
index 47a67c7..3fe8226 100644
--- a/compiler/jni/jni_cfi_test_expected.inc
+++ b/compiler/jni/jni_cfi_test_expected.inc
@@ -1,12 +1,9 @@
 // TODO These arrays should be generated automatically or have instructions for re-creation.
-// For now, the gc_is_marking offset can be adjusted by tweaking the last CL that made a
-// similar change.
-
 static constexpr uint8_t expected_asm_kThumb2[] = {
     0x2D, 0xE9, 0xE0, 0x4D, 0x2D, 0xED, 0x10, 0x8A, 0x81, 0xB0, 0x00, 0x90,
     0x19, 0x91, 0x8D, 0xED, 0x1A, 0x0A, 0x1B, 0x92, 0x1C, 0x93, 0x88, 0xB0,
     0x08, 0xB0, 0x01, 0xB0, 0xBD, 0xEC, 0x10, 0x8A, 0xBD, 0xE8, 0xE0, 0x4D,
-    0xD9, 0xF8, 0x20, 0x80, 0x70, 0x47,
+    0xD9, 0xF8, 0x24, 0x80, 0x70, 0x47,
 };
 static constexpr uint8_t expected_cfi_kThumb2[] = {
     0x44, 0x0E, 0x1C, 0x85, 0x07, 0x86, 0x06, 0x87, 0x05, 0x88, 0x04, 0x8A,
@@ -89,7 +86,7 @@
 // 0x00000024: .cfi_restore: r10
 // 0x00000024: .cfi_restore: r11
 // 0x00000024: .cfi_restore: r14
-// 0x00000024: ldr r8, [tr, #32] ; is_gc_marking
+// 0x00000024: ldr r8, [tr, #36] ; is_gc_marking
 // 0x00000028: bx lr
 // 0x0000002a: .cfi_restore_state
 // 0x0000002a: .cfi_def_cfa_offset: 112
@@ -104,7 +101,7 @@
     0xF3, 0x53, 0x45, 0xA9, 0xF5, 0x5B, 0x46, 0xA9, 0xF7, 0x63, 0x47, 0xA9,
     0xF9, 0x6B, 0x48, 0xA9, 0xFB, 0x73, 0x49, 0xA9, 0xFD, 0x7B, 0x4A, 0xA9,
     0xE8, 0x27, 0x41, 0x6D, 0xEA, 0x2F, 0x42, 0x6D, 0xEC, 0x37, 0x43, 0x6D,
-    0xEE, 0x3F, 0x44, 0x6D, 0x74, 0x22, 0x40, 0xB9, 0xFF, 0xC3, 0x02, 0x91,
+    0xEE, 0x3F, 0x44, 0x6D, 0x74, 0x26, 0x40, 0xB9, 0xFF, 0xC3, 0x02, 0x91,
     0xC0, 0x03, 0x5F, 0xD6,
 };
 static constexpr uint8_t expected_cfi_kArm64[] = {
@@ -191,7 +188,7 @@
 // 0x0000006c: ldp d14, d15, [sp, #64]
 // 0x00000070: .cfi_restore_extended: r78
 // 0x00000070: .cfi_restore_extended: r79
-// 0x00000070: ldr w20, [tr, #32] ; is_gc_marking
+// 0x00000070: ldr w20, [tr, #48] ; is_gc_marking
 // 0x00000074: add sp, sp, #0xb0 (176)
 // 0x00000078: .cfi_def_cfa_offset: 0
 // 0x00000078: ret
diff --git a/compiler/utils/assembler_thumb_test_expected.cc.inc b/compiler/utils/assembler_thumb_test_expected.cc.inc
index 6e0048e..aea7f14 100644
--- a/compiler/utils/assembler_thumb_test_expected.cc.inc
+++ b/compiler/utils/assembler_thumb_test_expected.cc.inc
@@ -76,7 +76,7 @@
   "      e4: f1bb 0f00     cmp.w r11, #0\n"
   "      e8: bf18          it ne\n"
   "      ea: 46e3          movne r11, r12\n"
-  "      ec: f8d9 c094     ldr.w r12, [r9, #148]\n"
+  "      ec: f8d9 c09c     ldr.w r12, [r9, #156]\n"
   "      f0: f1bc 0f00     cmp.w r12, #0\n"
   "      f4: d16f          bne 0x1d6     @ imm = #222\n"
   "      f6: f8cd c7ff     str.w r12, [sp, #2047]\n"
@@ -151,10 +151,10 @@
   "     206: b001          add sp, #4\n"
   "     208: ecbd 8a10     vpop {s16, s17, s18, s19, s20, s21, s22, s23, s24, s25, s26, s27, s28, s29, s30, s31}\n"
   "     20c: e8bd 4de0     pop.w {r5, r6, r7, r8, r10, r11, lr}\n"
-  "     210: f8d9 8020     ldr.w r8, [r9, #32]\n"
+  "     210: f8d9 8024     ldr.w r8, [r9, #36]\n"
   "     214: 4770          bx lr\n"
-  "     216: f8d9 0094     ldr.w r0, [r9, #148]\n"
-  "     21a: f8d9 e2c4     ldr.w lr, [r9, #708]\n"
+  "     216: f8d9 009c     ldr.w r0, [r9, #156]\n"
+  "     21a: f8d9 e2d0     ldr.w lr, [r9, #720]\n"
   "     21e: 47f0          blx lr\n"
 };
 
diff --git a/openjdkjvmti/ti_heap.cc b/openjdkjvmti/ti_heap.cc
index 80bfa0f..662f464 100644
--- a/openjdkjvmti/ti_heap.cc
+++ b/openjdkjvmti/ti_heap.cc
@@ -975,13 +975,6 @@
     jvmtiHeapReferenceKind GetReferenceKind(const art::RootInfo& info,
                                             jvmtiHeapReferenceInfo* ref_info)
         REQUIRES_SHARED(art::Locks::mutator_lock_) {
-      // We do not necessarily hold thread_list_lock_ here, but we may if we are called from
-      // VisitThreadRoots, which can happen from JVMTI FollowReferences. If it was acquired in
-      // ThreadList::VisitRoots, it's unsafe to temporarily release it. Thus we act as if we did
-      // not hold the thread_list_lock_ here, and relax CHECKs appropriately. If it does happen,
-      // we are in a SuspendAll situation with concurrent GC disabled, and should not need to run
-      // flip functions. TODO: Find a way to clean this up.
-
       // TODO: Fill in ref_info.
       memset(ref_info, 0, sizeof(jvmtiHeapReferenceInfo));
 
diff --git a/openjdkjvmti/ti_object.cc b/openjdkjvmti/ti_object.cc
index 3333a8a..f37df86 100644
--- a/openjdkjvmti/ti_object.cc
+++ b/openjdkjvmti/ti_object.cc
@@ -105,17 +105,13 @@
       notify_wait.push_back(jni->AddLocalReference<jthread>(thd->GetPeerFromOtherThread()));
       wait.push_back(jni->AddLocalReference<jthread>(thd->GetPeerFromOtherThread()));
     }
-    // Scan all threads to see which are waiting on this particular monitor.
-    std::list<art::Thread*> thread_list;
     {
-      // Since we're in a SuspendAll, exiting threads are not a concern.
+      // Scan all threads to see which are waiting on this particular monitor.
       art::MutexLock tll(self, *art::Locks::thread_list_lock_);
-      thread_list = art::Runtime::Current()->GetThreadList()->GetList();
-    }
-    for (art::Thread* thd : thread_list) {
-      if (thd != info.owner_ && target.Ptr() == thd->GetMonitorEnterObject()) {
-        art::mirror::Object* peer = thd->GetPeerFromOtherThread();
-        wait.push_back(jni->AddLocalReference<jthread>(peer));
+      for (art::Thread* thd : art::Runtime::Current()->GetThreadList()->GetList()) {
+        if (thd != info.owner_ && target.Ptr() == thd->GetMonitorEnterObject()) {
+          wait.push_back(jni->AddLocalReference<jthread>(thd->GetPeerFromOtherThread()));
+        }
       }
     }
   }
diff --git a/openjdkjvmti/ti_stack.cc b/openjdkjvmti/ti_stack.cc
index a560814..9af8861 100644
--- a/openjdkjvmti/ti_stack.cc
+++ b/openjdkjvmti/ti_stack.cc
@@ -363,13 +363,7 @@
     REQUIRES_SHARED(art::Locks::mutator_lock_) {
   // Note: requires the mutator lock as the checkpoint requires the mutator lock.
   GetAllStackTracesVectorClosure<Data> closure(max_frame_count, data);
-  // TODO(b/253671779): Replace this use of RunCheckpointUnchecked() with RunCheckpoint(). This is
-  // currently not possible, since the following undesirable call chain (abbreviated here) is then
-  // possible and exercised by current tests: (jvmti) GetAllStackTraces -> <this function> ->
-  // RunCheckpoint -> GetStackTraceVisitor -> EncodeMethodId -> Class::EnsureMethodIds ->
-  // Class::Alloc -> AllocObjectWithAllocator -> potentially suspends, or runs GC, etc. -> CHECK
-  // failure.
-  size_t barrier_count = art::Runtime::Current()->GetThreadList()->RunCheckpointUnchecked(&closure);
+  size_t barrier_count = art::Runtime::Current()->GetThreadList()->RunCheckpoint(&closure, nullptr);
   if (barrier_count == 0) {
     return;
   }
@@ -550,6 +544,7 @@
           // Found the thread.
           art::MutexLock mu(self, mutex);
 
+          threads.push_back(thread);
           thread_list_indices.push_back(index);
 
           frames.emplace_back(new std::vector<jvmtiFrameInfo>());
@@ -567,6 +562,7 @@
 
     // Storage. Only access directly after completion.
 
+    std::vector<art::Thread*> threads;
     std::vector<size_t> thread_list_indices;
 
     std::vector<std::unique_ptr<std::vector<jvmtiFrameInfo>>> frames;
@@ -604,10 +600,12 @@
     jvmtiStackInfo& stack_info = stack_info_array.get()[index];
     memset(&stack_info, 0, sizeof(jvmtiStackInfo));
 
+    art::Thread* self = data.threads[index];
     const std::vector<jvmtiFrameInfo>& thread_frames = *data.frames[index].get();
 
     // For the time being, set the thread to null. We don't have good ScopedLocalRef
     // infrastructure.
+    DCHECK(self->GetPeerFromOtherThread() != nullptr);
     stack_info.thread = nullptr;
     stack_info.state = JVMTI_THREAD_STATE_SUSPENDED;
 
diff --git a/openjdkjvmti/ti_thread.cc b/openjdkjvmti/ti_thread.cc
index 191b63c..13eebbf 100644
--- a/openjdkjvmti/ti_thread.cc
+++ b/openjdkjvmti/ti_thread.cc
@@ -289,9 +289,7 @@
 
     info_ptr->is_daemon = target->IsDaemon();
 
-    art::ObjPtr<art::mirror::Object> peer = target->LockedGetPeerFromOtherThread();
-    // *target may be invalid here since we may have temporarily released thread_list_lock_.
-    target = nullptr;  // Value should not be used.
+    art::ObjPtr<art::mirror::Object> peer = target->GetPeerFromOtherThread();
 
     // ThreadGroup.
     if (peer != nullptr) {
@@ -549,7 +547,6 @@
 
 // Suspends the current thread if it has any suspend requests on it.
 void ThreadUtil::SuspendCheck(art::Thread* self) {
-  DCHECK(!self->ReadFlag(art::ThreadFlag::kSuspensionImmune));
   art::ScopedObjectAccess soa(self);
   // Really this is only needed if we are in FastJNI and actually have the mutator_lock_ already.
   self->FullSuspendCheck();
@@ -643,30 +640,20 @@
 
   art::MutexLock mu(current, *art::Locks::thread_list_lock_);
   std::list<art::Thread*> thread_list = art::Runtime::Current()->GetThreadList()->GetList();
-  // We have to be careful with threads exiting while we build this list.
-  std::vector<art::ThreadExitFlag> tefs(thread_list.size());
-  auto i = tefs.begin();
-  for (art::Thread* thd : thread_list) {
-    thd->NotifyOnThreadExit(&*i++);
-  }
-  DCHECK(i == tefs.end());
 
   std::vector<art::ObjPtr<art::mirror::Object>> peers;
 
-  i = tefs.begin();
   for (art::Thread* thread : thread_list) {
-    art::ThreadExitFlag* tef = &*i++;
-    // Skip threads that have since exited or are still starting.
-    if (!tef->HasExited() && !thread->IsStillStarting()) {
-      // LockedGetPeerFromOtherThreads() may release lock!
-      art::ObjPtr<art::mirror::Object> peer = thread->LockedGetPeerFromOtherThread(tef);
-      if (peer != nullptr) {
-        peers.push_back(peer);
-      }
+    // Skip threads that are still starting.
+    if (thread->IsStillStarting()) {
+      continue;
     }
-    thread->UnregisterThreadExitFlag(tef);
+
+    art::ObjPtr<art::mirror::Object> peer = thread->GetPeerFromOtherThread();
+    if (peer != nullptr) {
+      peers.push_back(peer);
+    }
   }
-  DCHECK(i == tefs.end());
 
   if (peers.empty()) {
     *threads_count_ptr = 0;
@@ -678,8 +665,8 @@
       return data_result;
     }
     jthread* threads = reinterpret_cast<jthread*>(data);
-    for (size_t j = 0; j != peers.size(); ++j) {
-      threads[j] = soa.AddLocalReference<jthread>(peers[j]);
+    for (size_t i = 0; i != peers.size(); ++i) {
+      threads[i] = soa.AddLocalReference<jthread>(peers[i]);
     }
 
     *threads_count_ptr = static_cast<jint>(peers.size());
@@ -913,13 +900,17 @@
         }
       }
     }
+    bool timeout = true;
     art::Thread* ret_target = art::Runtime::Current()->GetThreadList()->SuspendThreadByPeer(
-        target_jthread, art::SuspendReason::kForUserCode);
-    if (ret_target == nullptr) {
+        target_jthread,
+        art::SuspendReason::kForUserCode,
+        &timeout);
+    if (ret_target == nullptr && !timeout) {
       // TODO It would be good to get more information about why exactly the thread failed to
       // suspend.
       return ERR(INTERNAL);
-    } else {
+    } else if (!timeout) {
+      // we didn't time out and got a result.
       return OK;
     }
     // We timed out. Just go around and try again.
@@ -936,11 +927,10 @@
       // This can only happen if we race with another thread to suspend 'self' and we lose.
       return ERR(THREAD_SUSPENDED);
     }
-    {
-      // IncrementSuspendCount normally needs thread_list_lock_ to ensure the thread stays
-      // around. In this case we are the target thread, so we fake it.
-      art::FakeMutexLock fmu(*art::Locks::thread_list_lock_);
-      self->IncrementSuspendCount(self, nullptr, nullptr, art::SuspendReason::kForUserCode);
+    // We shouldn't be able to fail this.
+    if (!self->ModifySuspendCount(self, +1, nullptr, art::SuspendReason::kForUserCode)) {
+      // TODO More specific error would be nice.
+      return ERR(INTERNAL);
     }
   }
   // Once we have requested the suspend we actually go to sleep. We need to do this after releasing
diff --git a/openjdkjvmti/ti_threadgroup.cc b/openjdkjvmti/ti_threadgroup.cc
index bfe6b51..120024e 100644
--- a/openjdkjvmti/ti_threadgroup.cc
+++ b/openjdkjvmti/ti_threadgroup.cc
@@ -171,26 +171,18 @@
   CHECK(thread_group != nullptr);
 
   art::MutexLock mu(art::Thread::Current(), *art::Locks::thread_list_lock_);
-  std::list<art::Thread*> thread_list = art::Runtime::Current()->GetThreadList()->GetList();
-  // We have to be careful with threads exiting while we build this list.
-  std::vector<art::ThreadExitFlag> tefs(thread_list.size());
-  auto i = tefs.begin();
-  for (art::Thread* thd : thread_list) {
-    thd->NotifyOnThreadExit(&*i++);
-  }
-  DCHECK(i == tefs.end());
-
-  i = tefs.begin();
-  for (art::Thread* t : thread_list) {
-    art::ThreadExitFlag* tef = &*i++;
-    art::ObjPtr<art::mirror::Object> peer = t->LockedGetPeerFromOtherThread(tef);
-    if (peer != nullptr && !tef->HasExited() && !t->IsStillStarting() &&
-        IsInDesiredThreadGroup(thread_group, peer)) {
+  for (art::Thread* t : art::Runtime::Current()->GetThreadList()->GetList()) {
+    if (t->IsStillStarting()) {
+      continue;
+    }
+    art::ObjPtr<art::mirror::Object> peer = t->GetPeerFromOtherThread();
+    if (peer == nullptr) {
+      continue;
+    }
+    if (IsInDesiredThreadGroup(thread_group, peer)) {
       thread_peers->push_back(peer);
     }
-    t->UnregisterThreadExitFlag(tef);
   }
-  DCHECK(i == tefs.end());
 }
 
 static void GetChildThreadGroups(art::Handle<art::mirror::Object> thread_group,
diff --git a/runtime/Android.bp b/runtime/Android.bp
index fcac3b4..4d16378 100644
--- a/runtime/Android.bp
+++ b/runtime/Android.bp
@@ -1080,7 +1080,6 @@
         "subtype_check_info_test.cc",
         "subtype_check_test.cc",
         "thread_pool_test.cc",
-        "thread_test.cc",
         "transaction_test.cc",
         "two_runtimes_test.cc",
         "vdex_file_test.cc",
diff --git a/runtime/base/locks.h b/runtime/base/locks.h
index e8c83fe..c15e5de 100644
--- a/runtime/base/locks.h
+++ b/runtime/base/locks.h
@@ -108,6 +108,10 @@
   kClassLinkerClassesLock,  // TODO rename.
   kSubtypeCheckLock,
   kBreakpointLock,
+  // This is a generic lock level for a lock meant to be gained after having a
+  // monitor lock.
+  kPostMonitorLock,
+  kMonitorLock,
   kMonitorListLock,
   kThreadListLock,
   kAllocTrackerLock,
@@ -121,10 +125,7 @@
   kRuntimeShutdownLock,
   kTraceLock,
   kHeapBitmapLock,
-  // This is a generic lock level for a lock meant to be gained after having a
-  // monitor lock.
-  kPostMonitorLock,
-  kMonitorLock,
+
   // This is a generic lock level for a top-level lock meant to be gained after having the
   // mutator_lock_.
   kPostMutatorTopLockLevel,
@@ -137,7 +138,7 @@
   kUserCodeSuspensionLock,
   kZygoteCreationLock,
 
-  // The highest valid lock level. Use this for locks that should only be acquired with no
+  // The highest valid lock level. Use this if there is code that should only be called with no
   // other locks held. Since this is the highest lock level we also allow it to be held even if the
   // runtime or current thread is not fully set-up yet (for example during thread attach). Note that
   // this lock also has special behavior around the mutator_lock_. Since the mutator_lock_ is not
diff --git a/runtime/base/mutex-inl.h b/runtime/base/mutex-inl.h
index 712b61d..dba1e12 100644
--- a/runtime/base/mutex-inl.h
+++ b/runtime/base/mutex-inl.h
@@ -60,44 +60,43 @@
   // The check below enumerates the cases where we expect not to be able to check the validity of
   // locks on a thread. Lock checking is disabled to avoid deadlock when checking shutdown lock.
   // TODO: tighten this check.
-  CHECK(!Locks::IsSafeToCallAbortRacy() ||
-        // Used during thread creation to avoid races with runtime shutdown. Thread::Current not
-        // yet established.
-        level == kRuntimeShutdownLock ||
-        // Thread Ids are allocated/released before threads are established.
-        level == kAllocatedThreadIdsLock ||
-        // Thread LDT's are initialized without Thread::Current established.
-        level == kModifyLdtLock ||
-        // Threads are unregistered while holding the thread list lock, during this process they
-        // no longer exist and so we expect an unlock with no self.
-        level == kThreadListLock ||
-        // Ignore logging which may or may not have set up thread data structures.
-        level == kLoggingLock ||
-        // When transitioning from suspended to runnable, a daemon thread might be in
-        // a situation where the runtime is shutting down. To not crash our debug locking
-        // mechanism we just pass null Thread* to the MutexLock during that transition
-        // (see Thread::TransitionFromSuspendedToRunnable).
-        level == kThreadSuspendCountLock ||
-        // Avoid recursive death.
-        level == kAbortLock ||
-        // Locks at the absolute top of the stack can be locked at any time.
-        level == kTopLockLevel ||
-        // The unexpected signal handler may be catching signals from any thread.
-        level == kUnexpectedSignalLock)
-      << level;
-}
-
-inline void BaseMutex::RegisterAsLocked(Thread* self, bool check) {
-  if (UNLIKELY(self == nullptr)) {
-    if (check) {
-      CheckUnattachedThread(level_);
-    }
-  } else {
-    RegisterAsLockedImpl(self, level_, check);
+  if (kDebugLocking) {
+    CHECK(!Locks::IsSafeToCallAbortRacy() ||
+          // Used during thread creation to avoid races with runtime shutdown. Thread::Current not
+          // yet established.
+          level == kRuntimeShutdownLock ||
+          // Thread Ids are allocated/released before threads are established.
+          level == kAllocatedThreadIdsLock ||
+          // Thread LDT's are initialized without Thread::Current established.
+          level == kModifyLdtLock ||
+          // Threads are unregistered while holding the thread list lock, during this process they
+          // no longer exist and so we expect an unlock with no self.
+          level == kThreadListLock ||
+          // Ignore logging which may or may not have set up thread data structures.
+          level == kLoggingLock ||
+          // When transitioning from suspended to runnable, a daemon thread might be in
+          // a situation where the runtime is shutting down. To not crash our debug locking
+          // mechanism we just pass null Thread* to the MutexLock during that transition
+          // (see Thread::TransitionFromSuspendedToRunnable).
+          level == kThreadSuspendCountLock ||
+          // Avoid recursive death.
+          level == kAbortLock ||
+          // Locks at the absolute top of the stack can be locked at any time.
+          level == kTopLockLevel ||
+          // The unexpected signal handler may be catching signals from any thread.
+          level == kUnexpectedSignalLock) << level;
   }
 }
 
-inline void BaseMutex::RegisterAsLockedImpl(Thread* self, LockLevel level, bool check) {
+inline void BaseMutex::RegisterAsLocked(Thread* self) {
+  if (UNLIKELY(self == nullptr)) {
+    CheckUnattachedThread(level_);
+    return;
+  }
+  RegisterAsLockedImpl(self, level_);
+}
+
+inline void BaseMutex::RegisterAsLockedImpl(Thread* self, LockLevel level) {
   DCHECK(self != nullptr);
   DCHECK_EQ(level_, level);
   // It would be nice to avoid this condition checking in the non-debug case,
@@ -108,7 +107,7 @@
   if (UNLIKELY(level == kThreadWaitLock) && self->GetHeldMutex(kThreadWaitLock) != nullptr) {
     level = kThreadWaitWakeLock;
   }
-  if (check) {
+  if (kDebugLocking) {
     // Check if a bad Mutex of this level or lower is held.
     bool bad_mutexes_held = false;
     // Specifically allow a kTopLockLevel lock to be gained when the current thread holds the
@@ -162,12 +161,10 @@
 
 inline void BaseMutex::RegisterAsUnlocked(Thread* self) {
   if (UNLIKELY(self == nullptr)) {
-    if (kDebugLocking) {
-      CheckUnattachedThread(level_);
-    }
-  } else {
-    RegisterAsUnlockedImpl(self, level_);
+    CheckUnattachedThread(level_);
+    return;
   }
+  RegisterAsUnlockedImpl(self , level_);
 }
 
 inline void BaseMutex::RegisterAsUnlockedImpl(Thread* self, LockLevel level) {
@@ -309,7 +306,7 @@
 }
 
 inline void MutatorMutex::TransitionFromSuspendedToRunnable(Thread* self) {
-  RegisterAsLockedImpl(self, kMutatorLock, kDebugLocking);
+  RegisterAsLockedImpl(self, kMutatorLock);
   AssertSharedHeld(self);
 }
 
diff --git a/runtime/base/mutex.cc b/runtime/base/mutex.cc
index 5b74bee..728dc84 100644
--- a/runtime/base/mutex.cc
+++ b/runtime/base/mutex.cc
@@ -246,12 +246,11 @@
 }
 
 void BaseMutex::CheckSafeToWait(Thread* self) {
-  if (!kDebugLocking) {
-    return;
-  }
   if (self == nullptr) {
     CheckUnattachedThread(level_);
-  } else {
+    return;
+  }
+  if (kDebugLocking) {
     CHECK(self->GetHeldMutex(level_) == this || level_ == kMonitorLock)
         << "Waiting on unacquired mutex: " << name_;
     bool bad_mutexes_held = false;
@@ -571,7 +570,6 @@
   }
 }
 
-template <bool kCheck>
 bool Mutex::ExclusiveTryLock(Thread* self) {
   DCHECK(self == nullptr || self == Thread::Current());
   if (kDebugLocking && !recursive_) {
@@ -602,7 +600,7 @@
 #endif
     DCHECK_EQ(GetExclusiveOwnerTid(), 0);
     exclusive_owner_.store(SafeGetTid(self), std::memory_order_relaxed);
-    RegisterAsLocked(self, kCheck);
+    RegisterAsLocked(self);
   }
   recursion_count_++;
   if (kDebugLocking) {
@@ -613,9 +611,6 @@
   return true;
 }
 
-template bool Mutex::ExclusiveTryLock<false>(Thread* self);
-template bool Mutex::ExclusiveTryLock<true>(Thread* self);
-
 bool Mutex::ExclusiveTryLockWithSpinning(Thread* self) {
   // Spin a small number of times, since this affects our ability to respond to suspension
   // requests. We spin repeatedly only if the mutex repeatedly becomes available and unavailable
@@ -722,12 +717,11 @@
 }
 
 void Mutex::Dump(std::ostream& os) const {
-  os << (recursive_ ? "recursive " : "non-recursive ") << name_
-     << " level=" << static_cast<int>(level_) << " rec=" << recursion_count_
-#if ART_USE_FUTEXES
-     << " state_and_contenders = " << std::hex << state_and_contenders_ << std::dec
-#endif
-     << " owner=" << GetExclusiveOwnerTid() << " ";
+  os << (recursive_ ? "recursive " : "non-recursive ")
+      << name_
+      << " level=" << static_cast<int>(level_)
+      << " rec=" << recursion_count_
+      << " owner=" << GetExclusiveOwnerTid() << " ";
   DumpContention(os);
 }
 
@@ -929,7 +923,7 @@
 }
 #endif
 
-bool ReaderWriterMutex::SharedTryLock(Thread* self, bool check) {
+bool ReaderWriterMutex::SharedTryLock(Thread* self) {
   DCHECK(self == nullptr || self == Thread::Current());
 #if ART_USE_FUTEXES
   bool done = false;
@@ -953,7 +947,7 @@
     PLOG(FATAL) << "pthread_mutex_trylock failed for " << name_;
   }
 #endif
-  RegisterAsLocked(self, check);
+  RegisterAsLocked(self);
   AssertSharedHeld(self);
   return true;
 }
diff --git a/runtime/base/mutex.h b/runtime/base/mutex.h
index dc9b885..87e9525 100644
--- a/runtime/base/mutex.h
+++ b/runtime/base/mutex.h
@@ -103,11 +103,10 @@
   BaseMutex(const char* name, LockLevel level);
   virtual ~BaseMutex();
 
-  // Add this mutex to those owned by self, and optionally perform lock order checking.  Caller
-  // may wish to disable checking for trylock calls that cannot result in deadlock.  For this call
-  // only, self may also be another suspended thread.
-  void RegisterAsLocked(Thread* self, bool check = kDebugLocking);
-  void RegisterAsLockedImpl(Thread* self, LockLevel level, bool check);
+  // Add this mutex to those owned by self, and perform appropriate checking.
+  // For this call only, self may also be another suspended thread.
+  void RegisterAsLocked(Thread* self);
+  void RegisterAsLockedImpl(Thread* self, LockLevel level);
 
   void RegisterAsUnlocked(Thread* self);
   void RegisterAsUnlockedImpl(Thread* self, LockLevel level);
@@ -184,10 +183,7 @@
   void ExclusiveLock(Thread* self) ACQUIRE();
   void Lock(Thread* self) ACQUIRE() {  ExclusiveLock(self); }
 
-  // Returns true if acquires exclusive access, false otherwise. The `check` argument specifies
-  // whether lock level checking should be performed.  Should be defaulted unless we are using
-  // TryLock instead of Lock for deadlock avoidance.
-  template <bool kCheck = kDebugLocking>
+  // Returns true if acquires exclusive access, false otherwise.
   bool ExclusiveTryLock(Thread* self) TRY_ACQUIRE(true);
   bool TryLock(Thread* self) TRY_ACQUIRE(true) { return ExclusiveTryLock(self); }
   // Equivalent to ExclusiveTryLock, but retry for a short period before giving up.
@@ -346,7 +342,7 @@
   void ReaderLock(Thread* self) ACQUIRE_SHARED() { SharedLock(self); }
 
   // Try to acquire share of ReaderWriterMutex.
-  bool SharedTryLock(Thread* self, bool check = kDebugLocking) SHARED_TRYLOCK_FUNCTION(true);
+  bool SharedTryLock(Thread* self) SHARED_TRYLOCK_FUNCTION(true);
 
   // Release a share of the access.
   void SharedUnlock(Thread* self) RELEASE_SHARED() ALWAYS_INLINE;
@@ -524,18 +520,6 @@
   DISALLOW_COPY_AND_ASSIGN(MutexLock);
 };
 
-// Pretend to acquire a mutex for checking purposes, without actually doing so. Use with
-// extreme caution when it is known the condition that the mutex would guard against cannot arise.
-class SCOPED_CAPABILITY FakeMutexLock {
- public:
-  explicit FakeMutexLock(Mutex& mu) ACQUIRE(mu) NO_THREAD_SAFETY_ANALYSIS {}
-
-  ~FakeMutexLock() RELEASE() NO_THREAD_SAFETY_ANALYSIS {}
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(FakeMutexLock);
-};
-
 // Scoped locker/unlocker for a ReaderWriterMutex that acquires read access to mu upon
 // construction and releases it upon destruction.
 class SCOPED_CAPABILITY ReaderMutexLock {
diff --git a/runtime/cha.cc b/runtime/cha.cc
index 0c548d3..8b77f76 100644
--- a/runtime/cha.cc
+++ b/runtime/cha.cc
@@ -237,10 +237,11 @@
       : barrier_(0),
         method_headers_(method_headers) {}
 
-  void Run(Thread* thread) override REQUIRES_SHARED(Locks::mutator_lock_) {
+  void Run(Thread* thread) override {
     // Note thread and self may not be equal if thread was already suspended at
     // the point of the request.
     Thread* self = Thread::Current();
+    ScopedObjectAccess soa(self);
     CHAStackVisitor visitor(thread, nullptr, method_headers_);
     visitor.WalkStack();
     barrier_.Pass(self);
diff --git a/runtime/debugger.cc b/runtime/debugger.cc
index e553515..9b0f49f 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -340,11 +340,7 @@
       Dbg::DdmSendThreadNotification(thread, CHUNK_TYPE("THCR"));
       finish_barrier.Pass(cls_self);
     });
-    // TODO(b/253671779): The above eventually results in calls to EventHandler::DispatchEvent,
-    // which does a ScopedThreadStateChange, which amounts to a thread state change inside the
-    // checkpoint run method. Hence the normal check would fail, and thus we specify Unchecked
-    // here.
-    size_t checkpoints = Runtime::Current()->GetThreadList()->RunCheckpointUnchecked(&fc);
+    size_t checkpoints = Runtime::Current()->GetThreadList()->RunCheckpoint(&fc);
     ScopedThreadSuspension sts(self, ThreadState::kWaitingForCheckPointsToRun);
     finish_barrier.Increment(self, checkpoints);
   }
diff --git a/runtime/entrypoints_order_test.cc b/runtime/entrypoints_order_test.cc
index a2d5a43..9b4af66 100644
--- a/runtime/entrypoints_order_test.cc
+++ b/runtime/entrypoints_order_test.cc
@@ -70,6 +70,7 @@
     EXPECT_OFFSET_DIFFP(Thread, tls32_, daemon, throwing_OutOfMemoryError, 4);
     EXPECT_OFFSET_DIFFP(Thread, tls32_, throwing_OutOfMemoryError, no_thread_suspension, 4);
     EXPECT_OFFSET_DIFFP(Thread, tls32_, no_thread_suspension, thread_exit_check_count, 4);
+    EXPECT_OFFSET_DIFFP(Thread, tls32_, thread_exit_check_count, is_transitioning_to_runnable, 4);
 
     // TODO: Better connection. Take alignment into account.
     EXPECT_OFFSET_DIFF_GT3(Thread, tls32_.thread_exit_check_count, tls64_.trace_clock_base, 4,
@@ -107,14 +108,13 @@
     EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, name, pthread_self, sizeof(void*));
     EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, pthread_self, last_no_thread_suspension_cause,
                         sizeof(void*));
-    EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, last_no_thread_suspension_cause,
-                        active_suspendall_barrier, sizeof(void*));
-    EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, active_suspendall_barrier,
-                        active_suspend1_barriers, sizeof(void*));
-    EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, active_suspend1_barriers, thread_local_pos, sizeof(void*));
+    EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, last_no_thread_suspension_cause, active_suspend_barriers,
+                        sizeof(void*));
+    EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, active_suspend_barriers, thread_local_start,
+                        sizeof(Thread::tls_ptr_sized_values::active_suspend_barriers));
+    EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, thread_local_start, thread_local_pos, sizeof(void*));
     EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, thread_local_pos, thread_local_end, sizeof(void*));
-    EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, thread_local_end, thread_local_start, sizeof(void*));
-    EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, thread_local_start, thread_local_limit, sizeof(void*));
+    EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, thread_local_end, thread_local_limit, sizeof(void*));
     EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, thread_local_limit, thread_local_objects, sizeof(void*));
     EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, thread_local_objects, checkpoint_function, sizeof(size_t));
     EXPECT_OFFSET_DIFFP(Thread, tlsPtr_, checkpoint_function, jni_entrypoints,
@@ -137,12 +137,11 @@
         Thread, tlsPtr_, top_reflective_handle_scope, method_trace_buffer, sizeof(void*));
     EXPECT_OFFSET_DIFFP(
         Thread, tlsPtr_, method_trace_buffer, method_trace_buffer_index, sizeof(void*));
-    EXPECT_OFFSET_DIFFP(
-        Thread, tlsPtr_, method_trace_buffer_index, thread_exit_flags, sizeof(void*));
     // The first field after tlsPtr_ is forced to a 16 byte alignment so it might have some space.
     auto offset_tlsptr_end = OFFSETOF_MEMBER(Thread, tlsPtr_) +
         sizeof(decltype(reinterpret_cast<Thread*>(16)->tlsPtr_));
-    CHECKED(offset_tlsptr_end - OFFSETOF_MEMBER(Thread, tlsPtr_.thread_exit_flags) == sizeof(void*),
+    CHECKED(offset_tlsptr_end - OFFSETOF_MEMBER(Thread, tlsPtr_.method_trace_buffer_index) ==
+                sizeof(void*),
             "async_exception last field");
   }
 
diff --git a/runtime/gc/collector/concurrent_copying.cc b/runtime/gc/collector/concurrent_copying.cc
index 90d8cd4..370e01b 100644
--- a/runtime/gc/collector/concurrent_copying.cc
+++ b/runtime/gc/collector/concurrent_copying.cc
@@ -480,8 +480,7 @@
   }
 
   void Run(Thread* thread) override REQUIRES_SHARED(Locks::mutator_lock_) {
-    // We are either running this in the target thread, or the target thread will wait for us
-    // before switching back to runnable.
+    // Note: self is not necessarily equal to thread since thread may be suspended.
     Thread* self = Thread::Current();
     CHECK(thread == self || thread->GetState() != ThreadState::kRunnable)
         << thread->GetState() << " thread " << thread << " self " << self;
@@ -496,6 +495,7 @@
     // We can use the non-CAS VisitRoots functions below because we update thread-local GC roots
     // only.
     thread->VisitRoots(this, kVisitRootFlagAllRoots);
+    concurrent_copying_->GetBarrier().Pass(self);
   }
 
   void VisitRoots(mirror::Object*** roots,
@@ -764,12 +764,17 @@
   }
   Thread* self = Thread::Current();
   Locks::mutator_lock_->AssertNotHeld(self);
+  gc_barrier_->Init(self, 0);
   ThreadFlipVisitor thread_flip_visitor(this, heap_->use_tlab_);
   FlipCallback flip_callback(this);
 
-  Runtime::Current()->GetThreadList()->FlipThreadRoots(
+  size_t barrier_count = Runtime::Current()->GetThreadList()->FlipThreadRoots(
       &thread_flip_visitor, &flip_callback, this, GetHeap()->GetGcPauseListener());
 
+  {
+    ScopedThreadStateChange tsc(self, ThreadState::kWaitingForCheckPointsToRun);
+    gc_barrier_->Increment(self, barrier_count);
+  }
   is_asserting_to_space_invariant_ = true;
   QuasiAtomic::ThreadFenceForConstructor();  // TODO: Remove?
   if (kVerboseMode) {
diff --git a/runtime/gc/collector/mark_compact.cc b/runtime/gc/collector/mark_compact.cc
index 27bb5f2..4da7faf 100644
--- a/runtime/gc/collector/mark_compact.cc
+++ b/runtime/gc/collector/mark_compact.cc
@@ -754,6 +754,7 @@
     CHECK(collector_->compacting_);
     thread->SweepInterpreterCache(collector_);
     thread->AdjustTlab(collector_->black_objs_slide_diff_);
+    collector_->GetBarrier().Pass(self);
   }
 
  private:
@@ -801,10 +802,15 @@
 
   {
     // Compaction pause
+    gc_barrier_.Init(self, 0);
     ThreadFlipVisitor visitor(this);
     FlipCallback callback(this);
-    runtime->GetThreadList()->FlipThreadRoots(
+    size_t barrier_count = runtime->GetThreadList()->FlipThreadRoots(
         &visitor, &callback, this, GetHeap()->GetGcPauseListener());
+    {
+      ScopedThreadStateChange tsc(self, ThreadState::kWaitingForCheckPointsToRun);
+      gc_barrier_.Increment(self, barrier_count);
+    }
   }
 
   if (IsValidFd(uffd_)) {
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index 4671b75..b4f703e 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -2730,122 +2730,113 @@
   }
   ScopedThreadStateChange tsc(self, ThreadState::kWaitingPerformingGc);
   Locks::mutator_lock_->AssertNotHeld(self);
-  SelfDeletingTask* clear;  // Unconditionally set below.
+  if (self->IsHandlingStackOverflow()) {
+    // If we are throwing a stack overflow error we probably don't have enough remaining stack
+    // space to run the GC.
+    // Count this as a GC in case someone is waiting for it to complete.
+    gcs_completed_.fetch_add(1, std::memory_order_release);
+    return collector::kGcTypeNone;
+  }
+  bool compacting_gc;
   {
-    // We should not ever become runnable and re-suspend while executing a GC.
-    // This would likely cause a deadlock if we acted on a suspension request.
-    // TODO: We really want to assert that we don't transition to kRunnable.
-    ScopedAssertNoThreadSuspension("Performing GC");
-    if (self->IsHandlingStackOverflow()) {
-      // If we are throwing a stack overflow error we probably don't have enough remaining stack
-      // space to run the GC.
-      // Count this as a GC in case someone is waiting for it to complete.
+    gc_complete_lock_->AssertNotHeld(self);
+    ScopedThreadStateChange tsc2(self, ThreadState::kWaitingForGcToComplete);
+    MutexLock mu(self, *gc_complete_lock_);
+    // Ensure there is only one GC at a time.
+    WaitForGcToCompleteLocked(gc_cause, self);
+    if (requested_gc_num != GC_NUM_ANY && !GCNumberLt(GetCurrentGcNum(), requested_gc_num)) {
+      // The appropriate GC was already triggered elsewhere.
+      return collector::kGcTypeNone;
+    }
+    compacting_gc = IsMovingGc(collector_type_);
+    // GC can be disabled if someone has a used GetPrimitiveArrayCritical.
+    if (compacting_gc && disable_moving_gc_count_ != 0) {
+      LOG(WARNING) << "Skipping GC due to disable moving GC count " << disable_moving_gc_count_;
+      // Again count this as a GC.
       gcs_completed_.fetch_add(1, std::memory_order_release);
       return collector::kGcTypeNone;
     }
-    bool compacting_gc;
-    {
-      gc_complete_lock_->AssertNotHeld(self);
-      ScopedThreadStateChange tsc2(self, ThreadState::kWaitingForGcToComplete);
-      MutexLock mu(self, *gc_complete_lock_);
-      // Ensure there is only one GC at a time.
-      WaitForGcToCompleteLocked(gc_cause, self);
-      if (requested_gc_num != GC_NUM_ANY && !GCNumberLt(GetCurrentGcNum(), requested_gc_num)) {
-        // The appropriate GC was already triggered elsewhere.
-        return collector::kGcTypeNone;
-      }
-      compacting_gc = IsMovingGc(collector_type_);
-      // GC can be disabled if someone has a used GetPrimitiveArrayCritical.
-      if (compacting_gc && disable_moving_gc_count_ != 0) {
-        LOG(WARNING) << "Skipping GC due to disable moving GC count " << disable_moving_gc_count_;
-        // Again count this as a GC.
-        gcs_completed_.fetch_add(1, std::memory_order_release);
-        return collector::kGcTypeNone;
-      }
-      if (gc_disabled_for_shutdown_) {
-        gcs_completed_.fetch_add(1, std::memory_order_release);
-        return collector::kGcTypeNone;
-      }
-      collector_type_running_ = collector_type_;
-      last_gc_cause_ = gc_cause;
+    if (gc_disabled_for_shutdown_) {
+      gcs_completed_.fetch_add(1, std::memory_order_release);
+      return collector::kGcTypeNone;
     }
-    if (gc_cause == kGcCauseForAlloc && runtime->HasStatsEnabled()) {
-      ++runtime->GetStats()->gc_for_alloc_count;
-      ++self->GetStats()->gc_for_alloc_count;
-    }
-    const size_t bytes_allocated_before_gc = GetBytesAllocated();
-
-    DCHECK_LT(gc_type, collector::kGcTypeMax);
-    DCHECK_NE(gc_type, collector::kGcTypeNone);
-
-    collector::GarbageCollector* collector = nullptr;
-    // TODO: Clean this up.
-    if (compacting_gc) {
-      DCHECK(current_allocator_ == kAllocatorTypeBumpPointer ||
-             current_allocator_ == kAllocatorTypeTLAB ||
-             current_allocator_ == kAllocatorTypeRegion ||
-             current_allocator_ == kAllocatorTypeRegionTLAB);
-      switch (collector_type_) {
-        case kCollectorTypeSS:
-          semi_space_collector_->SetFromSpace(bump_pointer_space_);
-          semi_space_collector_->SetToSpace(temp_space_);
-          semi_space_collector_->SetSwapSemiSpaces(true);
-          collector = semi_space_collector_;
-          break;
-        case kCollectorTypeCMC:
-          collector = mark_compact_;
-          break;
-        case kCollectorTypeCC:
-          collector::ConcurrentCopying* active_cc_collector;
-          if (use_generational_cc_) {
-            // TODO: Other threads must do the flip checkpoint before they start poking at
-            // active_concurrent_copying_collector_. So we should not concurrency here.
-            active_cc_collector = (gc_type == collector::kGcTypeSticky) ?
-                                      young_concurrent_copying_collector_ :
-                                      concurrent_copying_collector_;
-            active_concurrent_copying_collector_.store(active_cc_collector,
-                                                       std::memory_order_relaxed);
-            DCHECK(active_cc_collector->RegionSpace() == region_space_);
-            collector = active_cc_collector;
-          } else {
-            collector = active_concurrent_copying_collector_.load(std::memory_order_relaxed);
-          }
-          break;
-        default:
-          LOG(FATAL) << "Invalid collector type " << static_cast<size_t>(collector_type_);
-      }
-      // temp_space_ will be null for kCollectorTypeCMC.
-      if (temp_space_ != nullptr &&
-          collector != active_concurrent_copying_collector_.load(std::memory_order_relaxed)) {
-        temp_space_->GetMemMap()->Protect(PROT_READ | PROT_WRITE);
-        if (kIsDebugBuild) {
-          // Try to read each page of the memory map in case mprotect didn't work properly
-          // b/19894268.
-          temp_space_->GetMemMap()->TryReadable();
-        }
-        CHECK(temp_space_->IsEmpty());
-      }
-    } else if (current_allocator_ == kAllocatorTypeRosAlloc ||
-               current_allocator_ == kAllocatorTypeDlMalloc) {
-      collector = FindCollectorByGcType(gc_type);
-    } else {
-      LOG(FATAL) << "Invalid current allocator " << current_allocator_;
-    }
-
-    CHECK(collector != nullptr) << "Could not find garbage collector with collector_type="
-                                << static_cast<size_t>(collector_type_)
-                                << " and gc_type=" << gc_type;
-    collector->Run(gc_cause, clear_soft_references || runtime->IsZygote());
-    IncrementFreedEver();
-    RequestTrim(self);
-    // Collect cleared references.
-    clear = reference_processor_->CollectClearedReferences(self);
-    // Grow the heap so that we know when to perform the next GC.
-    GrowForUtilization(collector, bytes_allocated_before_gc);
-    old_native_bytes_allocated_.store(GetNativeBytes());
-    LogGC(gc_cause, collector);
-    FinishGC(self, gc_type);
+    collector_type_running_ = collector_type_;
+    last_gc_cause_ = gc_cause;
   }
+  if (gc_cause == kGcCauseForAlloc && runtime->HasStatsEnabled()) {
+    ++runtime->GetStats()->gc_for_alloc_count;
+    ++self->GetStats()->gc_for_alloc_count;
+  }
+  const size_t bytes_allocated_before_gc = GetBytesAllocated();
+
+  DCHECK_LT(gc_type, collector::kGcTypeMax);
+  DCHECK_NE(gc_type, collector::kGcTypeNone);
+
+  collector::GarbageCollector* collector = nullptr;
+  // TODO: Clean this up.
+  if (compacting_gc) {
+    DCHECK(current_allocator_ == kAllocatorTypeBumpPointer ||
+           current_allocator_ == kAllocatorTypeTLAB ||
+           current_allocator_ == kAllocatorTypeRegion ||
+           current_allocator_ == kAllocatorTypeRegionTLAB);
+    switch (collector_type_) {
+      case kCollectorTypeSS:
+        semi_space_collector_->SetFromSpace(bump_pointer_space_);
+        semi_space_collector_->SetToSpace(temp_space_);
+        semi_space_collector_->SetSwapSemiSpaces(true);
+        collector = semi_space_collector_;
+        break;
+      case kCollectorTypeCMC:
+        collector = mark_compact_;
+        break;
+      case kCollectorTypeCC:
+        collector::ConcurrentCopying* active_cc_collector;
+        if (use_generational_cc_) {
+          // TODO: Other threads must do the flip checkpoint before they start poking at
+          // active_concurrent_copying_collector_. So we should not concurrency here.
+          active_cc_collector = (gc_type == collector::kGcTypeSticky) ?
+                  young_concurrent_copying_collector_ : concurrent_copying_collector_;
+          active_concurrent_copying_collector_.store(active_cc_collector,
+                                                     std::memory_order_relaxed);
+          DCHECK(active_cc_collector->RegionSpace() == region_space_);
+          collector = active_cc_collector;
+        } else {
+          collector = active_concurrent_copying_collector_.load(std::memory_order_relaxed);
+        }
+        break;
+      default:
+        LOG(FATAL) << "Invalid collector type " << static_cast<size_t>(collector_type_);
+    }
+    // temp_space_ will be null for kCollectorTypeCMC.
+    if (temp_space_ != nullptr
+        && collector != active_concurrent_copying_collector_.load(std::memory_order_relaxed)) {
+      temp_space_->GetMemMap()->Protect(PROT_READ | PROT_WRITE);
+      if (kIsDebugBuild) {
+        // Try to read each page of the memory map in case mprotect didn't work properly b/19894268.
+        temp_space_->GetMemMap()->TryReadable();
+      }
+      CHECK(temp_space_->IsEmpty());
+    }
+  } else if (current_allocator_ == kAllocatorTypeRosAlloc ||
+      current_allocator_ == kAllocatorTypeDlMalloc) {
+    collector = FindCollectorByGcType(gc_type);
+  } else {
+    LOG(FATAL) << "Invalid current allocator " << current_allocator_;
+  }
+
+  CHECK(collector != nullptr)
+      << "Could not find garbage collector with collector_type="
+      << static_cast<size_t>(collector_type_) << " and gc_type=" << gc_type;
+  collector->Run(gc_cause, clear_soft_references || runtime->IsZygote());
+  IncrementFreedEver();
+  RequestTrim(self);
+  // Collect cleared references.
+  SelfDeletingTask* clear = reference_processor_->CollectClearedReferences(self);
+  // Grow the heap so that we know when to perform the next GC.
+  GrowForUtilization(collector, bytes_allocated_before_gc);
+  old_native_bytes_allocated_.store(GetNativeBytes());
+  LogGC(gc_cause, collector);
+  FinishGC(self, gc_type);
   // Actually enqueue all cleared references. Do this after the GC has officially finished since
   // otherwise we can deadlock.
   clear->Run(self);
@@ -3612,7 +3603,7 @@
   GcCause last_gc_cause = kGcCauseNone;
   uint64_t wait_start = NanoTime();
   while (collector_type_running_ != kCollectorTypeNone) {
-    if (!task_processor_->IsRunningThread(self)) {
+    if (self != task_processor_->GetRunningThread()) {
       // The current thread is about to wait for a currently running
       // collection to finish. If the waiting thread is not the heap
       // task daemon thread, the currently running collection is
@@ -3632,7 +3623,7 @@
     LOG(INFO) << "WaitForGcToComplete blocked " << cause << " on " << last_gc_cause << " for "
               << PrettyDuration(wait_time);
   }
-  if (!task_processor_->IsRunningThread(self)) {
+  if (self != task_processor_->GetRunningThread()) {
     // The current thread is about to run a collection. If the thread
     // is not the heap task daemon thread, it's considered as a
     // blocking GC (i.e., blocking itself).
diff --git a/runtime/gc/task_processor.cc b/runtime/gc/task_processor.cc
index e56dbd1..494cf2b 100644
--- a/runtime/gc/task_processor.cc
+++ b/runtime/gc/task_processor.cc
@@ -103,31 +103,9 @@
   return is_running_;
 }
 
-bool TaskProcessor::WaitForThread(Thread* self) {
-  // Waiting for too little time here may cause us to fail to get stack traces, since we can't
-  // safely do so without identifying a HeapTaskDaemon to avoid it. Waiting too long could
-  // conceivably deadlock if we somehow try to get a stack trace on the way to starting the
-  // HeapTaskDaemon. Under normal circumstances. this should terminate immediately, since
-  // HeapTaskDaemon should normally be running.
-  constexpr int kTotalWaitMillis = 100;
-  for (int i = 0; i < kTotalWaitMillis; ++i) {
-    if (is_running_) {
-      return true;
-    }
-    cond_.TimedWait(self, 1 /*msecs*/, 0 /*nsecs*/);
-  }
-  LOG(ERROR) << "No identifiable HeapTaskDaemon; unsafe to get thread stacks.";
-  return false;
-}
-
-bool TaskProcessor::IsRunningThread(Thread* t, bool wait) {
-  Thread* self = Thread::Current();
-  MutexLock mu(self, lock_);
-  if (wait && !WaitForThread(self)) {
-    // If Wait failed, either answer may be correct; in our case, true is safer.
-    return true;
-  }
-  return running_thread_ == t;
+Thread* TaskProcessor::GetRunningThread() const {
+  MutexLock mu(Thread::Current(), lock_);
+  return running_thread_;
 }
 
 void TaskProcessor::Stop(Thread* self) {
diff --git a/runtime/gc/task_processor.h b/runtime/gc/task_processor.h
index b9e6938..86e36ab 100644
--- a/runtime/gc/task_processor.h
+++ b/runtime/gc/task_processor.h
@@ -64,15 +64,9 @@
   bool IsRunning() const REQUIRES(!lock_);
   void UpdateTargetRunTime(Thread* self, HeapTask* target_time, uint64_t new_target_time)
       REQUIRES(!lock_);
-  // Is the given thread the task processor thread?
-  // If wait is true, and no thread has been registered via Start(), we briefly
-  // wait for one to be registered. If we time out, we return true.
-  bool IsRunningThread(Thread* t, bool wait = false) REQUIRES(!lock_);
+  Thread* GetRunningThread() const REQUIRES(!lock_);
 
  private:
-  // Wait briefly for running_thread_ to become non-null. Return false on timeout.
-  bool WaitForThread(Thread* self) REQUIRES(lock_);
-
   class CompareByTargetRunTime {
    public:
     bool operator()(const HeapTask* a, const HeapTask* b) const {
diff --git a/runtime/mirror/object.cc b/runtime/mirror/object.cc
index 995f06e..940b82d 100644
--- a/runtime/mirror/object.cc
+++ b/runtime/mirror/object.cc
@@ -185,8 +185,7 @@
   hash_code_seed.store(new_seed, std::memory_order_relaxed);
 }
 
-template <bool kAllowInflation>
-int32_t Object::IdentityHashCodeHelper() {
+int32_t Object::IdentityHashCode() {
   ObjPtr<Object> current_this = this;  // The this pointer may get invalidated by thread suspension.
   while (true) {
     LockWord lw = current_this->GetLockWord(false);
@@ -204,9 +203,6 @@
         break;
       }
       case LockWord::kThinLocked: {
-        if (!kAllowInflation) {
-          return 0;
-        }
         // Inflate the thin lock to a monitor and stick the hash code inside of the monitor. May
         // fail spuriously.
         Thread* self = Thread::Current();
@@ -234,12 +230,6 @@
   }
 }
 
-int32_t Object::IdentityHashCode() { return IdentityHashCodeHelper</* kAllowInflation= */ true>(); }
-
-int32_t Object::IdentityHashCodeNoInflation() {
-  return IdentityHashCodeHelper</* kAllowInflation= */ false>();
-}
-
 void Object::CheckFieldAssignmentImpl(MemberOffset field_offset, ObjPtr<Object> new_value) {
   ObjPtr<Class> c = GetClass();
   Runtime* runtime = Runtime::Current();
diff --git a/runtime/mirror/object.h b/runtime/mirror/object.h
index ecd8a48..54a17b1 100644
--- a/runtime/mirror/object.h
+++ b/runtime/mirror/object.h
@@ -138,16 +138,11 @@
       REQUIRES_SHARED(Locks::mutator_lock_)
       REQUIRES(!Roles::uninterruptible_);
 
-  // Returns a nonzero value that fits into lockword slot.
   int32_t IdentityHashCode()
       REQUIRES_SHARED(Locks::mutator_lock_)
       REQUIRES(!Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_);
 
-  // Identical to the above, but returns 0 if monitor inflation would otherwise be needed.
-  int32_t IdentityHashCodeNoInflation() REQUIRES_SHARED(Locks::mutator_lock_)
-      REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
-
   static constexpr MemberOffset MonitorOffset() {
     return OFFSET_OF_OBJECT_MEMBER(Object, monitor_);
   }
@@ -732,10 +727,6 @@
       REQUIRES_SHARED(Locks::mutator_lock_);
 
  private:
-  template <bool kAllowInflation>
-  int32_t IdentityHashCodeHelper() REQUIRES_SHARED(Locks::mutator_lock_)
-      REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
-
   // Get a field with acquire semantics.
   template<typename kSize>
   ALWAYS_INLINE kSize GetFieldAcquire(MemberOffset field_offset)
diff --git a/runtime/monitor.cc b/runtime/monitor.cc
index 4e9e4d9..3fed8d4 100644
--- a/runtime/monitor.cc
+++ b/runtime/monitor.cc
@@ -1005,7 +1005,7 @@
     if (monitor->num_waiters_.load(std::memory_order_relaxed) > 0) {
       return false;
     }
-    if (!monitor->monitor_lock_.ExclusiveTryLock</* check= */ false>(self)) {
+    if (!monitor->monitor_lock_.ExclusiveTryLock(self)) {
       // We cannot deflate a monitor that's currently held. It's unclear whether we should if
       // we could.
       return false;
@@ -1065,10 +1065,13 @@
     ThreadList* thread_list = Runtime::Current()->GetThreadList();
     // Suspend the owner, inflate. First change to blocked and give up mutator_lock_.
     self->SetMonitorEnterObject(obj.Get());
+    bool timed_out;
     Thread* owner;
     {
       ScopedThreadSuspension sts(self, ThreadState::kWaitingForLockInflation);
-      owner = thread_list->SuspendThreadByThreadId(owner_thread_id, SuspendReason::kInternal);
+      owner = thread_list->SuspendThreadByThreadId(owner_thread_id,
+                                                   SuspendReason::kInternal,
+                                                   &timed_out);
     }
     if (owner != nullptr) {
       // We succeeded in suspending the thread, check the lock's status didn't change.
diff --git a/runtime/mutator_gc_coord.md b/runtime/mutator_gc_coord.md
index 01e3ef0..aba8421 100644
--- a/runtime/mutator_gc_coord.md
+++ b/runtime/mutator_gc_coord.md
@@ -100,11 +100,7 @@
 
 Logically the mutator lock is held in shared/reader mode if ***either*** the
 underlying reader-writer lock is held in shared mode, ***or*** if a mutator is in
-runnable state. These two ways of holding the mutator mutex are ***not***
-equivalent: In particular, we rely on the garbage collector never actually
-entering a "runnable" state while active (see below). However, it often runs with
-the explicit mutator mutex in shared mode, thus blocking others from acquiring it
-in exclusive mode.
+runnable state.
 
 Suspension and checkpoint API
 -----------------------------
@@ -221,99 +217,13 @@
 checkpoints do not preclude client threads from being in the middle of an
 operation that involves a weak reference access, while nonempty checkpoints do.
 
-**Suspending the GC**
-Under unusual conditions, the GC can run on any thread. This means that when
-thread *A* suspends thread *B* for some other reason, Thread *B* might be
-running the garbage collector and conceivably thus cause it to block.  This
-would be very deadlock prone. If Thread *A* allocates while Thread *B* is
-suspended in the GC, and the allocation requires the GC's help to complete, we
-deadlock.
 
-Thus we ensure that the GC, together with anything else that can block GCs,
-cannot be blocked for thread suspension requests. This is accomplished by
-ensuring that it always appears to be in a suspended thread state. Since we
-only check for suspend requests when entering the runnable state, suspend
-requests go unnoticed until the GC completes. It may physically acquire and
-release the actual `mutator_lock_` in either shared or exclusive mode.
-
-Thread Suspension Mechanics
----------------------------
-
-Thread suspension is initiated by a registered thread, except that, for testing
-purposes, `SuspendAll` may be invoked with `self == nullptr`.  We never suspend
-the initiating thread, explicitly exclusing it from `SuspendAll()`, and failing
-`SuspendThreadBy...()` requests to that effect.
-
-The suspend calls invoke `IncrementSuspendCount()` to increment the thread
-suspend count for each thread. That adds a "suspend barrier" (atomic counter) to
-the per-thread list of such counters to decrement. It normally sets the
-`kSuspendRequest` ("should enter safepoint handler") and `kActiveSuspendBarrier`
-("need to notify us when suspended") flags.
-
-After setting these two flags, we check whether the thread is suspended and
-`kSuspendRequest` is still set. Since the thread is already suspended, it cannot
-be expected to respond to "pass the suspend barrier" (decrement the atomic
-counter) in a timely fashion.  Hence we do so on its behalf. This decrements
-the "barrier" and removes it from the thread's list of barriers to decrement,
-and clears `kActiveSuspendBarrier`. `kSuspendRequest` remains to ensure the
-thread doesn't prematurely return to runnable state.
-
-If `SuspendAllInternal()` does not immediately see a suspended state, then it is up
-to the target thread to decrement the suspend barrier.
-`TransitionFromRunnableToSuspended()` calls
-`TransitionToSuspendedAndRunCheckpoints()`, which changes the thread state
-and returns. `TransitionFromRunnableToSuspended()` then calls
-`CheckActiveSuspendBarriers()` to check for the `kActiveSuspendBarrier` flag
-and decrement the suspend barrier if set.
-
-The `suspend_count_lock_` is not consistently held in the target thread
-during this process.  Thus correctness in resolving the race between a
-suspension-requesting thread and a target thread voluntarily suspending relies
-on first requesting suspension, and then checking whether the target is
-already suspended, The detailed correctness argument is given in a comment
-inside `SuspendAllInternal()`. This also ensures that the barrier cannot be
-decremented after the stack frame holding the barrier goes away.
-
-This relies on the fact that the two stores in the two threads to the state and
-kActiveSuspendBarrier flag are ordered with respect to the later loads. That's
-guaranteed, since they are all stored in a single `atomic<>`. Thus even relaxed
-accesses are OK.
-
-The actual suspend barrier representation still varies between `SuspendAll()`
-and `SuspendThreadBy...()`.  The former relies on the fact that only one such
-barrier can be in use at a time, while the latter maintains a linked list of
-active suspend barriers for each target thread, relying on the fact that each
-one can appear on the list of only one thread, and we can thus use list nodes
-allocated in the stack frames of requesting threads.
-
-**Avoiding suspension cycles**
-
-Any thread can issue a `SuspendThreadByPeer()`, `SuspendThreadByThreadId()` or
-`SuspendAll()` request. But if Thread A increments Thread B's suspend count
-while Thread B increments Thread A's suspend count, and they then both suspend
-during a subsequent thread transition, we're deadlocked.
-
-For single-thread suspension requests, we refuse to initiate
-a suspend request from a registered thread that is also being asked to suspend
-(i.e. the suspend count is nonzero).  Instead the requestor waits for that
-condition to change.  This means that we cannot create a cycle in which each
-thread has asked to suspend the next one, and thus no thread can progress.  The
-required atomicity of the requestor suspend count check with setting the suspend
-count of the target(s) target is ensured by holding `suspend_count_lock_`.
-
-For `SuspendAll()`, we enforce a requirement that at most one `SuspendAll()`
-request is running at one time. We also set the `kSuspensionImmune` thread flag
-to prevent a single thread suspension of a thread currently between
-`SuspendAll()` and `ResumeAll()` calls. Thus once a `SuspendAll()` call starts,
-it will complete before it can be affected by suspension requests from other
-threads.
-
-[^1]: In the most recent versions of ART, compiler-generated code loads through
-    the address at `tlsPtr_.suspend_trigger`. A thread suspension is requested
-    by setting this to null, triggering a `SIGSEGV`, causing that thread to
-    check for GC cooperation requests. The older mechanism instead sets an
-    appropriate `ThreadFlag` entry to request suspension or a checkpoint. Note
-    that the actual checkpoint function value is set, along with the flag, while
-    holding `suspend_count_lock_`. If the target thread notices that a
-    checkpoint is requested, it then acquires the `suspend_count_lock_` to read
-    the checkpoint function.
+[^1]: Some comments in the code refer to a not-yet-really-implemented scheme in
+which the compiler-generated code would load through the address at
+`tlsPtr_.suspend_trigger`. A thread suspension is requested by setting this to
+null, triggering a `SIGSEGV`, causing that thread to check for GC cooperation
+requests. The real mechanism instead sets an appropriate `ThreadFlag` entry to
+request suspension or a checkpoint. Note that the actual checkpoint function
+value is set, along with the flag, while holding `suspend_count_lock_`. If the
+target thread notices that a checkpoint is requested, it then acquires
+the `suspend_count_lock_` to read the checkpoint function.
diff --git a/runtime/native/dalvik_system_VMStack.cc b/runtime/native/dalvik_system_VMStack.cc
index 596b2eb..71078c9 100644
--- a/runtime/native/dalvik_system_VMStack.cc
+++ b/runtime/native/dalvik_system_VMStack.cc
@@ -45,30 +45,35 @@
   ObjPtr<mirror::Object> decoded_peer = soa.Decode<mirror::Object>(peer);
   if (decoded_peer == soa.Self()->GetPeer()) {
     trace = fn(soa.Self(), soa);
-    return trace;
-  }
-  // Suspend thread to build stack trace.
-  ScopedThreadSuspension sts(soa.Self(), ThreadState::kNative);
-  Runtime* runtime = Runtime::Current();
-  ThreadList* thread_list = runtime->GetThreadList();
-  Thread* thread = thread_list->SuspendThreadByPeer(peer, SuspendReason::kInternal);
-  if (thread != nullptr) {
-    // If we were asked for the HeapTaskDaemon's stack trace, we went ahead and suspended it.
-    // It's usually already in a suspended state anyway. But we should immediately give up and
-    // resume it, since we must be able to allocate while generating the stack trace.
-    if (!runtime->GetHeap()->GetTaskProcessor()->IsRunningThread(thread, /*wait=*/true)) {
+  } else {
+    // Never allow suspending the heap task thread since it may deadlock if allocations are
+    // required for the stack trace.
+    Thread* heap_task_thread =
+        Runtime::Current()->GetHeap()->GetTaskProcessor()->GetRunningThread();
+    // heap_task_thread could be null if the daemons aren't yet started.
+    if (heap_task_thread != nullptr && decoded_peer == heap_task_thread->GetPeerFromOtherThread()) {
+      return nullptr;
+    }
+    // Suspend thread to build stack trace.
+    ScopedThreadSuspension sts(soa.Self(), ThreadState::kNative);
+    ThreadList* thread_list = Runtime::Current()->GetThreadList();
+    bool timed_out;
+    Thread* thread = thread_list->SuspendThreadByPeer(peer,
+                                                      SuspendReason::kInternal,
+                                                      &timed_out);
+    if (thread != nullptr) {
+      // Must be runnable to create returned array.
       {
-        // Must be runnable to create returned array.
         ScopedObjectAccess soa2(soa.Self());
         trace = fn(thread, soa);
       }
-      // Else either thread is the HeapTaskDaemon, or we couldn't identify the thread yet. The
-      // HeapTaskDaemon can appear in enumerations before it is registered with the task
-      // processor, and we don't wait indefinitely, so there is a tiny chance of the latter.
+      // Restart suspended thread.
+      bool resumed = thread_list->Resume(thread, SuspendReason::kInternal);
+      DCHECK(resumed);
+    } else if (timed_out) {
+      LOG(ERROR) << "Trying to get thread's stack failed as the thread failed to suspend within a "
+          "generous timeout.";
     }
-    // Restart suspended thread.
-    bool resumed = thread_list->Resume(thread, SuspendReason::kInternal);
-    DCHECK(resumed);
   }
   return trace;
 }
diff --git a/runtime/native/java_lang_Thread.cc b/runtime/native/java_lang_Thread.cc
index fd67a0a..570c554 100644
--- a/runtime/native/java_lang_Thread.cc
+++ b/runtime/native/java_lang_Thread.cc
@@ -147,8 +147,11 @@
   // thread list lock to avoid this, as setting the thread name causes mutator to lock/unlock
   // in the DDMS send code.
   ThreadList* thread_list = Runtime::Current()->GetThreadList();
+  bool timed_out;
   // Take suspend thread lock to avoid races with threads trying to suspend this one.
-  Thread* thread = thread_list->SuspendThreadByPeer(peer, SuspendReason::kInternal);
+  Thread* thread = thread_list->SuspendThreadByPeer(peer,
+                                                    SuspendReason::kInternal,
+                                                    &timed_out);
   if (thread != nullptr) {
     {
       ScopedObjectAccess soa(env);
@@ -156,6 +159,9 @@
     }
     bool resumed = thread_list->Resume(thread, SuspendReason::kInternal);
     DCHECK(resumed);
+  } else if (timed_out) {
+    LOG(ERROR) << "Trying to set thread name to '" << name.c_str() << "' failed as the thread "
+        "failed to suspend within a generous timeout.";
   }
 }
 
diff --git a/runtime/native/org_apache_harmony_dalvik_ddmc_DdmVmInternal.cc b/runtime/native/org_apache_harmony_dalvik_ddmc_DdmVmInternal.cc
index f20cd28..081ec20 100644
--- a/runtime/native/org_apache_harmony_dalvik_ddmc_DdmVmInternal.cc
+++ b/runtime/native/org_apache_harmony_dalvik_ddmc_DdmVmInternal.cc
@@ -59,6 +59,7 @@
     trace = Thread::InternalStackTraceToStackTraceElementArray(soa, internal_trace);
   } else {
     ThreadList* thread_list = Runtime::Current()->GetThreadList();
+    bool timed_out;
 
     // Check for valid thread
     if (thin_lock_id == ThreadList::kInvalidThreadId) {
@@ -66,7 +67,9 @@
     }
 
     // Suspend thread to build stack trace.
-    Thread* thread = thread_list->SuspendThreadByThreadId(thin_lock_id, SuspendReason::kInternal);
+    Thread* thread = thread_list->SuspendThreadByThreadId(thin_lock_id,
+                                                          SuspendReason::kInternal,
+                                                          &timed_out);
     if (thread != nullptr) {
       {
         ScopedObjectAccess soa(env);
@@ -76,6 +79,11 @@
       // Restart suspended thread.
       bool resumed = thread_list->Resume(thread, SuspendReason::kInternal);
       DCHECK(resumed);
+    } else {
+      if (timed_out) {
+        LOG(ERROR) << "Trying to get thread's stack by id failed as the thread failed to suspend "
+            "within a generous timeout.";
+      }
     }
   }
   return trace;
diff --git a/runtime/oat.h b/runtime/oat.h
index 48b7c5f..81ee6e4 100644
--- a/runtime/oat.h
+++ b/runtime/oat.h
@@ -44,8 +44,8 @@
 class PACKED(4) OatHeader {
  public:
   static constexpr std::array<uint8_t, 4> kOatMagic { { 'o', 'a', 't', '\n' } };
-  // Last oat version changed reason: Change suspend barrier data structure.
-  static constexpr std::array<uint8_t, 4> kOatVersion{{'2', '4', '0', '\0'}};
+  // Last oat version changed reason: Refactor OatQuickMethodHeader for assembly stubs.
+  static constexpr std::array<uint8_t, 4> kOatVersion{{'2', '3', '9', '\0'}};
 
   static constexpr const char* kDex2OatCmdLineKey = "dex2oat-cmdline";
   static constexpr const char* kDebuggableKey = "debuggable";
diff --git a/runtime/thread-inl.h b/runtime/thread-inl.h
index 3fd42a7..ce50471 100644
--- a/runtime/thread-inl.h
+++ b/runtime/thread-inl.h
@@ -17,6 +17,8 @@
 #ifndef ART_RUNTIME_THREAD_INL_H_
 #define ART_RUNTIME_THREAD_INL_H_
 
+#include "thread.h"
+
 #include "arch/instruction_set.h"
 #include "base/aborting.h"
 #include "base/casts.h"
@@ -26,10 +28,8 @@
 #include "jni/jni_env_ext.h"
 #include "managed_stack-inl.h"
 #include "obj_ptr-inl.h"
-#include "runtime.h"
+#include "suspend_reason.h"
 #include "thread-current-inl.h"
-#include "thread.h"
-#include "thread_list.h"
 #include "thread_pool.h"
 
 namespace art {
@@ -81,15 +81,12 @@
       break;
     } else if (state_and_flags.IsFlagSet(ThreadFlag::kCheckpointRequest)) {
       RunCheckpointFunction();
-    } else if (state_and_flags.IsFlagSet(ThreadFlag::kSuspendRequest) &&
-               !state_and_flags.IsFlagSet(ThreadFlag::kSuspensionImmune)) {
+    } else if (state_and_flags.IsFlagSet(ThreadFlag::kSuspendRequest)) {
       FullSuspendCheck(implicit);
       implicit = false;  // We do not need to `MadviseAwayAlternateSignalStack()` anymore.
-    } else if (state_and_flags.IsFlagSet(ThreadFlag::kEmptyCheckpointRequest)) {
-      RunEmptyCheckpoint();
     } else {
-      DCHECK(state_and_flags.IsFlagSet(ThreadFlag::kSuspensionImmune));
-      break;
+      DCHECK(state_and_flags.IsFlagSet(ThreadFlag::kEmptyCheckpointRequest));
+      RunEmptyCheckpoint();
     }
   }
   if (implicit) {
@@ -109,10 +106,9 @@
       if (kIsDebugBuild) {
         for (int i = kLockLevelCount - 1; i >= 0; --i) {
           BaseMutex* held_mutex = self->GetHeldMutex(static_cast<LockLevel>(i));
-          if (held_mutex != nullptr && held_mutex != GetMutatorLock() &&
-              held_mutex != cond_var_mutex &&
-              held_mutex != cp_placeholder_mutex_.load(std::memory_order_relaxed)) {
-            // placeholder_mutex may still be nullptr. That's OK.
+          if (held_mutex != nullptr &&
+              held_mutex != GetMutatorLock() &&
+              held_mutex != cond_var_mutex) {
             CHECK(Locks::IsExpectedOnWeakRefAccess(held_mutex))
                 << "Holding unexpected mutex " << held_mutex->GetName()
                 << " when accessing weak ref";
@@ -253,8 +249,7 @@
   }
 }
 
-inline void Thread::CheckActiveSuspendBarriers() {
-  DCHECK_NE(GetState(), ThreadState::kRunnable);
+inline void Thread::PassActiveSuspendBarriers() {
   while (true) {
     StateAndFlags state_and_flags = GetStateAndFlags(std::memory_order_relaxed);
     if (LIKELY(!state_and_flags.IsFlagSet(ThreadFlag::kCheckpointRequest) &&
@@ -262,7 +257,7 @@
                !state_and_flags.IsFlagSet(ThreadFlag::kActiveSuspendBarrier))) {
       break;
     } else if (state_and_flags.IsFlagSet(ThreadFlag::kActiveSuspendBarrier)) {
-      PassActiveSuspendBarriers();
+      PassActiveSuspendBarriers(this);
     } else {
       // Impossible
       LOG(FATAL) << "Fatal, thread transitioned into suspended without running the checkpoint";
@@ -270,20 +265,6 @@
   }
 }
 
-inline void Thread::AddSuspend1Barrier(WrappedSuspend1Barrier* suspend1_barrier) {
-  suspend1_barrier->next_ = tlsPtr_.active_suspend1_barriers;
-  tlsPtr_.active_suspend1_barriers = suspend1_barrier;
-}
-
-inline void Thread::RemoveFirstSuspend1Barrier() {
-  tlsPtr_.active_suspend1_barriers = tlsPtr_.active_suspend1_barriers->next_;
-}
-
-inline bool Thread::HasActiveSuspendBarrier() {
-  return tlsPtr_.active_suspend1_barriers != nullptr ||
-         tlsPtr_.active_suspendall_barrier != nullptr;
-}
-
 inline void Thread::TransitionFromRunnableToSuspended(ThreadState new_state) {
   // Note: JNI stubs inline a fast path of this method that transitions to suspended if
   // there are no flags set and then clears the `held_mutexes[kMutatorLock]` (this comes
@@ -299,7 +280,7 @@
   // Mark the release of the share of the mutator lock.
   GetMutatorLock()->TransitionFromRunnableToSuspended(this);
   // Once suspended - check the active suspend barrier flag
-  CheckActiveSuspendBarriers();
+  PassActiveSuspendBarriers();
 }
 
 inline ThreadState Thread::TransitionFromSuspendedToRunnable() {
@@ -309,7 +290,6 @@
   // inlined from the `GetMutatorLock()->TransitionFromSuspendedToRunnable(this)` below).
   // Therefore any code added here (other than debug build assertions) should be gated
   // on some flag being set, so that the JNI stub can take the slow path to get here.
-  DCHECK(this == Current());
   StateAndFlags old_state_and_flags = GetStateAndFlags(std::memory_order_relaxed);
   ThreadState old_state = old_state_and_flags.GetState();
   DCHECK_NE(old_state, ThreadState::kRunnable);
@@ -331,7 +311,7 @@
         break;
       }
     } else if (old_state_and_flags.IsFlagSet(ThreadFlag::kActiveSuspendBarrier)) {
-      PassActiveSuspendBarriers();
+      PassActiveSuspendBarriers(this);
     } else if (UNLIKELY(old_state_and_flags.IsFlagSet(ThreadFlag::kCheckpointRequest) ||
                         old_state_and_flags.IsFlagSet(ThreadFlag::kEmptyCheckpointRequest))) {
       // Checkpoint flags should not be set while in suspended state.
@@ -353,6 +333,7 @@
         thread_to_pass = this;
       }
       MutexLock mu(thread_to_pass, *Locks::thread_suspend_count_lock_);
+      ScopedTransitioningToRunnable scoped_transitioning_to_runnable(this);
       // Reload state and flags after locking the mutex.
       old_state_and_flags = GetStateAndFlags(std::memory_order_relaxed);
       DCHECK_EQ(old_state, old_state_and_flags.GetState());
@@ -364,15 +345,14 @@
         DCHECK_EQ(old_state, old_state_and_flags.GetState());
       }
       DCHECK_EQ(GetSuspendCount(), 0);
-    } else if (UNLIKELY(old_state_and_flags.IsFlagSet(ThreadFlag::kRunningFlipFunction))) {
-      DCHECK(!old_state_and_flags.IsFlagSet(ThreadFlag::kPendingFlipFunction));
-      // Do this before transitioning to runnable, both because we shouldn't wait in a runnable
-      // state, and so that the thread running the flip function can DCHECK we're not runnable.
+    } else if (UNLIKELY(old_state_and_flags.IsFlagSet(ThreadFlag::kRunningFlipFunction)) ||
+               UNLIKELY(old_state_and_flags.IsFlagSet(ThreadFlag::kWaitingForFlipFunction))) {
+      // It's possible that some thread runs this thread's flip-function in
+      // Thread::GetPeerFromOtherThread() even though it was runnable.
       WaitForFlipFunction(this);
-    } else if (old_state_and_flags.IsFlagSet(ThreadFlag::kPendingFlipFunction)) {
-      // Logically acquire mutator lock in shared mode.
-      DCHECK(!old_state_and_flags.IsFlagSet(ThreadFlag::kRunningFlipFunction));
-      if (EnsureFlipFunctionStarted(this, this, old_state_and_flags)) {
+    } else {
+      DCHECK(old_state_and_flags.IsFlagSet(ThreadFlag::kPendingFlipFunction));
+      if (EnsureFlipFunctionStarted(this, old_state_and_flags)) {
         break;
       }
     }
@@ -380,7 +360,6 @@
     old_state_and_flags = GetStateAndFlags(std::memory_order_relaxed);
     DCHECK_EQ(old_state, old_state_and_flags.GetState());
   }
-  DCHECK_EQ(this->GetState(), ThreadState::kRunnable);
   return static_cast<ThreadState>(old_state);
 }
 
@@ -459,66 +438,35 @@
   }
 }
 
-inline void Thread::IncrementSuspendCount(Thread* self,
-                                          AtomicInteger* suspendall_barrier,
-                                          WrappedSuspend1Barrier* suspend1_barrier,
-                                          SuspendReason reason) {
-  if (kIsDebugBuild) {
-    Locks::thread_suspend_count_lock_->AssertHeld(self);
-    if (this != self) {
-      Locks::thread_list_lock_->AssertHeld(self);
+inline bool Thread::ModifySuspendCount(Thread* self,
+                                       int delta,
+                                       AtomicInteger* suspend_barrier,
+                                       SuspendReason reason) {
+  if (delta > 0 &&
+      (((gUseUserfaultfd || gUseReadBarrier) && this != self) || suspend_barrier != nullptr)) {
+    // When delta > 0 (requesting a suspend), ModifySuspendCountInternal() may fail either if
+    // active_suspend_barriers is full or we are in the middle of a thread flip. Retry in a loop.
+    while (true) {
+      if (LIKELY(ModifySuspendCountInternal(self, delta, suspend_barrier, reason))) {
+        return true;
+      } else {
+        // Failure means the list of active_suspend_barriers is full or we are in the middle of a
+        // thread flip, we should release the thread_suspend_count_lock_ (to avoid deadlock) and
+        // wait till the target thread has executed or Thread::PassActiveSuspendBarriers() or the
+        // flip function. Note that we could not simply wait for the thread to change to a suspended
+        // state, because it might need to run checkpoint function before the state change or
+        // resumes from the resume_cond_, which also needs thread_suspend_count_lock_.
+        //
+        // The list of active_suspend_barriers is very unlikely to be full since more than
+        // kMaxSuspendBarriers threads need to execute SuspendAllInternal() simultaneously, and
+        // target thread stays in kRunnable in the mean time.
+        Locks::thread_suspend_count_lock_->ExclusiveUnlock(self);
+        NanoSleep(100000);
+        Locks::thread_suspend_count_lock_->ExclusiveLock(self);
+      }
     }
-  }
-  if (UNLIKELY(reason == SuspendReason::kForUserCode)) {
-    Locks::user_code_suspension_lock_->AssertHeld(self);
-  }
-
-  uint32_t flags = enum_cast<uint32_t>(ThreadFlag::kSuspendRequest);
-  if (suspendall_barrier != nullptr) {
-    DCHECK(suspend1_barrier == nullptr);
-    DCHECK(tlsPtr_.active_suspendall_barrier == nullptr);
-    tlsPtr_.active_suspendall_barrier = suspendall_barrier;
-    flags |= enum_cast<uint32_t>(ThreadFlag::kActiveSuspendBarrier);
-  } else if (suspend1_barrier != nullptr) {
-    AddSuspend1Barrier(suspend1_barrier);
-    flags |= enum_cast<uint32_t>(ThreadFlag::kActiveSuspendBarrier);
-  }
-
-  ++tls32_.suspend_count;
-  if (reason == SuspendReason::kForUserCode) {
-    ++tls32_.user_code_suspend_count;
-  }
-
-  // Two bits might be set simultaneously.
-  tls32_.state_and_flags.fetch_or(flags, std::memory_order_release);
-  TriggerSuspend();
-}
-
-inline void Thread::IncrementSuspendCount(Thread* self) {
-  IncrementSuspendCount(self, nullptr, nullptr, SuspendReason::kInternal);
-}
-
-inline void Thread::DecrementSuspendCount(Thread* self, bool for_user_code) {
-  DCHECK(ReadFlag(ThreadFlag::kSuspendRequest));
-  Locks::thread_suspend_count_lock_->AssertHeld(self);
-  if (UNLIKELY(tls32_.suspend_count <= 0)) {
-    UnsafeLogFatalForSuspendCount(self, this);
-    UNREACHABLE();
-  }
-  if (for_user_code) {
-    Locks::user_code_suspension_lock_->AssertHeld(self);
-    if (UNLIKELY(tls32_.user_code_suspend_count <= 0)) {
-      LOG(ERROR) << "user_code_suspend_count incorrect";
-      UnsafeLogFatalForSuspendCount(self, this);
-      UNREACHABLE();
-    }
-    --tls32_.user_code_suspend_count;
-  }
-
-  --tls32_.suspend_count;
-
-  if (tls32_.suspend_count == 0) {
-    AtomicClearFlag(ThreadFlag::kSuspendRequest, std::memory_order_release);
+  } else {
+    return ModifySuspendCountInternal(self, delta, suspend_barrier, reason);
   }
 }
 
@@ -551,93 +499,6 @@
   tlsPtr_.stack_end = tlsPtr_.stack_begin + GetStackOverflowReservedBytes(kRuntimeISA);
 }
 
-inline void Thread::NotifyOnThreadExit(ThreadExitFlag* tef) {
-  DCHECK_EQ(tef->exited_, false);
-  DCHECK(tlsPtr_.thread_exit_flags == nullptr || !tlsPtr_.thread_exit_flags->exited_);
-  tef->next_ = tlsPtr_.thread_exit_flags;
-  tlsPtr_.thread_exit_flags = tef;
-  if (tef->next_ != nullptr) {
-    DCHECK(!tef->next_->HasExited());
-    tef->next_->prev_ = tef;
-  }
-  tef->prev_ = nullptr;
-}
-
-inline void Thread::UnregisterThreadExitFlag(ThreadExitFlag* tef) {
-  if (tef->HasExited()) {
-    // List is no longer used; each client will deallocate its own ThreadExitFlag.
-    return;
-  }
-  DCHECK(IsRegistered(tef));
-  // Remove tef from the list.
-  if (tef->next_ != nullptr) {
-    tef->next_->prev_ = tef->prev_;
-  }
-  if (tef->prev_ == nullptr) {
-    DCHECK_EQ(tlsPtr_.thread_exit_flags, tef);
-    tlsPtr_.thread_exit_flags = tef->next_;
-  } else {
-    DCHECK_NE(tlsPtr_.thread_exit_flags, tef);
-    tef->prev_->next_ = tef->next_;
-  }
-  DCHECK(tlsPtr_.thread_exit_flags == nullptr || tlsPtr_.thread_exit_flags->prev_ == nullptr);
-}
-
-inline void Thread::DCheckUnregisteredEverywhere(ThreadExitFlag* first, ThreadExitFlag* last) {
-  if (!kIsDebugBuild) {
-    return;
-  }
-  Thread* self = Thread::Current();
-  MutexLock mu(self, *Locks::thread_list_lock_);
-  Runtime::Current()->GetThreadList()->ForEach([&](Thread* t) REQUIRES(Locks::thread_list_lock_) {
-    for (ThreadExitFlag* tef = t->tlsPtr_.thread_exit_flags; tef != nullptr; tef = tef->next_) {
-      CHECK(tef < first || tef > last)
-          << "tef = " << std::hex << tef << " first = " << first << std::dec;
-    }
-    // Also perform a minimal consistency check on each list.
-    ThreadExitFlag* flags = t->tlsPtr_.thread_exit_flags;
-    CHECK(flags == nullptr || flags->prev_ == nullptr);
-  });
-}
-
-inline bool Thread::IsRegistered(ThreadExitFlag* query_tef) {
-  for (ThreadExitFlag* tef = tlsPtr_.thread_exit_flags; tef != nullptr; tef = tef->next_) {
-    if (tef == query_tef) {
-      return true;
-    }
-  }
-  return false;
-}
-
-inline void Thread::DisallowPreMonitorMutexes() {
-  if (kIsDebugBuild) {
-    CHECK(this == Thread::Current());
-    CHECK(GetHeldMutex(kMonitorLock) == nullptr);
-    // Pretend we hold a kMonitorLock level mutex to detect disallowed mutex
-    // acquisitions by checkpoint Run() methods.  We don't normally register or thus check
-    // kMonitorLock level mutexes, but this is an exception.
-    Mutex* ph = cp_placeholder_mutex_.load(std::memory_order_acquire);
-    if (UNLIKELY(ph == nullptr)) {
-      Mutex* new_ph = new Mutex("checkpoint placeholder mutex", kMonitorLock);
-      if (LIKELY(cp_placeholder_mutex_.compare_exchange_strong(ph, new_ph))) {
-        ph = new_ph;
-      } else {
-        // ph now has the value set by another thread.
-        delete new_ph;
-      }
-    }
-    SetHeldMutex(kMonitorLock, ph);
-  }
-}
-
-// Undo the effect of the previous call. Again only invoked by the thread itself.
-inline void Thread::AllowPreMonitorMutexes() {
-  if (kIsDebugBuild) {
-    CHECK_EQ(GetHeldMutex(kMonitorLock), cp_placeholder_mutex_.load(std::memory_order_relaxed));
-    SetHeldMutex(kMonitorLock, nullptr);
-  }
-}
-
 }  // namespace art
 
 #endif  // ART_RUNTIME_THREAD_INL_H_
diff --git a/runtime/thread.cc b/runtime/thread.cc
index ccdcebd..73008eb 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -140,12 +140,14 @@
 const size_t Thread::kStackOverflowImplicitCheckSize = GetStackOverflowReservedBytes(kRuntimeISA);
 bool (*Thread::is_sensitive_thread_hook_)() = nullptr;
 Thread* Thread::jit_sensitive_thread_ = nullptr;
-std::atomic<Mutex*> Thread::cp_placeholder_mutex_(nullptr);
 #ifndef __BIONIC__
 thread_local Thread* Thread::self_tls_ = nullptr;
 #endif
 
 static constexpr bool kVerifyImageObjectsMarked = kIsDebugBuild;
+// Amount of time (in microseconds) that we sleep if another thread is running
+// flip function of the thread that we are interested in.
+static constexpr size_t kSuspendTimeDuringFlip = 5'000;
 
 // For implicit overflow checks we reserve an extra piece of memory at the bottom
 // of the stack (lowest memory).  The higher portion of the memory
@@ -1470,7 +1472,7 @@
 }
 
 // Attempt to rectify locks so that we dump thread list with required locks before exiting.
-void Thread::UnsafeLogFatalForSuspendCount(Thread* self, Thread* thread) NO_THREAD_SAFETY_ANALYSIS {
+static void UnsafeLogFatalForSuspendCount(Thread* self, Thread* thread) NO_THREAD_SAFETY_ANALYSIS {
   LOG(ERROR) << *thread << " suspend count already zero.";
   Locks::thread_suspend_count_lock_->Unlock(self);
   if (!Locks::mutator_lock_->IsSharedHeld(self)) {
@@ -1488,55 +1490,141 @@
   std::ostringstream ss;
   Runtime::Current()->GetThreadList()->Dump(ss);
   LOG(FATAL) << ss.str();
-  UNREACHABLE();
 }
 
-bool Thread::PassActiveSuspendBarriers() {
-  DCHECK_EQ(this, Thread::Current());
-  DCHECK_NE(GetState(), ThreadState::kRunnable);
-  // Grab the suspend_count lock and copy the current set of barriers. Then clear the list and the
-  // flag. The IncrementSuspendCount function requires the lock so we prevent a race between setting
-  // the kActiveSuspendBarrier flag and clearing it.
-  // TODO: Consider doing this without the temporary vector. That code will be a bit
-  // tricky, since the WrappedSuspend1Barrier may disappear once the barrier is decremented.
-  std::vector<AtomicInteger*> pass_barriers{};
-  {
-    MutexLock mu(this, *Locks::thread_suspend_count_lock_);
-    if (!ReadFlag(ThreadFlag::kActiveSuspendBarrier)) {
-      // quick exit test: the barriers have already been claimed - this is possible as there may
-      // be a race to claim and it doesn't matter who wins.
-      // All of the callers of this function (except the SuspendAllInternal) will first test the
-      // kActiveSuspendBarrier flag without lock. Here double-check whether the barrier has been
-      // passed with the suspend_count lock.
+bool Thread::ModifySuspendCountInternal(Thread* self,
+                                        int delta,
+                                        AtomicInteger* suspend_barrier,
+                                        SuspendReason reason) {
+  if (kIsDebugBuild) {
+    DCHECK(delta == -1 || delta == +1)
+          << reason << " " << delta << " " << this;
+    Locks::thread_suspend_count_lock_->AssertHeld(self);
+    if (this != self && !IsSuspended()) {
+      Locks::thread_list_lock_->AssertHeld(self);
+    }
+  }
+  // User code suspensions need to be checked more closely since they originate from code outside of
+  // the runtime's control.
+  if (UNLIKELY(reason == SuspendReason::kForUserCode)) {
+    Locks::user_code_suspension_lock_->AssertHeld(self);
+    if (UNLIKELY(delta + tls32_.user_code_suspend_count < 0)) {
+      LOG(ERROR) << "attempting to modify suspend count in an illegal way.";
       return false;
     }
-    if (tlsPtr_.active_suspendall_barrier != nullptr) {
-      // We have at most one active active_suspendall_barrier. See thread.h comment.
-      pass_barriers.push_back(tlsPtr_.active_suspendall_barrier);
-      tlsPtr_.active_suspendall_barrier = nullptr;
+  }
+  if (UNLIKELY(delta < 0 && tls32_.suspend_count <= 0)) {
+    UnsafeLogFatalForSuspendCount(self, this);
+    return false;
+  }
+
+  if (delta > 0 && this != self && tlsPtr_.flip_function != nullptr) {
+    // Force retry of a suspend request if it's in the middle of a thread flip to avoid a
+    // deadlock. b/31683379.
+    return false;
+  }
+
+  uint32_t flags = enum_cast<uint32_t>(ThreadFlag::kSuspendRequest);
+  if (delta > 0 && suspend_barrier != nullptr) {
+    uint32_t available_barrier = kMaxSuspendBarriers;
+    for (uint32_t i = 0; i < kMaxSuspendBarriers; ++i) {
+      if (tlsPtr_.active_suspend_barriers[i] == nullptr) {
+        available_barrier = i;
+        break;
+      }
     }
-    for (WrappedSuspend1Barrier* w = tlsPtr_.active_suspend1_barriers; w != nullptr; w = w->next_) {
-      pass_barriers.push_back(&(w->barrier_));
+    if (available_barrier == kMaxSuspendBarriers) {
+      // No barrier spaces available, we can't add another.
+      return false;
     }
-    tlsPtr_.active_suspend1_barriers = nullptr;
+    tlsPtr_.active_suspend_barriers[available_barrier] = suspend_barrier;
+    flags |= enum_cast<uint32_t>(ThreadFlag::kActiveSuspendBarrier);
+  }
+
+  tls32_.suspend_count += delta;
+  switch (reason) {
+    case SuspendReason::kForUserCode:
+      tls32_.user_code_suspend_count += delta;
+      break;
+    case SuspendReason::kInternal:
+      break;
+  }
+
+  if (tls32_.suspend_count == 0) {
+    AtomicClearFlag(ThreadFlag::kSuspendRequest);
+  } else {
+    // Two bits might be set simultaneously.
+    tls32_.state_and_flags.fetch_or(flags, std::memory_order_seq_cst);
+    TriggerSuspend();
+  }
+  return true;
+}
+
+bool Thread::PassActiveSuspendBarriers(Thread* self) {
+  // Grab the suspend_count lock and copy the current set of
+  // barriers. Then clear the list and the flag. The ModifySuspendCount
+  // function requires the lock so we prevent a race between setting
+  // the kActiveSuspendBarrier flag and clearing it.
+  AtomicInteger* pass_barriers[kMaxSuspendBarriers];
+  {
+    MutexLock mu(self, *Locks::thread_suspend_count_lock_);
+    if (!ReadFlag(ThreadFlag::kActiveSuspendBarrier)) {
+      // quick exit test: the barriers have already been claimed - this is
+      // possible as there may be a race to claim and it doesn't matter
+      // who wins.
+      // All of the callers of this function (except the SuspendAllInternal)
+      // will first test the kActiveSuspendBarrier flag without lock. Here
+      // double-check whether the barrier has been passed with the
+      // suspend_count lock.
+      return false;
+    }
+
+    for (uint32_t i = 0; i < kMaxSuspendBarriers; ++i) {
+      pass_barriers[i] = tlsPtr_.active_suspend_barriers[i];
+      tlsPtr_.active_suspend_barriers[i] = nullptr;
+    }
     AtomicClearFlag(ThreadFlag::kActiveSuspendBarrier);
   }
 
   uint32_t barrier_count = 0;
-  for (AtomicInteger* barrier : pass_barriers) {
-    ++barrier_count;
-    int32_t old_val = barrier->fetch_sub(1, std::memory_order_release);
-    CHECK_GT(old_val, 0) << "Unexpected value for PassActiveSuspendBarriers(): " << old_val;
+  for (uint32_t i = 0; i < kMaxSuspendBarriers; i++) {
+    AtomicInteger* pending_threads = pass_barriers[i];
+    if (pending_threads != nullptr) {
+      bool done = false;
+      do {
+        int32_t cur_val = pending_threads->load(std::memory_order_relaxed);
+        CHECK_GT(cur_val, 0) << "Unexpected value for PassActiveSuspendBarriers(): " << cur_val;
+        // Reduce value by 1.
+        done = pending_threads->CompareAndSetWeakRelaxed(cur_val, cur_val - 1);
 #if ART_USE_FUTEXES
-    if (old_val == 1) {
-      futex(barrier->Address(), FUTEX_WAKE_PRIVATE, INT_MAX, nullptr, nullptr, 0);
-    }
+        if (done && (cur_val - 1) == 0) {  // Weak CAS may fail spuriously.
+          futex(pending_threads->Address(), FUTEX_WAKE_PRIVATE, INT_MAX, nullptr, nullptr, 0);
+        }
 #endif
+      } while (!done);
+      ++barrier_count;
+    }
   }
   CHECK_GT(barrier_count, 0U);
   return true;
 }
 
+void Thread::ClearSuspendBarrier(AtomicInteger* target) {
+  CHECK(ReadFlag(ThreadFlag::kActiveSuspendBarrier));
+  bool clear_flag = true;
+  for (uint32_t i = 0; i < kMaxSuspendBarriers; ++i) {
+    AtomicInteger* ptr = tlsPtr_.active_suspend_barriers[i];
+    if (ptr == target) {
+      tlsPtr_.active_suspend_barriers[i] = nullptr;
+    } else if (ptr != nullptr) {
+      clear_flag = false;
+    }
+  }
+  if (LIKELY(clear_flag)) {
+    AtomicClearFlag(ThreadFlag::kActiveSuspendBarrier);
+  }
+}
+
 void Thread::RunCheckpointFunction() {
   DCHECK_EQ(Thread::Current(), this);
   CHECK(!GetStateAndFlags(std::memory_order_relaxed).IsAnyOfFlagsSet(FlipFunctionFlags()));
@@ -1571,26 +1659,28 @@
 }
 
 bool Thread::RequestCheckpoint(Closure* function) {
-  bool success;
-  do {
-    StateAndFlags old_state_and_flags = GetStateAndFlags(std::memory_order_relaxed);
-    if (old_state_and_flags.GetState() != ThreadState::kRunnable) {
-      return false;  // Fail, thread is suspended and so can't run a checkpoint.
-    }
-    StateAndFlags new_state_and_flags = old_state_and_flags;
-    new_state_and_flags.SetFlag(ThreadFlag::kCheckpointRequest);
-    success = tls32_.state_and_flags.CompareAndSetWeakSequentiallyConsistent(
-        old_state_and_flags.GetValue(), new_state_and_flags.GetValue());
-  } while (!success);
-  // Succeeded setting checkpoint flag, now insert the actual checkpoint.
-  if (tlsPtr_.checkpoint_function == nullptr) {
-    tlsPtr_.checkpoint_function = function;
-  } else {
-    checkpoint_overflow_.push_back(function);
+  StateAndFlags old_state_and_flags = GetStateAndFlags(std::memory_order_relaxed);
+  if (old_state_and_flags.GetState() != ThreadState::kRunnable) {
+    return false;  // Fail, thread is suspended and so can't run a checkpoint.
   }
-  DCHECK(ReadFlag(ThreadFlag::kCheckpointRequest));
-  TriggerSuspend();
-  return true;
+
+  // We must be runnable to request a checkpoint.
+  DCHECK_EQ(old_state_and_flags.GetState(), ThreadState::kRunnable);
+  StateAndFlags new_state_and_flags = old_state_and_flags;
+  new_state_and_flags.SetFlag(ThreadFlag::kCheckpointRequest);
+  bool success = tls32_.state_and_flags.CompareAndSetStrongSequentiallyConsistent(
+      old_state_and_flags.GetValue(), new_state_and_flags.GetValue());
+  if (success) {
+    // Succeeded setting checkpoint flag, now insert the actual checkpoint.
+    if (tlsPtr_.checkpoint_function == nullptr) {
+      tlsPtr_.checkpoint_function = function;
+    } else {
+      checkpoint_overflow_.push_back(function);
+    }
+    CHECK(ReadFlag(ThreadFlag::kCheckpointRequest));
+    TriggerSuspend();
+  }
+  return success;
 }
 
 bool Thread::RequestEmptyCheckpoint() {
@@ -1622,8 +1712,8 @@
     barrier_.Pass(self);
   }
 
-  void Wait(Thread* self, ThreadState wait_state) {
-    if (wait_state != ThreadState::kRunnable) {
+  void Wait(Thread* self, ThreadState suspend_state) {
+    if (suspend_state != ThreadState::kRunnable) {
       barrier_.Increment<Barrier::kDisallowHoldingLocks>(self, 1);
     } else {
       barrier_.Increment<Barrier::kAllowHoldingLocks>(self, 1);
@@ -1636,9 +1726,9 @@
 };
 
 // RequestSynchronousCheckpoint releases the thread_list_lock_ as a part of its execution.
-bool Thread::RequestSynchronousCheckpoint(Closure* function, ThreadState wait_state) {
+bool Thread::RequestSynchronousCheckpoint(Closure* function, ThreadState suspend_state) {
   Thread* self = Thread::Current();
-  if (this == self) {
+  if (this == Thread::Current()) {
     Locks::thread_list_lock_->AssertExclusiveHeld(self);
     // Unlock the tll before running so that the state is the same regardless of thread.
     Locks::thread_list_lock_->ExclusiveUnlock(self);
@@ -1649,289 +1739,213 @@
 
   // The current thread is not this thread.
 
-  VerifyState();
-
-  Locks::thread_list_lock_->AssertExclusiveHeld(self);
-  // If target "this" thread is runnable, try to schedule a checkpoint. Do some gymnastics to not
-  // hold the suspend-count lock for too long.
-  if (GetState() == ThreadState::kRunnable) {
-    BarrierClosure barrier_closure(function);
-    bool installed = false;
-    {
-      MutexLock mu(self, *Locks::thread_suspend_count_lock_);
-      installed = RequestCheckpoint(&barrier_closure);
-    }
-    if (installed) {
-      // Relinquish the thread-list lock. We should not wait holding any locks. We cannot
-      // reacquire it since we don't know if 'this' hasn't been deleted yet.
-      Locks::thread_list_lock_->ExclusiveUnlock(self);
-      ScopedThreadStateChange sts(self, wait_state);
-      // Wait state can be kRunnable, in which case, for lock ordering purposes, it's as if we ran
-      // the closure ourselves. This means that the target thread should not acquire a pre-mutator
-      // lock without running the checkpoint, and the closure should not acquire a pre-mutator
-      // lock or suspend.
-      barrier_closure.Wait(self, wait_state);
-      return true;
-    }
-    // No longer runnable. Fall-through.
+  if (GetState() == ThreadState::kTerminated) {
+    Locks::thread_list_lock_->ExclusiveUnlock(self);
+    return false;
   }
 
-  // Target "this" thread was not runnable. Suspend it, hopefully redundantly,
-  // but it might have become runnable in the meantime.
-  // Although this is a thread suspension, the target thread only blocks while we run the
-  // checkpoint, which is presumed to terminate quickly even if other threads are blocked.
-  // Note: IncrementSuspendCount also expects the thread_list_lock to be held unless this == self.
-  {
-    bool is_suspended = false;
-    WrappedSuspend1Barrier wrapped_barrier{};
+  struct ScopedThreadListLockUnlock {
+    explicit ScopedThreadListLockUnlock(Thread* self_in) RELEASE(*Locks::thread_list_lock_)
+        : self_thread(self_in) {
+      Locks::thread_list_lock_->AssertHeld(self_thread);
+      Locks::thread_list_lock_->Unlock(self_thread);
+    }
 
-    {
-      MutexLock suspend_count_mu(self, *Locks::thread_suspend_count_lock_);
-      // If wait_state is kRunnable, function may not suspend. We thus never block because
-      // we ourselves are being asked to suspend.
-      if (UNLIKELY(wait_state != ThreadState::kRunnable && self->GetSuspendCount() != 0)) {
-        // We are being asked to suspend while we are suspending another thread that may be
-        // responsible for our suspension. This is likely to result in deadlock if we each
-        // block on the suspension request. Instead we wait for the situation to change.
-        ThreadExitFlag target_status;
-        NotifyOnThreadExit(&target_status);
-        for (int iter_count = 1; self->GetSuspendCount() != 0; ++iter_count) {
-          Locks::thread_suspend_count_lock_->ExclusiveUnlock(self);
-          Locks::thread_list_lock_->ExclusiveUnlock(self);
-          {
-            ScopedThreadStateChange sts(self, wait_state);
-            usleep(ThreadList::kThreadSuspendSleepUs);
-          }
-          CHECK_LT(iter_count, ThreadList::kMaxSuspendRetries);
-          Locks::thread_list_lock_->ExclusiveLock(self);
-          if (target_status.HasExited()) {
-            Locks::thread_list_lock_->ExclusiveUnlock(self);
-            DCheckUnregisteredEverywhere(&target_status, &target_status);
-            return false;
-          }
-          Locks::thread_suspend_count_lock_->ExclusiveLock(self);
-        }
-        UnregisterThreadExitFlag(&target_status);
+    ~ScopedThreadListLockUnlock() ACQUIRE(*Locks::thread_list_lock_) {
+      Locks::thread_list_lock_->AssertNotHeld(self_thread);
+      Locks::thread_list_lock_->Lock(self_thread);
+    }
+
+    Thread* self_thread;
+  };
+
+  for (;;) {
+    Locks::thread_list_lock_->AssertExclusiveHeld(self);
+    // If this thread is runnable, try to schedule a checkpoint. Do some gymnastics to not hold the
+    // suspend-count lock for too long.
+    if (GetState() == ThreadState::kRunnable) {
+      BarrierClosure barrier_closure(function);
+      bool installed = false;
+      {
+        MutexLock mu(self, *Locks::thread_suspend_count_lock_);
+        installed = RequestCheckpoint(&barrier_closure);
       }
-      IncrementSuspendCount(self, nullptr, &wrapped_barrier, SuspendReason::kInternal);
-      VerifyState();
-      DCHECK_GT(GetSuspendCount(), 0);
-      DCHECK_EQ(self->GetSuspendCount(), 0);
-      // Since we've incremented the suspend count, "this" thread can no longer disappear.
-      Locks::thread_list_lock_->ExclusiveUnlock(self);
-      if (IsSuspended()) {
-        // See the discussion in mutator_gc_coord.md and SuspendAllInternal for the race here.
-        RemoveFirstSuspend1Barrier();
-        if (!HasActiveSuspendBarrier()) {
-          AtomicClearFlag(ThreadFlag::kActiveSuspendBarrier);
-        }
-        is_suspended = true;
+      if (installed) {
+        // Relinquish the thread-list lock. We should not wait holding any locks. We cannot
+        // reacquire it since we don't know if 'this' hasn't been deleted yet.
+        Locks::thread_list_lock_->ExclusiveUnlock(self);
+        ScopedThreadStateChange sts(self, suspend_state);
+        barrier_closure.Wait(self, suspend_state);
+        return true;
+      }
+      // Fall-through.
+    }
+
+    // This thread is not runnable, make sure we stay suspended, then run the checkpoint.
+    // Note: ModifySuspendCountInternal also expects the thread_list_lock to be held in
+    //       certain situations.
+    {
+      MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
+
+      if (!ModifySuspendCount(self, +1, nullptr, SuspendReason::kInternal)) {
+        // Just retry the loop.
+        sched_yield();
+        continue;
       }
     }
-    if (!is_suspended) {
-      bool success =
-          Runtime::Current()->GetThreadList()->WaitForSuspendBarrier(&wrapped_barrier.barrier_);
-      CHECK(success);
+
+    {
+      // Release for the wait. The suspension will keep us from being deleted. Reacquire after so
+      // that we can call ModifySuspendCount without racing against ThreadList::Unregister.
+      ScopedThreadListLockUnlock stllu(self);
+      {
+        ScopedThreadStateChange sts(self, suspend_state);
+        while (GetState() == ThreadState::kRunnable) {
+          // We became runnable again. Wait till the suspend triggered in ModifySuspendCount
+          // moves us to suspended.
+          sched_yield();
+        }
+      }
+      // Ensure that the flip function for this thread, if pending, is finished *before*
+      // the checkpoint function is run. Otherwise, we may end up with both `to' and 'from'
+      // space references on the stack, confusing the GC's thread-flip logic. The caller is
+      // runnable so can't have a pending flip function.
+      DCHECK_EQ(self->GetState(), ThreadState::kRunnable);
+      DCHECK(
+          !self->GetStateAndFlags(std::memory_order_relaxed).IsAnyOfFlagsSet(FlipFunctionFlags()));
+      EnsureFlipFunctionStarted(self);
+      while (GetStateAndFlags(std::memory_order_acquire).IsAnyOfFlagsSet(FlipFunctionFlags())) {
+        usleep(kSuspendTimeDuringFlip);
+      }
+
+      function->Run(this);
     }
 
-    // Ensure that the flip function for this thread, if pending, is finished *before*
-    // the checkpoint function is run. Otherwise, we may end up with both `to' and 'from'
-    // space references on the stack, confusing the GC's thread-flip logic. The caller is
-    // runnable so can't have a pending flip function.
-    DCHECK_EQ(self->GetState(), ThreadState::kRunnable);
-    DCHECK(IsSuspended());
-    DCHECK(!self->GetStateAndFlags(std::memory_order_relaxed).IsAnyOfFlagsSet(FlipFunctionFlags()));
-    EnsureFlipFunctionStarted(self, this);
-    // Since we're runnable, and kPendingFlipFunction is set with all threads suspended, it
-    // cannot be set again here. Thus kRunningFlipFunction is either already set after the
-    // EnsureFlipFunctionStarted call, or will not be set before we call Run().
-    if (ReadFlag(ThreadFlag::kRunningFlipFunction)) {
-      WaitForFlipFunction(self);
+    {
+      MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
+
+      DCHECK_NE(GetState(), ThreadState::kRunnable);
+      bool updated = ModifySuspendCount(self, -1, nullptr, SuspendReason::kInternal);
+      DCHECK(updated);
+      // Imitate ResumeAll, the thread may be waiting on Thread::resume_cond_ since we raised its
+      // suspend count. Now the suspend_count_ is lowered so we must do the broadcast.
+      Thread::resume_cond_->Broadcast(self);
     }
-    function->Run(this);
-  }
 
-  {
-    MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
-    DCHECK_NE(GetState(), ThreadState::kRunnable);
-    DCHECK_GT(GetSuspendCount(), 0);
-    DecrementSuspendCount(self);
-    resume_cond_->Broadcast(self);
-  }
+    // Release the thread_list_lock_ to be consistent with the barrier-closure path.
+    Locks::thread_list_lock_->ExclusiveUnlock(self);
 
-  Locks::thread_list_lock_->AssertNotHeld(self);
-  return true;
+    return true;  // We're done, break out of the loop.
+  }
 }
 
 void Thread::SetFlipFunction(Closure* function) {
   // This is called with all threads suspended, except for the calling thread.
   DCHECK(IsSuspended() || Thread::Current() == this);
   DCHECK(function != nullptr);
-  DCHECK(GetFlipFunction() == nullptr);
-  tlsPtr_.flip_function.store(function, std::memory_order_relaxed);
+  DCHECK(tlsPtr_.flip_function == nullptr);
+  tlsPtr_.flip_function = function;
   DCHECK(!GetStateAndFlags(std::memory_order_relaxed).IsAnyOfFlagsSet(FlipFunctionFlags()));
   AtomicSetFlag(ThreadFlag::kPendingFlipFunction, std::memory_order_release);
 }
 
-bool Thread::EnsureFlipFunctionStarted(Thread* self,
-                                       Thread* target,
-                                       StateAndFlags old_state_and_flags,
-                                       ThreadExitFlag* tef,
-                                       bool* finished) {
-  //  Note: If tef is non-null, *target may have been destroyed. We have to be careful about
-  //  accessing it. That is the reason this is static and not a member function.
-  DCHECK(self == Current());
-  bool check_exited = (tef != nullptr);
-  // Check that the thread can't unexpectedly exit while we are running.
-  DCHECK(self == target || check_exited || target->ReadFlag(ThreadFlag::kSuspendRequest) ||
-         Locks::thread_list_lock_->IsExclusiveHeld(self))
-      << *target;
+bool Thread::EnsureFlipFunctionStarted(Thread* self, StateAndFlags old_state_and_flags) {
   bool become_runnable;
-  auto maybe_release = [=]() NO_THREAD_SAFETY_ANALYSIS /* conditionally unlocks */ {
-    if (check_exited) {
-      Locks::thread_list_lock_->Unlock(self);
-    }
-  };
-  auto set_finished = [=](bool value) {
-    if (finished != nullptr) {
-      *finished = value;
-    }
-  };
-
-  if (check_exited) {
-    Locks::thread_list_lock_->Lock(self);
-    if (tef->HasExited()) {
-      Locks::thread_list_lock_->Unlock(self);
-      set_finished(true);
-      return false;
-    }
-  }
-  target->VerifyState();
   if (old_state_and_flags.GetValue() == 0) {
     become_runnable = false;
-    old_state_and_flags = target->GetStateAndFlags(std::memory_order_relaxed);
+    old_state_and_flags = GetStateAndFlags(std::memory_order_relaxed);
   } else {
     become_runnable = true;
-    DCHECK(!check_exited);
-    DCHECK(target == self);
-    DCHECK(old_state_and_flags.IsFlagSet(ThreadFlag::kPendingFlipFunction));
     DCHECK(!old_state_and_flags.IsFlagSet(ThreadFlag::kSuspendRequest));
   }
-  while (true) {
-    DCHECK(!check_exited || (Locks::thread_list_lock_->IsExclusiveHeld(self) && !tef->HasExited()));
-    if (!old_state_and_flags.IsFlagSet(ThreadFlag::kPendingFlipFunction)) {
-      maybe_release();
-      set_finished(!old_state_and_flags.IsFlagSet(ThreadFlag::kRunningFlipFunction));
-      return false;
-    }
+
+  while (old_state_and_flags.IsFlagSet(ThreadFlag::kPendingFlipFunction)) {
     DCHECK(!old_state_and_flags.IsFlagSet(ThreadFlag::kRunningFlipFunction));
     StateAndFlags new_state_and_flags =
         old_state_and_flags.WithFlag(ThreadFlag::kRunningFlipFunction)
                            .WithoutFlag(ThreadFlag::kPendingFlipFunction);
     if (become_runnable) {
-      DCHECK_EQ(self, target);
+      DCHECK_EQ(self, this);
       DCHECK_NE(self->GetState(), ThreadState::kRunnable);
       new_state_and_flags = new_state_and_flags.WithState(ThreadState::kRunnable);
     }
-    if (target->tls32_.state_and_flags.CompareAndSetWeakAcquire(old_state_and_flags.GetValue(),
-                                                                new_state_and_flags.GetValue())) {
+    if (tls32_.state_and_flags.CompareAndSetWeakAcquire(old_state_and_flags.GetValue(),
+                                                        new_state_and_flags.GetValue())) {
       if (become_runnable) {
-        self->GetMutatorLock()->TransitionFromSuspendedToRunnable(self);
+        GetMutatorLock()->TransitionFromSuspendedToRunnable(this);
       }
       art::Locks::mutator_lock_->AssertSharedHeld(self);
-      maybe_release();
-      // Thread will not go away while kRunningFlipFunction is set.
-      target->RunFlipFunction(self);
-      // At this point, no flip function flags should be set. It's unsafe to DCHECK that, since
-      // the thread may now have exited.
-      set_finished(true);
-      return become_runnable;
+      RunFlipFunction(self, /*notify=*/ true);
+      DCHECK(!GetStateAndFlags(std::memory_order_relaxed).IsAnyOfFlagsSet(FlipFunctionFlags()));
+      return true;
+    } else {
+      old_state_and_flags = GetStateAndFlags(std::memory_order_relaxed);
+      if (become_runnable && old_state_and_flags.IsFlagSet(ThreadFlag::kSuspendRequest)) {
+        break;
+      }
     }
-    if (become_runnable) {
-      DCHECK(!check_exited);  // We didn't acquire thread_list_lock_ .
-      // Let caller retry.
-      return false;
-    }
-    old_state_and_flags = target->GetStateAndFlags(std::memory_order_acquire);
   }
-  // Unreachable.
+  return false;
 }
 
-void Thread::RunFlipFunction(Thread* self) {
-  // This function is called either by the thread running `ThreadList::FlipThreadRoots()` or when
-  // a thread becomes runnable, after we've successfully set the kRunningFlipFunction ThreadFlag.
-  DCHECK(ReadFlag(ThreadFlag::kRunningFlipFunction));
+void Thread::RunFlipFunction(Thread* self, bool notify) {
+  // This function is called for suspended threads and by the thread running
+  // `ThreadList::FlipThreadRoots()` after we've successfully set the flag
+  // `ThreadFlag::kRunningFlipFunction`. This flag is not set if the thread is
+  // running the flip function right after transitioning to Runnable as
+  // no other thread may run checkpoints on a thread that's actually Runnable.
+  DCHECK_EQ(notify, ReadFlag(ThreadFlag::kRunningFlipFunction));
 
-  Closure* flip_function = GetFlipFunction();
-  tlsPtr_.flip_function.store(nullptr, std::memory_order_relaxed);
+  Closure* flip_function = tlsPtr_.flip_function;
+  tlsPtr_.flip_function = nullptr;
   DCHECK(flip_function != nullptr);
-  VerifyState();
   flip_function->Run(this);
-  DCHECK(!ReadFlag(ThreadFlag::kPendingFlipFunction));
-  VerifyState();
-  AtomicClearFlag(ThreadFlag::kRunningFlipFunction, std::memory_order_release);
-  // From here on this thread may go away, and it is no longer safe to access.
 
-  // Notify all threads that are waiting for completion.
-  // TODO: Should we create a separate mutex and condition variable instead
-  // of piggy-backing on the `thread_suspend_count_lock_` and `resume_cond_`?
-  MutexLock mu(self, *Locks::thread_suspend_count_lock_);
-  resume_cond_->Broadcast(self);
+  if (notify) {
+    // Clear the `ThreadFlag::kRunningFlipFunction` and `ThreadFlag::kWaitingForFlipFunction`.
+    // Check if the latter was actually set, indicating that there is at least one waiting thread.
+    constexpr uint32_t kFlagsToClear = enum_cast<uint32_t>(ThreadFlag::kRunningFlipFunction) |
+                                       enum_cast<uint32_t>(ThreadFlag::kWaitingForFlipFunction);
+    StateAndFlags old_state_and_flags(
+        tls32_.state_and_flags.fetch_and(~kFlagsToClear, std::memory_order_release));
+    if (old_state_and_flags.IsFlagSet(ThreadFlag::kWaitingForFlipFunction)) {
+      // Notify all threads that are waiting for completion (at least one).
+      // TODO: Should we create a separate mutex and condition variable instead
+      // of piggy-backing on the `thread_suspend_count_lock_` and `resume_cond_`?
+      MutexLock mu(self, *Locks::thread_suspend_count_lock_);
+      resume_cond_->Broadcast(self);
+    }
+  }
 }
 
-void Thread::WaitForFlipFunction(Thread* self) const {
+void Thread::WaitForFlipFunction(Thread* self) {
   // Another thread is running the flip function. Wait for it to complete.
   // Check the flag while holding the mutex so that we do not miss the broadcast.
   // Repeat the check after waiting to guard against spurious wakeups (and because
   // we share the `thread_suspend_count_lock_` and `resume_cond_` with other code).
-  // Check that the thread can't unexpectedly exit while we are running.
-  DCHECK(self == this || ReadFlag(ThreadFlag::kSuspendRequest) ||
-         Locks::thread_list_lock_->IsExclusiveHeld(self));
   MutexLock mu(self, *Locks::thread_suspend_count_lock_);
   while (true) {
     StateAndFlags old_state_and_flags = GetStateAndFlags(std::memory_order_acquire);
     DCHECK(!old_state_and_flags.IsFlagSet(ThreadFlag::kPendingFlipFunction));
     if (!old_state_and_flags.IsFlagSet(ThreadFlag::kRunningFlipFunction)) {
-      return;
+      DCHECK(!old_state_and_flags.IsAnyOfFlagsSet(FlipFunctionFlags()));
+      break;
     }
-    // We sometimes hold mutator lock here. OK since the flip function must complete quickly.
-    resume_cond_->WaitHoldingLocks(self);
-  }
-}
-
-void Thread::WaitForFlipFunctionTestingExited(Thread* self, ThreadExitFlag* tef) {
-  Locks::thread_list_lock_->Lock(self);
-  if (tef->HasExited()) {
-    Locks::thread_list_lock_->Unlock(self);
-    return;
-  }
-  // We need to hold suspend_count_lock_ to avoid missed wakeups when the flip function finishes.
-  // We need to hold thread_list_lock_ because the tef test result is only valid while we hold the
-  // lock, and once kRunningFlipFunction is no longer set, "this" may be deallocated. Hence the
-  // complicated locking dance.
-  MutexLock mu(self, *Locks::thread_suspend_count_lock_);
-  while (true) {
-    StateAndFlags old_state_and_flags = GetStateAndFlags(std::memory_order_acquire);
-    DCHECK(!old_state_and_flags.IsFlagSet(ThreadFlag::kPendingFlipFunction));
-    Locks::thread_list_lock_->Unlock(self);  // So we can wait or return.
-    if (!old_state_and_flags.IsFlagSet(ThreadFlag::kRunningFlipFunction)) {
-      return;
+    if (!old_state_and_flags.IsFlagSet(ThreadFlag::kWaitingForFlipFunction)) {
+      // Mark that there is a waiting thread.
+      StateAndFlags new_state_and_flags =
+          old_state_and_flags.WithFlag(ThreadFlag::kWaitingForFlipFunction);
+      if (!tls32_.state_and_flags.CompareAndSetWeakRelaxed(old_state_and_flags.GetValue(),
+                                                           new_state_and_flags.GetValue())) {
+        continue;  // Retry.
+      }
     }
-    resume_cond_->WaitHoldingLocks(self);
-    Locks::thread_suspend_count_lock_->Unlock(self);  // To re-lock thread_list_lock.
-    Locks::thread_list_lock_->Lock(self);
-    Locks::thread_suspend_count_lock_->Lock(self);
-    if (tef->HasExited()) {
-      Locks::thread_list_lock_->Unlock(self);
-      return;
-    }
+    resume_cond_->Wait(self);
   }
 }
 
 void Thread::FullSuspendCheck(bool implicit) {
   ScopedTrace trace(__FUNCTION__);
-  DCHECK(!ReadFlag(ThreadFlag::kSuspensionImmune));
-  DCHECK(this == Thread::Current());
   VLOG(threads) << this << " self-suspending";
   // Make thread appear suspended to other threads, release mutator_lock_.
   // Transition to suspended and back to runnable, re-acquire share on mutator_lock_.
@@ -2246,27 +2260,19 @@
     if (obj == nullptr) {
       os << msg << "an unknown object";
     } else {
-      const std::string pretty_type(obj->PrettyTypeOf());
-      // It's often unsafe to allow lock inflation here. We may be the only runnable thread, or
-      // this may be called from a checkpoint. We get the hashcode on a best effort basis.
-      static constexpr int kNumRetries = 3;
-      static constexpr int kSleepMicros = 10;
-      int32_t hash_code;
-      for (int i = 0;; ++i) {
-        hash_code = obj->IdentityHashCodeNoInflation();
-        if (hash_code != 0 || i == kNumRetries) {
-          break;
-        }
-        usleep(kSleepMicros);
-      }
-      if (hash_code == 0) {
-        os << msg
-           << StringPrintf("<@addr=0x%" PRIxPTR "> (a %s)",
-                           reinterpret_cast<intptr_t>(obj.Ptr()),
-                           pretty_type.c_str());
+      if ((obj->GetLockWord(true).GetState() == LockWord::kThinLocked) &&
+          Locks::mutator_lock_->IsExclusiveHeld(Thread::Current())) {
+        // Getting the identity hashcode here would result in lock inflation and suspension of the
+        // current thread, which isn't safe if this is the only runnable thread.
+        os << msg << StringPrintf("<@addr=0x%" PRIxPTR "> (a %s)",
+                                  reinterpret_cast<intptr_t>(obj.Ptr()),
+                                  obj->PrettyTypeOf().c_str());
       } else {
-        // - waiting on <0x608c468> (a java.lang.Class<java.lang.ref.ReferenceQueue>)
-        os << msg << StringPrintf("<0x%08x> (a %s)", hash_code, pretty_type.c_str());
+        // - waiting on <0x6008c468> (a java.lang.Class<java.lang.ref.ReferenceQueue>)
+        // Call PrettyTypeOf before IdentityHashCode since IdentityHashCode can cause thread
+        // suspension and move pretty_object.
+        const std::string pretty_type(obj->PrettyTypeOf());
+        os << msg << StringPrintf("<0x%08x> (a %s)", obj->IdentityHashCode(), pretty_type.c_str());
       }
     }
     if (owner_tid != ThreadList::kInvalidThreadId) {
@@ -2470,21 +2476,6 @@
       soa.Self(), thread_group_object, thread_object);
 }
 
-void Thread::SignalExitFlags() {
-  ThreadExitFlag* next;
-  for (ThreadExitFlag* tef = tlsPtr_.thread_exit_flags; tef != nullptr; tef = next) {
-    DCHECK(!tef->exited_);
-    tef->exited_ = true;
-    next = tef->next_;
-    if (kIsDebugBuild) {
-      ThreadExitFlag* const garbage_tef = reinterpret_cast<ThreadExitFlag*>(1);
-      // Link fields should no longer be used.
-      tef->prev_ = tef->next_ = garbage_tef;
-    }
-  }
-  tlsPtr_.thread_exit_flags = nullptr;  // Now unused.
-}
-
 Thread::Thread(bool daemon)
     : tls32_(daemon),
       wait_monitor_(nullptr),
@@ -2509,10 +2500,12 @@
             tlsPtr_.rosalloc_runs + kNumRosAllocThreadLocalSizeBracketsInThread,
             gc::allocator::RosAlloc::GetDedicatedFullRun());
   tlsPtr_.checkpoint_function = nullptr;
-  tlsPtr_.active_suspendall_barrier = nullptr;
-  tlsPtr_.active_suspend1_barriers = nullptr;
-  tlsPtr_.flip_function.store(nullptr, std::memory_order_relaxed);
+  for (uint32_t i = 0; i < kMaxSuspendBarriers; ++i) {
+    tlsPtr_.active_suspend_barriers[i] = nullptr;
+  }
+  tlsPtr_.flip_function = nullptr;
   tlsPtr_.thread_local_mark_stack = nullptr;
+  tls32_.is_transitioning_to_runnable = false;
   ResetTlab();
 }
 
@@ -2654,11 +2647,10 @@
   CHECK_NE(GetState(), ThreadState::kRunnable);
   CHECK(!ReadFlag(ThreadFlag::kCheckpointRequest));
   CHECK(!ReadFlag(ThreadFlag::kEmptyCheckpointRequest));
-  CHECK(!ReadFlag(ThreadFlag::kSuspensionImmune));
   CHECK(tlsPtr_.checkpoint_function == nullptr);
   CHECK_EQ(checkpoint_overflow_.size(), 0u);
-  // A pending flip function request is OK. FlipThreadRoots will have been notified that we
-  // exited, and nobody will attempt to process the request.
+  CHECK(tlsPtr_.flip_function == nullptr);
+  CHECK_EQ(tls32_.is_transitioning_to_runnable, false);
 
   // Make sure we processed all deoptimization requests.
   CHECK(tlsPtr_.deoptimization_context_stack == nullptr) << "Missed deoptimization";
@@ -4747,47 +4739,11 @@
 }
 
 mirror::Object* Thread::GetPeerFromOtherThread() {
-  Thread* self = Thread::Current();
-  if (this == self) {
-    // We often call this on every thread, including ourselves.
-    return GetPeer();
-  }
-  // If "this" thread is not suspended, it could disappear.
-  DCHECK(IsSuspended()) << *this;
   DCHECK(tlsPtr_.jpeer == nullptr);
-  // Some JVMTI code may unfortunately hold thread_list_lock_, but if it does, it should hold the
-  // mutator lock in exclusive mode, and we should not have a pending flip function.
-  if (kIsDebugBuild && Locks::thread_list_lock_->IsExclusiveHeld(self)) {
-    Locks::mutator_lock_->AssertExclusiveHeld(self);
-    CHECK(!ReadFlag(ThreadFlag::kPendingFlipFunction));
-  }
   // Ensure that opeer is not obsolete.
-  EnsureFlipFunctionStarted(self, this);
-  if (ReadFlag(ThreadFlag::kRunningFlipFunction)) {
-    // Does not release mutator lock. Hence no new flip requests can be issued.
-    WaitForFlipFunction(self);
-  }
-  return tlsPtr_.opeer;
-}
-
-mirror::Object* Thread::LockedGetPeerFromOtherThread(ThreadExitFlag* tef) {
-  DCHECK(tlsPtr_.jpeer == nullptr);
-  Thread* self = Thread::Current();
-  Locks::thread_list_lock_->AssertHeld(self);
-  if (ReadFlag(ThreadFlag::kPendingFlipFunction)) {
-    // It is unsafe to call EnsureFlipFunctionStarted with thread_list_lock_. Thus we temporarily
-    // release it, taking care to handle the case in which "this" thread disapppears while we no
-    // longer hold it.
-    Locks::thread_list_lock_->Unlock(self);
-    EnsureFlipFunctionStarted(self, this, StateAndFlags(0), tef);
-    Locks::thread_list_lock_->Lock(self);
-    if (tef->HasExited()) {
-      return nullptr;
-    }
-  }
-  if (ReadFlag(ThreadFlag::kRunningFlipFunction)) {
-    // Does not release mutator lock. Hence no new flip requests can be issued.
-    WaitForFlipFunction(self);
+  EnsureFlipFunctionStarted(Thread::Current());
+  while (GetStateAndFlags(std::memory_order_acquire).IsAnyOfFlagsSet(FlipFunctionFlags())) {
+    usleep(kSuspendTimeDuringFlip);
   }
   return tlsPtr_.opeer;
 }
diff --git a/runtime/thread.h b/runtime/thread.h
index e439565..e1503ae 100644
--- a/runtime/thread.h
+++ b/runtime/thread.h
@@ -47,7 +47,6 @@
 #include "reflective_handle_scope.h"
 #include "runtime_globals.h"
 #include "runtime_stats.h"
-#include "suspend_reason.h"
 #include "thread_state.h"
 
 namespace unwindstack {
@@ -102,6 +101,7 @@
 class ScopedObjectAccessAlreadyRunnable;
 class ShadowFrame;
 class StackedShadowFrameRecord;
+enum class SuspendReason : char;
 class Thread;
 class ThreadList;
 enum VisitRootFlags : uint8_t;
@@ -133,35 +133,25 @@
   kEmptyCheckpointRequest = 1u << 2,
 
   // Register that at least 1 suspend barrier needs to be passed.
-  // Changes to this flag are guarded by suspend_count_lock_ .
   kActiveSuspendBarrier = 1u << 3,
 
   // Marks that a "flip function" needs to be executed on this thread.
-  // Set only while holding thread_list_lock_.
   kPendingFlipFunction = 1u << 4,
 
   // Marks that the "flip function" is being executed by another thread.
   //
-  // This is used to guard against multiple threads trying to run the
+  // This is used to guards against multiple threads trying to run the
   // "flip function" for the same thread while the thread is suspended.
   //
-  // Set when we have some way to ensure that the thread cannot disappear out from under us,
-  // Either:
-  //   1) Set by the thread itself,
-  //   2) by a thread holding thread_list_lock_, or
-  //   3) while the target has a pending suspension request.
-  // Once set, prevents a thread from exiting.
+  // This is not needed when the thread is running the flip function
+  // on its own after transitioning to Runnable.
   kRunningFlipFunction = 1u << 5,
 
-  // We are responsible for resuming all other threads. We ignore suspension requests,
-  // but not checkpoint requests, until a more opportune time. GC code should
-  // in any case not check for such requests; other clients of SuspendAll might.
-  // Prevents a situation in which we are asked to suspend just before we suspend all
-  // other threads, and then notice the suspension request and suspend ourselves,
-  // leading to deadlock. Guarded by suspend_count_lock_ .
-  // TODO(b/296639267): Generalize use to prevent SuspendAll from blocking
-  // in-progress GC.
-  kSuspensionImmune = 1u << 6,
+  // Marks that a thread is wating for "flip function" to complete.
+  //
+  // This is used to check if we need to broadcast the completion of the
+  // "flip function" to other threads. See also `kRunningFlipFunction`.
+  kWaitingForFlipFunction = 1u << 6,
 
   // Request that compiled JNI stubs do not transition to Native or Runnable with
   // inlined code, but take a slow path for monitoring method entry and exit events.
@@ -197,31 +187,6 @@
   kDisabled
 };
 
-// See Thread.tlsPtr_.active_suspend1_barriers below for explanation.
-struct WrappedSuspend1Barrier {
-  WrappedSuspend1Barrier() : barrier_(1), next_(nullptr) {}
-  AtomicInteger barrier_;
-  struct WrappedSuspend1Barrier* next_ GUARDED_BY(Locks::thread_suspend_count_lock_);
-};
-
-// Mostly opaque structure allocated by the client of NotifyOnThreadExit.  Allows a client to
-// check whether the thread still exists after temporarily releasing thread_list_lock_, usually
-// because we need to wait for something.
-class ThreadExitFlag {
- public:
-  ThreadExitFlag() : exited_(false) {}
-  bool HasExited() REQUIRES(Locks::thread_list_lock_) { return exited_; }
-
- private:
-  // All ThreadExitFlags associated with a thread and with exited_ == false are in a doubly linked
-  // list.  tlsPtr_.thread_exit_flags points to the first element.  first.prev_ and last.next_ are
-  // null. This list contains no ThreadExitFlags with exited_ == true;
-  ThreadExitFlag* next_ GUARDED_BY(Locks::thread_list_lock_);
-  ThreadExitFlag* prev_ GUARDED_BY(Locks::thread_list_lock_);
-  bool exited_ GUARDED_BY(Locks::thread_list_lock_);
-  friend class Thread;
-};
-
 // This should match RosAlloc::kNumThreadLocalSizeBrackets.
 static constexpr size_t kNumRosAllocThreadLocalSizeBracketsInThread = 16;
 
@@ -355,10 +320,7 @@
   }
 
   bool IsSuspended() const {
-    // We need to ensure that once we return true, all prior accesses to the Java data by "this"
-    // thread are complete. Hence we need "acquire" ordering here, and "release" when the flags
-    // are set.
-    StateAndFlags state_and_flags = GetStateAndFlags(std::memory_order_acquire);
+    StateAndFlags state_and_flags = GetStateAndFlags(std::memory_order_relaxed);
     return state_and_flags.GetState() != ThreadState::kRunnable &&
            state_and_flags.IsFlagSet(ThreadFlag::kSuspendRequest);
   }
@@ -374,35 +336,20 @@
     return tls32_.define_class_counter;
   }
 
-  // Increment suspend count and optionally install at most one suspend barrier.
-  // Must hold thread_list_lock, OR be called with self == this, so that the Thread cannot
-  // disappear while we're running. If it's known that this == self, and thread_list_lock_
-  // is not held, FakeMutexLock should be used to fake-acquire thread_list_lock_ for
-  // static checking purposes.
+  // If delta > 0 and (this != self or suspend_barrier is not null), this function may temporarily
+  // release thread_suspend_count_lock_ internally.
   ALWAYS_INLINE
-  void IncrementSuspendCount(Thread* self,
-                             AtomicInteger* suspendall_barrier,
-                             WrappedSuspend1Barrier* suspend1_barrier,
-                             SuspendReason reason) REQUIRES(Locks::thread_suspend_count_lock_)
-      REQUIRES(Locks::thread_list_lock_);
-
-  // The same, but default reason to kInternal, and barriers to nullptr.
-  ALWAYS_INLINE void IncrementSuspendCount(Thread* self) REQUIRES(Locks::thread_suspend_count_lock_)
-      REQUIRES(Locks::thread_list_lock_);
-
-  // Follows one of the above calls. For_user_code indicates if SuspendReason was kForUserCode.
-  ALWAYS_INLINE void DecrementSuspendCount(Thread* self, bool for_user_code = false)
+  bool ModifySuspendCount(Thread* self,
+                          int delta,
+                          AtomicInteger* suspend_barrier,
+                          SuspendReason reason)
+      WARN_UNUSED
       REQUIRES(Locks::thread_suspend_count_lock_);
 
- private:
-  NO_RETURN static void UnsafeLogFatalForSuspendCount(Thread* self, Thread* thread);
-
- public:
   // Requests a checkpoint closure to run on another thread. The closure will be run when the
   // thread notices the request, either in an explicit runtime CheckSuspend() call, or in a call
   // originating from a compiler generated suspend point check. This returns true if the closure
-  // was added and will (eventually) be executed. It returns false if this was impossible
-  // because the thread was suspended, and we thus did nothing.
+  // was added and will (eventually) be executed. It returns false otherwise.
   //
   // Since multiple closures can be queued and some closures can delay other threads from running,
   // no closure should attempt to suspend another thread while running.
@@ -416,37 +363,27 @@
 
   // RequestSynchronousCheckpoint releases the thread_list_lock_ as a part of its execution. This is
   // due to the fact that Thread::Current() needs to go to sleep to allow the targeted thread to
-  // execute the checkpoint for us if it is Runnable. The wait_state is the state that the thread
+  // execute the checkpoint for us if it is Runnable. The suspend_state is the state that the thread
   // will go into while it is awaiting the checkpoint to be run.
-  // The closure may be run on Thread::Current() on behalf of "this" thread.
-  // Thus for lock ordering purposes, the closure should be runnable by the caller. This also
-  // sometimes makes it reasonable to pass ThreadState::kRunnable as wait_state: We may wait on
-  // a condition variable for the "this" thread to act, but for lock ordering purposes, this is
-  // exactly as though Thread::Current() had run the closure.
+  // NB Passing ThreadState::kRunnable may cause the current thread to wait in a condition variable
+  // while holding the mutator_lock_.  Callers should ensure that this will not cause any problems
+  // for the closure or the rest of the system.
   // NB Since multiple closures can be queued and some closures can delay other threads from running
   // no closure should attempt to suspend another thread while running.
   bool RequestSynchronousCheckpoint(Closure* function,
-                                    ThreadState wait_state = ThreadState::kWaiting)
-      REQUIRES_SHARED(Locks::mutator_lock_) RELEASE(Locks::thread_list_lock_)
-          REQUIRES(!Locks::thread_suspend_count_lock_);
+                                    ThreadState suspend_state = ThreadState::kWaiting)
+      REQUIRES_SHARED(Locks::mutator_lock_)
+      RELEASE(Locks::thread_list_lock_)
+      REQUIRES(!Locks::thread_suspend_count_lock_);
 
   bool RequestEmptyCheckpoint()
       REQUIRES(Locks::thread_suspend_count_lock_);
 
-  Closure* GetFlipFunction() { return tlsPtr_.flip_function.load(std::memory_order_relaxed); }
-
   // Set the flip function. This is done with all threads suspended, except for the calling thread.
-  void SetFlipFunction(Closure* function) REQUIRES(Locks::thread_suspend_count_lock_)
-      REQUIRES(Locks::thread_list_lock_);
+  void SetFlipFunction(Closure* function);
 
-  // Wait for the flip function to complete if still running on another thread. Assumes the "this"
-  // thread remains live.
-  void WaitForFlipFunction(Thread* self) const REQUIRES(!Locks::thread_suspend_count_lock_);
-
-  // An enhanced version of the above that uses tef to safely return if the thread exited in the
-  // meantime.
-  void WaitForFlipFunctionTestingExited(Thread* self, ThreadExitFlag* tef)
-      REQUIRES(!Locks::thread_suspend_count_lock_, !Locks::thread_list_lock_);
+  // Wait for the flip function to complete if still running on another thread.
+  void WaitForFlipFunction(Thread* self);
 
   gc::accounting::AtomicStack<mirror::Object>* GetThreadLocalMarkStack() {
     CHECK(gUseReadBarrier);
@@ -468,7 +405,6 @@
 
   // Called when thread detected that the thread_suspend_count_ was non-zero. Gives up share of
   // mutator_lock_ and waits until it is resumed and thread_suspend_count_ is zero.
-  // Should be called only when the kSuspensionImmune flag is clear. Requires this == Current();
   void FullSuspendCheck(bool implicit = false)
       REQUIRES(!Locks::thread_suspend_count_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);
@@ -583,33 +519,10 @@
   // GetPeer is not safe if called on another thread in the middle of the thread flip and
   // the thread's stack may have not been flipped yet and peer may be a from-space (stale) ref.
   // This function will force a flip for the other thread if necessary.
-  // Since we hold a shared mutator lock, a new flip function cannot be concurrently installed.
-  // The target thread must be suspended, so that it cannot disappear during the call.
-  // We should ideally not hold thread_list_lock_ . GetReferenceKind in ti_heap.cc, currently does
-  // hold it, but in a context in which we do not invoke EnsureFlipFunctionStarted().
+  // Since we hold a shared mutator lock, a new flip function cannot be concurrently
+  // installed
   mirror::Object* GetPeerFromOtherThread() REQUIRES_SHARED(Locks::mutator_lock_);
 
-  // A version of the above that requires thread_list_lock_, but does not require the thread to
-  // be suspended. This may temporarily release thread_list_lock_. It thus needs a ThreadExitFlag
-  // describing the thread's status, so we can tell if it exited in the interim. Returns null if
-  // the thread exited.
-  mirror::Object* LockedGetPeerFromOtherThread(ThreadExitFlag* tef)
-      REQUIRES_SHARED(Locks::mutator_lock_) REQUIRES(Locks::thread_list_lock_);
-
-  // A convenience version of the above that creates the ThreadExitFlag locally. This is often
-  // unsafe if more than one thread is being processed. A prior call may have released
-  // thread_list_lock_, and thus the NotifyOnThreadExit() call here could see a deallocated
-  // Thread. We must hold the thread_list_lock continuously between obtaining the Thread*
-  // and calling NotifyOnThreadExit().
-  mirror::Object* LockedGetPeerFromOtherThread() REQUIRES_SHARED(Locks::mutator_lock_)
-      REQUIRES(Locks::thread_list_lock_) {
-    ThreadExitFlag tef;
-    NotifyOnThreadExit(&tef);
-    mirror::Object* result = LockedGetPeerFromOtherThread(&tef);
-    UnregisterThreadExitFlag(&tef);
-    return result;
-  }
-
   bool HasPeer() const {
     return tlsPtr_.jpeer != nullptr || tlsPtr_.opeer != nullptr;
   }
@@ -731,33 +644,6 @@
   void NotifyThreadGroup(ScopedObjectAccessAlreadyRunnable& soa, jobject thread_group = nullptr)
       REQUIRES_SHARED(Locks::mutator_lock_);
 
-  // Request notification when this thread is unregistered, typically because it has exited.
-  //
-  // The ThreadExitFlag status is only changed when we remove the thread from the thread list,
-  // which we only do once no suspend requests are outstanding, and no flip-functions are still
-  // running.
-  //
-  // The caller must allocate a fresh ThreadExitFlag, and pass it in. The caller is responsible
-  // for either waiting until the thread has exited, or unregistering the ThreadExitFlag, and
-  // then, and only then, deallocating the ThreadExitFlag.  (This scheme avoids an allocation and
-  // questions about what to do if the allocation fails. Allows detection of thread exit after
-  // temporary release of thread_list_lock_)
-  void NotifyOnThreadExit(ThreadExitFlag* tef) REQUIRES(Locks::thread_list_lock_);
-  void UnregisterThreadExitFlag(ThreadExitFlag* tef) REQUIRES(Locks::thread_list_lock_);
-
-  // Is the ThreadExitFlag currently registered in this thread, which has not yet terminated?
-  // Intended only for testing.
-  bool IsRegistered(ThreadExitFlag* query_tef) REQUIRES(Locks::thread_list_lock_);
-
-  // For debuggable builds, CHECK that neither first nor last, nor any ThreadExitFlag with an
-  // address in-between, is currently registered with any thread.
-  static void DCheckUnregisteredEverywhere(ThreadExitFlag* first, ThreadExitFlag* last)
-      REQUIRES(!Locks::thread_list_lock_);
-
-  // Called when thread is unregistered. May be called repeatedly, in which case only newly
-  // registered clients are processed.
-  void SignalExitFlags() REQUIRES(Locks::thread_list_lock_);
-
   // JNI methods
   JNIEnvExt* GetJniEnv() const {
     return tlsPtr_.jni_env;
@@ -876,17 +762,6 @@
   void VisitReflectiveTargets(ReflectiveValueVisitor* visitor)
       REQUIRES(Locks::mutator_lock_);
 
-  // Check that the thread state is valid. Try to fail if the thread has erroneously terminated.
-  // Note that once the thread has been terminated, it can also be deallocated.  But even if the
-  // thread state has been overwritten, the value is unlikely to be in the correct range.
-  void VerifyState() {
-    if (kIsDebugBuild) {
-      ThreadState state = GetState();
-      StateAndFlags::ValidateThreadState(state);
-      DCHECK_NE(state, ThreadState::kTerminated);
-    }
-  }
-
   void VerifyStack() REQUIRES_SHARED(Locks::mutator_lock_) {
     if (kVerifyStack) {
       VerifyStackImpl();
@@ -1366,24 +1241,18 @@
     tlsPtr_.held_mutexes[level] = mutex;
   }
 
-  // Possibly check that no mutexes at level kMonitorLock or above are subsequently acquired.
-  // Only invoked by the thread itself.
-  void DisallowPreMonitorMutexes();
-
-  // Undo the effect of the previous call. Again only invoked by the thread itself.
-  void AllowPreMonitorMutexes();
+  void ClearSuspendBarrier(AtomicInteger* target)
+      REQUIRES(Locks::thread_suspend_count_lock_);
 
   bool ReadFlag(ThreadFlag flag) const {
     return GetStateAndFlags(std::memory_order_relaxed).IsFlagSet(flag);
   }
 
   void AtomicSetFlag(ThreadFlag flag, std::memory_order order = std::memory_order_seq_cst) {
-    // Since we discard the returned value, memory_order_release will often suffice.
     tls32_.state_and_flags.fetch_or(enum_cast<uint32_t>(flag), order);
   }
 
   void AtomicClearFlag(ThreadFlag flag, std::memory_order order = std::memory_order_seq_cst) {
-    // Since we discard the returned value, memory_order_release will often suffice.
     tls32_.state_and_flags.fetch_and(~enum_cast<uint32_t>(flag), order);
   }
 
@@ -1476,6 +1345,14 @@
   bool ProtectStack(bool fatal_on_error = true);
   bool UnprotectStack();
 
+  bool IsTransitioningToRunnable() const {
+    return tls32_.is_transitioning_to_runnable;
+  }
+
+  void SetIsTransitioningToRunnable(bool value) {
+    tls32_.is_transitioning_to_runnable = value;
+  }
+
   uint32_t DecrementForceInterpreterCount() REQUIRES(Locks::thread_list_lock_) {
     return --tls32_.force_interpreter_count;
   }
@@ -1586,7 +1463,8 @@
 
   static constexpr uint32_t FlipFunctionFlags() {
     return enum_cast<uint32_t>(ThreadFlag::kPendingFlipFunction) |
-           enum_cast<uint32_t>(ThreadFlag::kRunningFlipFunction);
+           enum_cast<uint32_t>(ThreadFlag::kRunningFlipFunction) |
+           enum_cast<uint32_t>(ThreadFlag::kWaitingForFlipFunction);
   }
 
   static constexpr uint32_t StoredThreadStateValue(ThreadState state) {
@@ -1607,23 +1485,8 @@
   }
 
  private:
-  // We pretend to acquire this while running a checkpoint to detect lock ordering issues.
-  // Initialized lazily.
-  static std::atomic<Mutex*> cp_placeholder_mutex_;
-
   explicit Thread(bool daemon);
-
-  // A successfully started thread is only deleted by the thread itself.
-  // Threads are deleted after they have been removed from the thread list while holding
-  // suspend_count_lock_ and thread_list_lock_. We refuse to do this while either kSuspendRequest
-  // or kRunningFlipFunction are set. We can prevent Thread destruction by holding either of those
-  // locks, ensuring that either of those flags are set, or possibly by registering and checking a
-  // ThreadExitFlag.
   ~Thread() REQUIRES(!Locks::mutator_lock_, !Locks::thread_suspend_count_lock_);
-
-  // Thread destruction actions that do not invalidate the thread. Checkpoints and flip_functions
-  // may still be called on this Thread object, though not by this thread, during and after the
-  // Destroy() call.
   void Destroy(bool should_run_callbacks);
 
   // Deletes and clears the tlsPtr_.jpeer field. Done in a way so that both it and opeer cannot be
@@ -1650,8 +1513,7 @@
 
   // Avoid use, callers should use SetState.
   // Used only by `Thread` destructor and stack trace collection in semi-space GC (currently
-  // disabled by `kStoreStackTraces = false`). May not be called on a runnable thread other
-  // than Thread::Current().
+  // disabled by `kStoreStackTraces = false`).
   // NO_THREAD_SAFETY_ANALYSIS: This function is "Unsafe" and can be called in
   // different states, so clang cannot perform the thread safety analysis.
   ThreadState SetStateUnsafe(ThreadState new_state) NO_THREAD_SAFETY_ANALYSIS {
@@ -1660,19 +1522,19 @@
     if (old_state == new_state) {
       // Nothing to do.
     } else if (old_state == ThreadState::kRunnable) {
-      DCHECK_EQ(this, Thread::Current());
       // Need to run pending checkpoint and suspend barriers. Run checkpoints in runnable state in
       // case they need to use a ScopedObjectAccess. If we are holding the mutator lock and a SOA
       // attempts to TransitionFromSuspendedToRunnable, it results in a deadlock.
       TransitionToSuspendedAndRunCheckpoints(new_state);
       // Since we transitioned to a suspended state, check the pass barrier requests.
-      CheckActiveSuspendBarriers();
+      PassActiveSuspendBarriers();
     } else {
       while (true) {
         StateAndFlags new_state_and_flags = old_state_and_flags;
         new_state_and_flags.SetState(new_state);
         if (LIKELY(tls32_.state_and_flags.CompareAndSetWeakAcquire(
-                old_state_and_flags.GetValue(), new_state_and_flags.GetValue()))) {
+                                              old_state_and_flags.GetValue(),
+                                              new_state_and_flags.GetValue()))) {
           break;
         }
         // Reload state and flags.
@@ -1739,25 +1601,8 @@
       REQUIRES(!Locks::thread_suspend_count_lock_, !Roles::uninterruptible_)
       REQUIRES_SHARED(Locks::mutator_lock_);
 
-  // Call PassActiveSuspendBarriers() if there are active barriers. Only called on current thread.
-  ALWAYS_INLINE void CheckActiveSuspendBarriers()
-      REQUIRES(!Locks::thread_suspend_count_lock_, !Locks::mutator_lock_, !Roles::uninterruptible_);
-
-  // Decrement all "suspend barriers" for the current thread, notifying threads that requested our
-  // suspension. Only called on current thread, when suspended. If suspend_count_ > 0 then we
-  // promise that we are and will remain "suspended" until the suspend count is decremented.
-  bool PassActiveSuspendBarriers()
-      REQUIRES(!Locks::thread_suspend_count_lock_, !Locks::mutator_lock_);
-
-  // Add an entry to active_suspend1_barriers.
-  ALWAYS_INLINE void AddSuspend1Barrier(WrappedSuspend1Barrier* suspend1_barrier)
-      REQUIRES(Locks::thread_suspend_count_lock_);
-
-  // Remove last-added entry from active_suspend1_barriers.
-  // Only makes sense if we're still holding thread_suspend_count_lock_ since insertion.
-  ALWAYS_INLINE void RemoveFirstSuspend1Barrier() REQUIRES(Locks::thread_suspend_count_lock_);
-
-  ALWAYS_INLINE bool HasActiveSuspendBarrier() REQUIRES(Locks::thread_suspend_count_lock_);
+  ALWAYS_INLINE void PassActiveSuspendBarriers()
+      REQUIRES(!Locks::thread_suspend_count_lock_, !Roles::uninterruptible_);
 
   // Registers the current thread as the jit sensitive thread. Should be called just once.
   static void SetJitSensitiveThread() {
@@ -1773,6 +1618,13 @@
     is_sensitive_thread_hook_ = is_sensitive_thread_hook;
   }
 
+  bool ModifySuspendCountInternal(Thread* self,
+                                  int delta,
+                                  AtomicInteger* suspend_barrier,
+                                  SuspendReason reason)
+      WARN_UNUSED
+      REQUIRES(Locks::thread_suspend_count_lock_);
+
   // Runs a single checkpoint function. If there are no more pending checkpoint functions it will
   // clear the kCheckpointRequest flag. The caller is responsible for calling this in a loop until
   // the kCheckpointRequest flag is cleared.
@@ -1781,6 +1633,9 @@
       REQUIRES_SHARED(Locks::mutator_lock_);
   void RunEmptyCheckpoint();
 
+  bool PassActiveSuspendBarriers(Thread* self)
+      REQUIRES(!Locks::thread_suspend_count_lock_);
+
   // Install the protected region for implicit stack checks.
   void InstallImplicitProtection();
 
@@ -1857,6 +1712,7 @@
       return ThreadStateField::Encode(state);
     }
 
+   private:
     static constexpr void ValidateThreadState(ThreadState state) {
       if (kIsDebugBuild && state != ThreadState::kRunnable) {
         CHECK_GE(state, ThreadState::kTerminated);
@@ -1883,31 +1739,21 @@
   // Format state and flags as a hex string. For diagnostic output.
   std::string StateAndFlagsAsHexString() const;
 
-  // Run the flip function and notify other threads that may have tried
+  // Run the flip function and, if requested, notify other threads that may have tried
   // to do that concurrently.
-  void RunFlipFunction(Thread* self) REQUIRES_SHARED(Locks::mutator_lock_);
+  void RunFlipFunction(Thread* self, bool notify) REQUIRES_SHARED(Locks::mutator_lock_);
 
-  // Ensure that thread flip function for thread target started running. If no other thread is
-  // executing it, the calling thread shall run the flip function and then notify other threads
+  // Ensure that thread flip function started running. If no other thread is executing
+  // it, the calling thread shall run the flip function and then notify other threads
   // that have tried to do that concurrently. After this function returns, the
-  // `ThreadFlag::kPendingFlipFunction` is cleared but another thread may still be running the
-  // flip function as indicated by the `ThreadFlag::kRunningFlipFunction`. Optional arguments:
-  //  - old_state_and_flags indicates the current and state and flags value for the thread, with
-  //    at least kPendingFlipFunction set. The thread should logically acquire the
-  //    mutator lock before running the flip function.  A special zero value indicates that the
-  //    thread already holds the mutator lock, and the actual state_and_flags must be read.
-  //    A non-zero value implies this == Current().
-  //  - If tef is non-null, we check that the target thread has not yet exited, as indicated by
-  //    tef. In that case, we acquire thread_list_lock_ as needed.
-  //  - If finished is non-null, we assign to *finished to indicate whether the flip was known to
-  //    be completed when we returned.
-  //  Returns true if and only if we acquired the mutator lock (which implies that we ran the flip
-  //  function after finding old_state_and_flags unchanged).
-  static bool EnsureFlipFunctionStarted(Thread* self,
-                                        Thread* target,
-                                        StateAndFlags old_state_and_flags = StateAndFlags(0),
-                                        ThreadExitFlag* tef = nullptr,
-                                        /*out*/ bool* finished = nullptr)
+  // `ThreadFlag::kPendingFlipFunction` is cleared but another thread may still
+  // run the flip function as indicated by the `ThreadFlag::kRunningFlipFunction`.
+  // A non-zero 'old_state_and_flags' indicates that the thread should logically
+  // acquire mutator lock if we win the race to run the flip function, if a
+  // suspend request is not already set. A zero 'old_state_and_flags' indicates
+  // we already hold the mutator lock.
+  // Returns true if this call ran the flip function.
+  bool EnsureFlipFunctionStarted(Thread* self, StateAndFlags old_state_and_flags = StateAndFlags(0))
       TRY_ACQUIRE_SHARED(true, Locks::mutator_lock_);
 
   static void ThreadExitCallback(void* arg);
@@ -1953,6 +1799,7 @@
           throwing_OutOfMemoryError(false),
           no_thread_suspension(0),
           thread_exit_check_count(0),
+          is_transitioning_to_runnable(false),
           is_gc_marking(false),
           is_deopt_check_required(false),
           weak_ref_access_enabled(WeakRefAccessState::kVisiblyEnabled),
@@ -1962,7 +1809,8 @@
           make_visibly_initialized_counter(0),
           define_class_counter(0),
           num_name_readers(0),
-          shared_method_hotness(kSharedMethodHotnessThreshold) {}
+          shared_method_hotness(kSharedMethodHotnessThreshold)
+        {}
 
     // The state and flags field must be changed atomically so that flag values aren't lost.
     // See `StateAndFlags` for bit assignments of `ThreadFlag` and `ThreadState` values.
@@ -1998,6 +1846,11 @@
     // How many times has our pthread key's destructor been called?
     uint32_t thread_exit_check_count;
 
+    // True if the thread is in TransitionFromSuspendedToRunnable(). This is used to distinguish the
+    // non-runnable threads (eg. kNative, kWaiting) that are about to transition to runnable from
+    // the rest of them.
+    bool32_t is_transitioning_to_runnable;
+
     // True if the GC is in the marking phase. This is used for the CC collector only. This is
     // thread local so that we can simplify the logic to check for the fast path of read barriers of
     // GC roots.
@@ -2102,11 +1955,9 @@
                                name(nullptr),
                                pthread_self(0),
                                last_no_thread_suspension_cause(nullptr),
-                               active_suspendall_barrier(nullptr),
-                               active_suspend1_barriers(nullptr),
+                               thread_local_start(nullptr),
                                thread_local_pos(nullptr),
                                thread_local_end(nullptr),
-                               thread_local_start(nullptr),
                                thread_local_limit(nullptr),
                                thread_local_objects(0),
                                checkpoint_function(nullptr),
@@ -2118,8 +1969,7 @@
                                async_exception(nullptr),
                                top_reflective_handle_scope(nullptr),
                                method_trace_buffer(nullptr),
-                               method_trace_buffer_index(0),
-                               thread_exit_flags(nullptr) {
+                               method_trace_buffer_index(0) {
       std::fill(held_mutexes, held_mutexes + kLockLevelCount, nullptr);
     }
 
@@ -2217,37 +2067,27 @@
     // If no_thread_suspension_ is > 0, what is causing that assertion.
     const char* last_no_thread_suspension_cause;
 
-    // After a thread observes a suspend request and enters a suspended state,
-    // it notifies the requestor by arriving at a "suspend barrier". This consists of decrementing
-    // the atomic integer representing the barrier. (This implementation was introduced in 2015 to
-    // minimize cost. There may be other options.) These atomic integer barriers are always
-    // stored on the requesting thread's stack. They are referenced from the target thread's
-    // data structure in one of two ways; in either case the data structure referring to these
-    // barriers is guarded by suspend_count_lock:
-    // 1. A SuspendAll barrier is directly referenced from the target thread. Only one of these
-    // can be active at a time:
-    AtomicInteger* active_suspendall_barrier GUARDED_BY(Locks::thread_suspend_count_lock_);
-    // 2. For individual thread suspensions, active barriers are embedded in a struct that is used
-    // to link together all suspend requests for this thread. Unlike the SuspendAll case, each
-    // barrier is referenced by a single target thread, and thus can appear only on a single list.
-    // The struct as a whole is still stored on the requesting thread's stack.
-    WrappedSuspend1Barrier* active_suspend1_barriers GUARDED_BY(Locks::thread_suspend_count_lock_);
+    // Pending barriers that require passing or NULL if non-pending. Installation guarding by
+    // Locks::thread_suspend_count_lock_.
+    // They work effectively as art::Barrier, but implemented directly using AtomicInteger and futex
+    // to avoid additional cost of a mutex and a condition variable, as used in art::Barrier.
+    AtomicInteger* active_suspend_barriers[kMaxSuspendBarriers];
+
+    // Thread-local allocation pointer. Moved here to force alignment for thread_local_pos on ARM.
+    uint8_t* thread_local_start;
 
     // thread_local_pos and thread_local_end must be consecutive for ldrd and are 8 byte aligned for
     // potentially better performance.
     uint8_t* thread_local_pos;
     uint8_t* thread_local_end;
 
-    // Thread-local allocation pointer. Can be moved above the preceding two to correct alignment.
-    uint8_t* thread_local_start;
-
     // Thread local limit is how much we can expand the thread local buffer to, it is greater or
     // equal to thread_local_end.
     uint8_t* thread_local_limit;
 
     size_t thread_local_objects;
 
-    // Pending checkpoint function or null if non-pending. If this checkpoint is set and someone
+    // Pending checkpoint function or null if non-pending. If this checkpoint is set and someone\
     // requests another checkpoint, it goes to the checkpoint overflow list.
     Closure* checkpoint_function GUARDED_BY(Locks::thread_suspend_count_lock_);
 
@@ -2270,9 +2110,8 @@
     // Support for Mutex lock hierarchy bug detection.
     BaseMutex* held_mutexes[kLockLevelCount];
 
-    // The function used for thread flip.  Set while holding Locks::thread_suspend_count_lock_ and
-    // with all other threads suspended.  May be cleared while being read.
-    std::atomic<Closure*> flip_function;
+    // The function used for thread flip.
+    Closure* flip_function;
 
     union {
       // Thread-local mark stack for the concurrent copying collector.
@@ -2292,9 +2131,6 @@
 
     // The index of the next free entry in method_trace_buffer.
     size_t method_trace_buffer_index;
-
-    // Pointer to the first node of an intrusively doubly-linked list of ThreadExitFlags.
-    ThreadExitFlag* thread_exit_flags GUARDED_BY(Locks::thread_list_lock_);
   } tlsPtr_;
 
   // Small thread-local cache to be used from the interpreter.
@@ -2442,6 +2278,20 @@
   Thread* const self_;
 };
 
+class ScopedTransitioningToRunnable : public ValueObject {
+ public:
+  explicit ScopedTransitioningToRunnable(Thread* self)
+      : self_(self) {
+    DCHECK_EQ(self, Thread::Current());
+    self_->SetIsTransitioningToRunnable(true);
+  }
+
+  ~ScopedTransitioningToRunnable() { self_->SetIsTransitioningToRunnable(false); }
+
+ private:
+  Thread* const self_;
+};
+
 class ThreadLifecycleCallback {
  public:
   virtual ~ThreadLifecycleCallback() {}
diff --git a/runtime/thread_list.cc b/runtime/thread_list.cc
index 40c35d1..ce7035f 100644
--- a/runtime/thread_list.cc
+++ b/runtime/thread_list.cc
@@ -67,6 +67,10 @@
 using android::base::StringPrintf;
 
 static constexpr uint64_t kLongThreadSuspendThreshold = MsToNs(5);
+// Use 0 since we want to yield to prevent blocking for an unpredictable amount of time.
+static constexpr useconds_t kThreadSuspendInitialSleepUs = 0;
+static constexpr useconds_t kThreadSuspendMaxYieldUs = 3000;
+static constexpr useconds_t kThreadSuspendMaxSleepUs = 5000;
 
 // Whether we should try to dump the native stack of unattached threads. See commit ed8b723 for
 // some history.
@@ -75,7 +79,7 @@
 ThreadList::ThreadList(uint64_t thread_suspend_timeout_ns)
     : suspend_all_count_(0),
       unregistering_count_(0),
-      suspend_all_histogram_("suspend all histogram", 16, 64),
+      suspend_all_historam_("suspend all histogram", 16, 64),
       long_suspend_(false),
       shut_down_(false),
       thread_suspend_timeout_ns_(thread_suspend_timeout_ns),
@@ -136,10 +140,10 @@
   {
     ScopedObjectAccess soa(Thread::Current());
     // Only print if we have samples.
-    if (suspend_all_histogram_.SampleSize() > 0) {
+    if (suspend_all_historam_.SampleSize() > 0) {
       Histogram<uint64_t>::CumulativeData data;
-      suspend_all_histogram_.CreateHistogram(&data);
-      suspend_all_histogram_.PrintConfidenceIntervals(os, 0.99, data);  // Dump time to suspend.
+      suspend_all_historam_.CreateHistogram(&data);
+      suspend_all_historam_.PrintConfidenceIntervals(os, 0.99, data);  // Dump time to suspend.
     }
   }
   bool dump_native_stack = Runtime::Current()->GetDumpNativeStackOnSigQuit();
@@ -275,11 +279,11 @@
   }
 }
 
-void ThreadList::AssertOtherThreadsAreSuspended(Thread* self) {
+void ThreadList::AssertThreadsAreSuspended(Thread* self, Thread* ignore1, Thread* ignore2) {
   MutexLock mu(self, *Locks::thread_list_lock_);
   MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
   for (const auto& thread : list_) {
-    if (thread != self) {
+    if (thread != ignore1 && thread != ignore2) {
       CHECK(thread->IsSuspended())
             << "\nUnsuspended thread: <<" << *thread << "\n"
             << "self: <<" << *Thread::Current();
@@ -306,9 +310,20 @@
 }
 #endif
 
-size_t ThreadList::RunCheckpoint(Closure* checkpoint_function,
-                                 Closure* callback,
-                                 bool allow_lock_checking) {
+// Unlike suspending all threads where we can wait to acquire the mutator_lock_, suspending an
+// individual thread requires polling. delay_us is the requested sleep wait. If delay_us is 0 then
+// we use sched_yield instead of calling usleep.
+// Although there is the possibility, here and elsewhere, that usleep could return -1 and
+// errno = EINTR, there should be no problem if interrupted, so we do not check.
+static void ThreadSuspendSleep(useconds_t delay_us) {
+  if (delay_us == 0) {
+    sched_yield();
+  } else {
+    usleep(delay_us);
+  }
+}
+
+size_t ThreadList::RunCheckpoint(Closure* checkpoint_function, Closure* callback) {
   Thread* self = Thread::Current();
   Locks::mutator_lock_->AssertNotExclusiveHeld(self);
   Locks::thread_list_lock_->AssertNotHeld(self);
@@ -317,12 +332,9 @@
   std::vector<Thread*> suspended_count_modified_threads;
   size_t count = 0;
   {
-    // Call a checkpoint function for each thread. We directly invoke the function on behalf of
-    // suspended threads.
+    // Call a checkpoint function for each thread, threads which are suspended get their checkpoint
+    // manually called.
     MutexLock mu(self, *Locks::thread_list_lock_);
-    if (kIsDebugBuild && allow_lock_checking) {
-      self->DisallowPreMonitorMutexes();
-    }
     MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
     count = list_.size();
     for (const auto& thread : list_) {
@@ -333,35 +345,39 @@
             // This thread will run its checkpoint some time in the near future.
             if (requested_suspend) {
               // The suspend request is now unnecessary.
-              thread->DecrementSuspendCount(self);
+              bool updated =
+                  thread->ModifySuspendCount(self, -1, nullptr, SuspendReason::kInternal);
+              DCHECK(updated);
               requested_suspend = false;
             }
             break;
           } else {
-            // The thread was, and probably still is, suspended.
-            if (!requested_suspend) {
-              // This does not risk suspension cycles: We may have a pending suspension request,
-              // but it cannot block us: Checkpoint Run() functions may not suspend, thus we cannot
-              // be blocked from decrementing the count again.
-              thread->IncrementSuspendCount(self);
-              requested_suspend = true;
+            // The thread is probably suspended, try to make sure that it stays suspended.
+            if (thread->GetState() == ThreadState::kRunnable) {
+              // Spurious fail, try again.
+              continue;
             }
-            if (thread->IsSuspended()) {
-              // We saw it suspended after incrementing suspend count, so it will stay that way.
+            if (!requested_suspend) {
+              bool updated =
+                  thread->ModifySuspendCount(self, +1, nullptr, SuspendReason::kInternal);
+              DCHECK(updated);
+              requested_suspend = true;
+              if (thread->IsSuspended()) {
+                break;
+              }
+              // The thread raced us to become Runnable. Try to RequestCheckpoint() again.
+            } else {
+              // The thread previously raced our suspend request to become Runnable but
+              // since it is suspended again, it must honor that suspend request now.
+              DCHECK(thread->IsSuspended());
               break;
             }
           }
-          // We only get here if the thread entered kRunnable again. Retry immediately.
         }
-        // At this point, either the thread was runnable, and will run the checkpoint itself,
-        // or requested_suspend is true, and the thread is safely suspended.
         if (requested_suspend) {
-          DCHECK(thread->IsSuspended());
           suspended_count_modified_threads.push_back(thread);
         }
       }
-      // Thread either has honored or will honor the checkpoint, or it has been added to
-      // suspended_count_modified_threads.
     }
     // Run the callback to be called inside this critical section.
     if (callback != nullptr) {
@@ -382,9 +398,9 @@
         // We know for sure that the thread is suspended at this point.
         DCHECK(thread->IsSuspended());
         if (mutator_lock_held) {
-          // Make sure there is no pending flip function before running Java-heap-accessing
-          // checkpoint on behalf of thread.
-          Thread::EnsureFlipFunctionStarted(self, thread);
+          // Make sure there is no pending flip function before running checkpoint
+          // on behalf of thread.
+          thread->EnsureFlipFunctionStarted(self);
           if (thread->GetStateAndFlags(std::memory_order_acquire)
                   .IsAnyOfFlagsSet(Thread::FlipFunctionFlags())) {
             // There is another thread running the flip function for 'thread'.
@@ -392,12 +408,12 @@
             repeat = true;
             continue;
           }
-        }  // O.w. the checkpoint will not access Java data structures, and doesn't care whether
-           // the flip function has been called.
+        }
         checkpoint_function->Run(thread);
         {
           MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
-          thread->DecrementSuspendCount(self);
+          bool updated = thread->ModifySuspendCount(self, -1, nullptr, SuspendReason::kInternal);
+          DCHECK(updated);
         }
         // We are done with 'thread' so set it to nullptr so that next outer
         // loop iteration, if any, skips 'thread'.
@@ -416,9 +432,6 @@
     Thread::resume_cond_->Broadcast(self);
   }
 
-  if (kIsDebugBuild && allow_lock_checking) {
-    self->AllowPreMonitorMutexes();
-  }
   return count;
 }
 
@@ -527,156 +540,113 @@
   }
 }
 
-// Separate function to disable just the right amount of thread-safety analysis.
-ALWAYS_INLINE void AcquireMutatorLockSharedUncontended(Thread* self)
-    ACQUIRE_SHARED(*Locks::mutator_lock_) NO_THREAD_SAFETY_ANALYSIS {
-  bool success = Locks::mutator_lock_->SharedTryLock(self, /*check=*/false);
-  CHECK(success);
-}
-
 // A checkpoint/suspend-all hybrid to switch thread roots from
 // from-space to to-space refs. Used to synchronize threads at a point
 // to mark the initiation of marking while maintaining the to-space
 // invariant.
-void ThreadList::FlipThreadRoots(Closure* thread_flip_visitor,
-                                 Closure* flip_callback,
-                                 gc::collector::GarbageCollector* collector,
-                                 gc::GcPauseListener* pause_listener) {
+size_t ThreadList::FlipThreadRoots(Closure* thread_flip_visitor,
+                                   Closure* flip_callback,
+                                   gc::collector::GarbageCollector* collector,
+                                   gc::GcPauseListener* pause_listener) {
   TimingLogger::ScopedTiming split("ThreadListFlip", collector->GetTimings());
   Thread* self = Thread::Current();
   Locks::mutator_lock_->AssertNotHeld(self);
   Locks::thread_list_lock_->AssertNotHeld(self);
   Locks::thread_suspend_count_lock_->AssertNotHeld(self);
   CHECK_NE(self->GetState(), ThreadState::kRunnable);
+  size_t runnable_thread_count = 0;
+  std::vector<Thread*> other_threads;
 
   collector->GetHeap()->ThreadFlipBegin(self);  // Sync with JNI critical calls.
 
   // ThreadFlipBegin happens before we suspend all the threads, so it does not
   // count towards the pause.
   const uint64_t suspend_start_time = NanoTime();
-  VLOG(threads) << "Suspending all for thread flip";
-  SuspendAllInternal(self);
+  SuspendAllInternal(self, self, nullptr);
   if (pause_listener != nullptr) {
     pause_listener->StartPause();
   }
 
   // Run the flip callback for the collector.
   Locks::mutator_lock_->ExclusiveLock(self);
-  suspend_all_histogram_.AdjustAndAddValue(NanoTime() - suspend_start_time);
+  suspend_all_historam_.AdjustAndAddValue(NanoTime() - suspend_start_time);
   flip_callback->Run(self);
+  // Releasing mutator-lock *before* setting up flip function in the threads
+  // leaves a gap for another thread trying to suspend all threads. That thread
+  // gets to run with mutator-lock, thereby accessing the heap, without running
+  // its flip function. It's not a problem with CC as the gc-thread hasn't
+  // started marking yet and the from-space is accessible. By delaying releasing
+  // mutator-lock until after the flip function are running on all threads we
+  // fix that without increasing pause time, except for any thread that might be
+  // trying to suspend all. Even though the change works irrespective of the GC,
+  // it has been limited to userfaultfd GC to keep the change behind the flag.
+  //
+  // TODO: It's a temporary change as aosp/2377951 is going to clean-up at a
+  // broad scale, including not allowing concurrent suspend-all.
 
-  std::vector<Thread*> flipping_threads;  // All suspended threads. Includes us.
-  int thread_count;
-  // Flipping threads might exit between the time we resume them and try to run the flip function.
-  // Track that in a parallel vector.
-  std::unique_ptr<ThreadExitFlag[]> exit_flags;
+  // Resume runnable threads.
   {
     TimingLogger::ScopedTiming split2("ResumeRunnableThreads", collector->GetTimings());
     MutexLock mu(self, *Locks::thread_list_lock_);
     MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
-    thread_count = list_.size();
-    exit_flags.reset(new ThreadExitFlag[thread_count]);
-    flipping_threads.resize(thread_count, nullptr);
-    int i = 1;
+    --suspend_all_count_;
     for (Thread* thread : list_) {
       // Set the flip function for all threads because once we start resuming any threads,
       // they may need to run the flip function on behalf of other threads, even this one.
-      DCHECK(thread == self || thread->IsSuspended());
       thread->SetFlipFunction(thread_flip_visitor);
-      // Put ourselves first, so other threads are more likely to have finished before we get
-      // there.
-      int thread_index = thread == self ? 0 : i++;
-      flipping_threads[thread_index] = thread;
-      thread->NotifyOnThreadExit(&exit_flags[thread_index]);
+      if (thread == self) {
+        continue;
+      }
+      // Resume early the threads that were runnable but are suspended just for this thread flip or
+      // about to transition from non-runnable (eg. kNative at the SOA entry in a JNI function) to
+      // runnable (both cases waiting inside Thread::TransitionFromSuspendedToRunnable), or waiting
+      // for the thread flip to end at the JNI critical section entry (kWaitingForGcThreadFlip),
+      ThreadState state = thread->GetState();
+      if ((state == ThreadState::kWaitingForGcThreadFlip || thread->IsTransitioningToRunnable()) &&
+          thread->GetSuspendCount() == 1) {
+        // The thread will resume right after the broadcast.
+        bool updated = thread->ModifySuspendCount(self, -1, nullptr, SuspendReason::kInternal);
+        DCHECK(updated);
+        ++runnable_thread_count;
+      } else {
+        other_threads.push_back(thread);
+      }
     }
-    DCHECK(i == thread_count);
-  }
-
-  if (pause_listener != nullptr) {
-    pause_listener->EndPause();
-  }
-  // Any new threads created after this will be created by threads that already ran their flip
-  // functions. In the normal GC use case in which the flip function converts all local references
-  // to to-space references, these newly created threads will also see only to-space references.
-
-  // Resume threads, making sure that we do not release suspend_count_lock_ until we've reacquired
-  // the mutator_lock_ in shared mode, and decremented suspend_all_count_.  This avoids a
-  // concurrent SuspendAll, and ensures that newly started threads see a correct value of
-  // suspend_all_count.
-  {
-    MutexLock mu(self, *Locks::thread_list_lock_);
-    Locks::thread_suspend_count_lock_->Lock(self);
-    ResumeAllInternal(self);
+    Thread::resume_cond_->Broadcast(self);
   }
 
   collector->RegisterPause(NanoTime() - suspend_start_time);
-
-  // Since all threads were suspended, they will attempt to run the flip function before
-  // reentering a runnable state. We will also attempt to run the flip functions ourselves.  Any
-  // intervening checkpoint request will do the same.  Exactly one of those flip function attempts
-  // will succeed, and the target thread will not be able to reenter a runnable state until one of
-  // them does.
-
-  // Try to run the closure on the other threads.
-  TimingLogger::ScopedTiming split3("RunningThreadFlips", collector->GetTimings());
-  // Reacquire the mutator lock while holding suspend_count_lock. This cannot fail, since we
-  // do not acquire the mutator lock unless suspend_all_count was read as 0 while holding
-  // suspend_count_lock. We did not release suspend_count_lock since releasing the mutator
-  // lock.
-  AcquireMutatorLockSharedUncontended(self);
-
-  Locks::thread_suspend_count_lock_->Unlock(self);
-  // Concurrent SuspendAll may now see zero suspend_all_count_, but block on mutator_lock_.
-
+  if (pause_listener != nullptr) {
+    pause_listener->EndPause();
+  }
   collector->GetHeap()->ThreadFlipEnd(self);
 
-  for (int i = 0; i < thread_count; ++i) {
-    bool finished;
-    Thread::EnsureFlipFunctionStarted(
-        self, flipping_threads[i], Thread::StateAndFlags(0), &exit_flags[i], &finished);
-    if (finished) {
-      MutexLock mu2(self, *Locks::thread_list_lock_);
-      flipping_threads[i]->UnregisterThreadExitFlag(&exit_flags[i]);
-      flipping_threads[i] = nullptr;
+  // Try to run the closure on the other threads.
+  {
+    TimingLogger::ScopedTiming split3("FlipOtherThreads", collector->GetTimings());
+    for (Thread* thread : other_threads) {
+      thread->EnsureFlipFunctionStarted(self);
+      DCHECK(!thread->ReadFlag(ThreadFlag::kPendingFlipFunction));
     }
-  }
-  // Make sure all flips complete before we return.
-  for (int i = 0; i < thread_count; ++i) {
-    if (UNLIKELY(flipping_threads[i] != nullptr)) {
-      flipping_threads[i]->WaitForFlipFunctionTestingExited(self, &exit_flags[i]);
-      MutexLock mu2(self, *Locks::thread_list_lock_);
-      flipping_threads[i]->UnregisterThreadExitFlag(&exit_flags[i]);
-    }
+    // Try to run the flip function for self.
+    self->EnsureFlipFunctionStarted(self);
+    DCHECK(!self->ReadFlag(ThreadFlag::kPendingFlipFunction));
   }
 
-  Thread::DCheckUnregisteredEverywhere(&exit_flags[0], &exit_flags[thread_count - 1]);
+  Locks::mutator_lock_->ExclusiveUnlock(self);
 
-  Locks::mutator_lock_->SharedUnlock(self);
-}
-
-bool ThreadList::WaitForSuspendBarrier(AtomicInteger* barrier) {
-#if ART_USE_FUTEXES
-  timespec wait_timeout;
-  InitTimeSpec(false, CLOCK_MONOTONIC, NsToMs(thread_suspend_timeout_ns_), 0, &wait_timeout);
-#endif
-  while (true) {
-    int32_t cur_val = barrier->load(std::memory_order_acquire);
-    if (cur_val <= 0) {
-      CHECK_EQ(cur_val, 0);
-      return true;
+  // Resume other threads.
+  {
+    TimingLogger::ScopedTiming split4("ResumeOtherThreads", collector->GetTimings());
+    MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
+    for (const auto& thread : other_threads) {
+      bool updated = thread->ModifySuspendCount(self, -1, nullptr, SuspendReason::kInternal);
+      DCHECK(updated);
     }
-#if ART_USE_FUTEXES
-    if (futex(barrier->Address(), FUTEX_WAIT_PRIVATE, cur_val, &wait_timeout, nullptr, 0) != 0) {
-      if (errno == ETIMEDOUT) {
-        return false;
-      } else if (errno != EAGAIN && errno != EINTR) {
-        PLOG(FATAL) << "futex wait for suspend barrier failed";
-      }
-    }
-#endif
-    // Else spin wait. This is likely to be slow, but ART_USE_FUTEXES is set on Linux,
-    // including all targets.
+    Thread::resume_cond_->Broadcast(self);
   }
+
+  return runnable_thread_count + other_threads.size() + 1;  // +1 for self.
 }
 
 void ThreadList::SuspendAll(const char* cause, bool long_suspend) {
@@ -691,7 +661,7 @@
     ScopedTrace trace("Suspending mutator threads");
     const uint64_t start_time = NanoTime();
 
-    SuspendAllInternal(self);
+    SuspendAllInternal(self, self);
     // All threads are known to have suspended (but a thread may still own the mutator lock)
     // Make sure this thread grabs exclusive access to the mutator lock and its protected data.
 #if HAVE_TIMED_RWLOCK
@@ -715,21 +685,16 @@
 
     const uint64_t end_time = NanoTime();
     const uint64_t suspend_time = end_time - start_time;
-    suspend_all_histogram_.AdjustAndAddValue(suspend_time);
+    suspend_all_historam_.AdjustAndAddValue(suspend_time);
     if (suspend_time > kLongThreadSuspendThreshold) {
       LOG(WARNING) << "Suspending all threads took: " << PrettyDuration(suspend_time);
     }
 
     if (kDebugLocking) {
       // Debug check that all threads are suspended.
-      AssertOtherThreadsAreSuspended(self);
+      AssertThreadsAreSuspended(self, self);
     }
   }
-
-  // SuspendAllInternal blocks if we are in the middle of a flip.
-  DCHECK(!self->ReadFlag(ThreadFlag::kPendingFlipFunction));
-  DCHECK(!self->ReadFlag(ThreadFlag::kRunningFlipFunction));
-
   ATraceBegin((std::string("Mutator threads suspended for ") + cause).c_str());
 
   if (self != nullptr) {
@@ -740,9 +705,10 @@
 }
 
 // Ensures all threads running Java suspend and that those not running Java don't start.
-void ThreadList::SuspendAllInternal(Thread* self, SuspendReason reason) {
-  // self can be nullptr if this is an unregistered thread.
-  const uint64_t start_time = NanoTime();
+void ThreadList::SuspendAllInternal(Thread* self,
+                                    Thread* ignore1,
+                                    Thread* ignore2,
+                                    SuspendReason reason) {
   Locks::mutator_lock_->AssertNotExclusiveHeld(self);
   Locks::thread_list_lock_->AssertNotHeld(self);
   Locks::thread_suspend_count_lock_->AssertNotHeld(self);
@@ -760,97 +726,91 @@
 
   // The atomic counter for number of threads that need to pass the barrier.
   AtomicInteger pending_threads;
-
-  for (int iter_count = 1;; ++iter_count) {
-    {
-      MutexLock mu(self, *Locks::thread_list_lock_);
-      MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
-      if (suspend_all_count_ == 0) {
-        // Never run multiple SuspendAlls concurrently.
-        // If we are asked to suspend ourselves, we proceed anyway, but must ignore suspend
-        // request from other threads until we resume them.
-        bool found_myself = false;
-        // Update global suspend all state for attaching threads.
-        ++suspend_all_count_;
-        pending_threads.store(list_.size() - (self == nullptr ? 0 : 1), std::memory_order_relaxed);
-        // Increment everybody else's suspend count.
-        for (const auto& thread : list_) {
-          if (thread == self) {
-            found_myself = true;
-          } else {
-            VLOG(threads) << "requesting thread suspend: " << *thread;
-            DCHECK_EQ(suspend_all_count_, 1);
-            thread->IncrementSuspendCount(self, &pending_threads, nullptr, reason);
-            if (thread->IsSuspended()) {
-              // Effectively pass the barrier on behalf of the already suspended thread.
-              // The thread itself cannot yet have acted on our request since we still hold the
-              // suspend_count_lock_, and it will notice that kActiveSuspendBarrier has already
-              // been cleared if and when it acquires the lock in PassActiveSuspendBarriers().
-              DCHECK_EQ(thread->tlsPtr_.active_suspendall_barrier, &pending_threads);
-              pending_threads.fetch_sub(1, std::memory_order_seq_cst);
-              thread->tlsPtr_.active_suspendall_barrier = nullptr;
-              if (!thread->HasActiveSuspendBarrier()) {
-                thread->AtomicClearFlag(ThreadFlag::kActiveSuspendBarrier);
-              }
-            }
-            // else:
-            // The target thread was not yet suspended, and hence will be forced to execute
-            // TransitionFromRunnableToSuspended shortly. Since we set the kSuspendRequest flag
-            // before checking, and it checks kActiveSuspendBarrier after noticing kSuspendRequest,
-            // it must notice kActiveSuspendBarrier when it does. Thus it is guaranteed to
-            // decrement the suspend barrier. We're relying on store; load ordering here, but
-            // that's not a problem, since state and flags all reside in the same atomic, and
-            // are thus properly ordered, even for relaxed accesses.
-          }
-        }
-        self->AtomicSetFlag(ThreadFlag::kSuspensionImmune, std::memory_order_relaxed);
-        DCHECK(self == nullptr || found_myself);
-        break;
-      }
-    }
-    if (iter_count >= kMaxSuspendRetries) {
-      LOG(FATAL) << "Too many SuspendAll retries: " << iter_count;
-    } else {
-      MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
-      DCHECK_LE(suspend_all_count_, 1);
-      if (suspend_all_count_ != 0) {
-        // This may take a while, and we're not runnable, and thus would otherwise not block.
-        Thread::resume_cond_->WaitHoldingLocks(self);
-        continue;
-      }
-    }
-    // We're already not runnable, so an attempt to suspend us should succeed.
+  uint32_t num_ignored = 0;
+  if (ignore1 != nullptr) {
+    ++num_ignored;
   }
-
-  if (!WaitForSuspendBarrier(&pending_threads)) {
-    const uint64_t wait_time = NanoTime() - start_time;
+  if (ignore2 != nullptr && ignore1 != ignore2) {
+    ++num_ignored;
+  }
+  {
     MutexLock mu(self, *Locks::thread_list_lock_);
     MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
-    std::ostringstream oss;
+    // Update global suspend all state for attaching threads.
+    ++suspend_all_count_;
+    pending_threads.store(list_.size() - num_ignored, std::memory_order_relaxed);
+    // Increment everybody's suspend count (except those that should be ignored).
     for (const auto& thread : list_) {
-      if (thread != self && !thread->IsSuspended()) {
-        oss << std::endl << "Thread not suspended: " << *thread;
+      if (thread == ignore1 || thread == ignore2) {
+        continue;
+      }
+      VLOG(threads) << "requesting thread suspend: " << *thread;
+      bool updated = thread->ModifySuspendCount(self, +1, &pending_threads, reason);
+      DCHECK(updated);
+
+      // Must install the pending_threads counter first, then check thread->IsSuspend() and clear
+      // the counter. Otherwise there's a race with Thread::TransitionFromRunnableToSuspended()
+      // that can lead a thread to miss a call to PassActiveSuspendBarriers().
+      if (thread->IsSuspended()) {
+        // Only clear the counter for the current thread.
+        thread->ClearSuspendBarrier(&pending_threads);
+        pending_threads.fetch_sub(1, std::memory_order_seq_cst);
       }
     }
-    LOG(::android::base::FATAL) << "Timed out waiting for threads to suspend, waited for "
-                                << PrettyDuration(wait_time) << oss.str();
+  }
+
+  // Wait for the barrier to be passed by all runnable threads. This wait
+  // is done with a timeout so that we can detect problems.
+#if ART_USE_FUTEXES
+  timespec wait_timeout;
+  InitTimeSpec(false, CLOCK_MONOTONIC, NsToMs(thread_suspend_timeout_ns_), 0, &wait_timeout);
+#endif
+  const uint64_t start_time = NanoTime();
+  while (true) {
+    int32_t cur_val = pending_threads.load(std::memory_order_relaxed);
+    if (LIKELY(cur_val > 0)) {
+#if ART_USE_FUTEXES
+      if (futex(pending_threads.Address(), FUTEX_WAIT_PRIVATE, cur_val, &wait_timeout, nullptr, 0)
+          != 0) {
+        if ((errno == EAGAIN) || (errno == EINTR)) {
+          // EAGAIN and EINTR both indicate a spurious failure, try again from the beginning.
+          continue;
+        }
+        if (errno == ETIMEDOUT) {
+          const uint64_t wait_time = NanoTime() - start_time;
+          MutexLock mu(self, *Locks::thread_list_lock_);
+          MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
+          std::ostringstream oss;
+          for (const auto& thread : list_) {
+            if (thread == ignore1 || thread == ignore2) {
+              continue;
+            }
+            if (!thread->IsSuspended()) {
+              oss << std::endl << "Thread not suspended: " << *thread;
+            }
+          }
+          LOG(kIsDebugBuild ? ::android::base::FATAL : ::android::base::ERROR)
+              << "Timed out waiting for threads to suspend, waited for "
+              << PrettyDuration(wait_time)
+              << oss.str();
+        } else {
+          PLOG(FATAL) << "futex wait failed for SuspendAllInternal()";
+        }
+      }  // else re-check pending_threads in the next iteration (this may be a spurious wake-up).
+#else
+      // Spin wait. This is likely to be slow, but on most architecture ART_USE_FUTEXES is set.
+      UNUSED(start_time);
+#endif
+    } else {
+      CHECK_EQ(cur_val, 0);
+      break;
+    }
   }
 }
 
 void ThreadList::ResumeAll() {
   Thread* self = Thread::Current();
-  if (kDebugLocking) {
-    // Debug check that all threads are suspended.
-    AssertOtherThreadsAreSuspended(self);
-  }
-  MutexLock mu(self, *Locks::thread_list_lock_);
-  MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
-  ResumeAllInternal(self);
-}
 
-// Holds thread_list_lock_ and suspend_count_lock_
-void ThreadList::ResumeAllInternal(Thread* self) {
-  DCHECK_NE(self->GetState(), ThreadState::kRunnable);
   if (self != nullptr) {
     VLOG(threads) << *self << " ResumeAll starting";
   } else {
@@ -861,31 +821,37 @@
 
   ScopedTrace trace("Resuming mutator threads");
 
+  if (kDebugLocking) {
+    // Debug check that all threads are suspended.
+    AssertThreadsAreSuspended(self, self);
+  }
+
   long_suspend_ = false;
 
   Locks::mutator_lock_->ExclusiveUnlock(self);
-
-  // Decrement the suspend counts for all threads.
-  for (const auto& thread : list_) {
-    if (thread != self) {
-      thread->DecrementSuspendCount(self);
+  {
+    MutexLock mu(self, *Locks::thread_list_lock_);
+    MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
+    // Update global suspend all state for attaching threads.
+    --suspend_all_count_;
+    // Decrement the suspend counts for all threads.
+    for (const auto& thread : list_) {
+      if (thread == self) {
+        continue;
+      }
+      bool updated = thread->ModifySuspendCount(self, -1, nullptr, SuspendReason::kInternal);
+      DCHECK(updated);
     }
-  }
 
-  // Update global suspend all state for attaching threads. Unblocks other SuspendAlls once
-  // suspend_count_lock_ is released.
-  --suspend_all_count_;
-  self->AtomicClearFlag(ThreadFlag::kSuspensionImmune, std::memory_order_relaxed);
-  // Pending suspend requests for us will be handled when we become Runnable again.
-
-  // Broadcast a notification to all suspended threads, some or all of
-  // which may choose to wake up.  No need to wait for them.
-  if (self != nullptr) {
-    VLOG(threads) << *self << " ResumeAll waking others";
-  } else {
-    VLOG(threads) << "Thread[null] ResumeAll waking others";
+    // Broadcast a notification to all suspended threads, some or all of
+    // which may choose to wake up.  No need to wait for them.
+    if (self != nullptr) {
+      VLOG(threads) << *self << " ResumeAll waking others";
+    } else {
+      VLOG(threads) << "Thread[null] ResumeAll waking others";
+    }
+    Thread::resume_cond_->Broadcast(self);
   }
-  Thread::resume_cond_->Broadcast(self);
 
   if (self != nullptr) {
     VLOG(threads) << *self << " ResumeAll complete";
@@ -919,7 +885,11 @@
           << ") thread not within thread list";
       return false;
     }
-    thread->DecrementSuspendCount(self, /*for_user_code=*/(reason == SuspendReason::kForUserCode));
+    if (UNLIKELY(!thread->ModifySuspendCount(self, -1, nullptr, reason))) {
+      LOG(ERROR) << "Resume(" << reinterpret_cast<void*>(thread)
+                 << ") could not modify suspend count.";
+      return false;
+    }
   }
 
   {
@@ -945,19 +915,40 @@
   }
 }
 
-Thread* ThreadList::SuspendThreadByPeer(jobject peer, SuspendReason reason) {
-  bool is_suspended = false;
+Thread* ThreadList::SuspendThreadByPeer(jobject peer,
+                                        SuspendReason reason,
+                                        bool* timed_out) {
+  bool request_suspension = true;
+  const uint64_t start_time = NanoTime();
+  int self_suspend_count = 0;
+  useconds_t sleep_us = kThreadSuspendInitialSleepUs;
+  *timed_out = false;
   Thread* const self = Thread::Current();
+  Thread* suspended_thread = nullptr;
   VLOG(threads) << "SuspendThreadByPeer starting";
-  Thread* thread;
-  WrappedSuspend1Barrier wrapped_barrier{};
-  for (int iter_count = 1;; ++iter_count) {
+  while (true) {
+    Thread* thread;
     {
-      // Note: this will transition to runnable and potentially suspend.
+      // Note: this will transition to runnable and potentially suspend. We ensure only one thread
+      // is requesting another suspend, to avoid deadlock, by requiring this function be called
+      // holding Locks::thread_list_suspend_thread_lock_. Its important this thread suspend rather
+      // than request thread suspension, to avoid potential cycles in threads requesting each other
+      // suspend.
       ScopedObjectAccess soa(self);
       MutexLock thread_list_mu(self, *Locks::thread_list_lock_);
       thread = Thread::FromManagedThread(soa, peer);
       if (thread == nullptr) {
+        if (suspended_thread != nullptr) {
+          MutexLock suspend_count_mu(self, *Locks::thread_suspend_count_lock_);
+          // If we incremented the suspend count but the thread reset its peer, we need to
+          // re-decrement it since it is shutting down and may deadlock the runtime in
+          // ThreadList::WaitForOtherNonDaemonThreadsToExit.
+          bool updated = suspended_thread->ModifySuspendCount(soa.Self(),
+                                                              -1,
+                                                              nullptr,
+                                                              reason);
+          DCHECK(updated);
+        }
         ThreadSuspendByPeerWarning(soa,
                                    ::android::base::WARNING,
                                     "No such thread for suspend",
@@ -965,64 +956,84 @@
         return nullptr;
       }
       if (!Contains(thread)) {
+        CHECK(suspended_thread == nullptr);
         VLOG(threads) << "SuspendThreadByPeer failed for unattached thread: "
             << reinterpret_cast<void*>(thread);
         return nullptr;
       }
-      // IsSuspended on the current thread will fail as the current thread is changed into
-      // Runnable above. As the suspend count is now raised if this is the current thread
-      // it will self suspend on transition to Runnable, making it hard to work with. It's simpler
-      // to just explicitly handle the current thread in the callers to this code.
-      CHECK_NE(thread, self) << "Attempt to suspend the current thread for the debugger";
       VLOG(threads) << "SuspendThreadByPeer found thread: " << *thread;
       {
         MutexLock suspend_count_mu(self, *Locks::thread_suspend_count_lock_);
-        if (LIKELY(self->GetSuspendCount() == 0)) {
-          thread->IncrementSuspendCount(self, nullptr, &wrapped_barrier, reason);
-          if (thread->IsSuspended()) {
-            // See the discussion in mutator_gc_coord.md and SuspendAllInternal for the race here.
-            thread->RemoveFirstSuspend1Barrier();
-            if (!thread->HasActiveSuspendBarrier()) {
-              thread->AtomicClearFlag(ThreadFlag::kActiveSuspendBarrier);
-            }
-            is_suspended = true;
+        if (request_suspension) {
+          if (self->GetSuspendCount() > 0) {
+            // We hold the suspend count lock but another thread is trying to suspend us. Its not
+            // safe to try to suspend another thread in case we get a cycle. Start the loop again
+            // which will allow this thread to be suspended.
+            ++self_suspend_count;
+            continue;
           }
-          DCHECK_GT(thread->GetSuspendCount(), 0);
-          break;
+          CHECK(suspended_thread == nullptr);
+          suspended_thread = thread;
+          bool updated = suspended_thread->ModifySuspendCount(self, +1, nullptr, reason);
+          DCHECK(updated);
+          request_suspension = false;
+        } else {
+          // If the caller isn't requesting suspension, a suspension should have already occurred.
+          CHECK_GT(thread->GetSuspendCount(), 0);
         }
-        // Else we hold the suspend count lock but another thread is trying to suspend us,
-        // making it unsafe to try to suspend another thread in case we get a cycle.
-        // We start the loop again, which will allow this thread to be suspended.
+        // IsSuspended on the current thread will fail as the current thread is changed into
+        // Runnable above. As the suspend count is now raised if this is the current thread
+        // it will self suspend on transition to Runnable, making it hard to work with. It's simpler
+        // to just explicitly handle the current thread in the callers to this code.
+        CHECK_NE(thread, self) << "Attempt to suspend the current thread for the debugger";
+        // If thread is suspended (perhaps it was already not Runnable but didn't have a suspend
+        // count, or else we've waited and it has self suspended) or is the current thread, we're
+        // done.
+        if (thread->IsSuspended()) {
+          VLOG(threads) << "SuspendThreadByPeer thread suspended: " << *thread;
+          if (ATraceEnabled()) {
+            std::string name;
+            thread->GetThreadName(name);
+            ATraceBegin(StringPrintf("SuspendThreadByPeer suspended %s for peer=%p", name.c_str(),
+                                      peer).c_str());
+          }
+          return thread;
+        }
+        const uint64_t total_delay = NanoTime() - start_time;
+        if (total_delay >= thread_suspend_timeout_ns_) {
+          if (suspended_thread == nullptr) {
+            ThreadSuspendByPeerWarning(soa,
+                                       ::android::base::FATAL,
+                                       "Failed to issue suspend request",
+                                       peer);
+          } else {
+            CHECK_EQ(suspended_thread, thread);
+            LOG(WARNING) << "Suspended thread state_and_flags: "
+                         << suspended_thread->StateAndFlagsAsHexString()
+                         << ", self_suspend_count = " << self_suspend_count;
+            // Explicitly release thread_suspend_count_lock_; we haven't held it for long, so
+            // seeing threads blocked on it is not informative.
+            Locks::thread_suspend_count_lock_->Unlock(self);
+            ThreadSuspendByPeerWarning(soa,
+                                       ::android::base::FATAL,
+                                       "Thread suspension timed out",
+                                       peer);
+          }
+          UNREACHABLE();
+        } else if (sleep_us == 0 &&
+            total_delay > static_cast<uint64_t>(kThreadSuspendMaxYieldUs) * 1000) {
+          // We have spun for kThreadSuspendMaxYieldUs time, switch to sleeps to prevent
+          // excessive CPU usage.
+          sleep_us = kThreadSuspendMaxYieldUs / 2;
+        }
       }
+      // Release locks and come out of runnable state.
     }
-    // All locks are released, and we should quickly exit the suspend-unfriendly state. Retry.
-    if (iter_count >= kMaxSuspendRetries) {
-      LOG(FATAL) << "Too many suspend retries";
-    }
-    usleep(kThreadSuspendSleepUs);
-  }
-  // Now wait for target to decrement suspend barrier.
-  if (is_suspended || WaitForSuspendBarrier(&wrapped_barrier.barrier_)) {
-    // wrapped_barrier.barrier_ has been decremented and will no longer be accessed.
-    VLOG(threads) << "SuspendThreadByPeer thread suspended: " << *thread;
-    if (ATraceEnabled()) {
-      std::string name;
-      thread->GetThreadName(name);
-      ATraceBegin(
-          StringPrintf("SuspendThreadByPeer suspended %s for peer=%p", name.c_str(), peer).c_str());
-    }
-    DCHECK(thread->IsSuspended());
-    return thread;
-  } else {
-    LOG(WARNING) << "Suspended thread state_and_flags: " << thread->StateAndFlagsAsHexString();
-    // thread still has a pointer to wrapped_barrier. Returning and continuing would be unsafe
-    // without additional cleanup.
-    {
-      ScopedObjectAccess soa(self);
-      ThreadSuspendByPeerWarning(
-          soa, ::android::base::FATAL, "SuspendThreadByPeer timed out", peer);
-    }
-    UNREACHABLE();
+    VLOG(threads) << "SuspendThreadByPeer waiting to allow thread chance to suspend";
+    ThreadSuspendSleep(sleep_us);
+    // This may stay at 0 if sleep_us == 0, but this is WAI since we want to avoid using usleep at
+    // all if possible. This shouldn't be an issue since time to suspend should always be small.
+    sleep_us = std::min(sleep_us * 2, kThreadSuspendMaxSleepUs);
   }
 }
 
@@ -1032,74 +1043,101 @@
   LOG(severity) << StringPrintf("%s: %d", message, thread_id);
 }
 
-Thread* ThreadList::SuspendThreadByThreadId(uint32_t thread_id, SuspendReason reason) {
-  bool is_suspended = false;
+Thread* ThreadList::SuspendThreadByThreadId(uint32_t thread_id,
+                                            SuspendReason reason,
+                                            bool* timed_out) {
+  const uint64_t start_time = NanoTime();
+  useconds_t sleep_us = kThreadSuspendInitialSleepUs;
+  *timed_out = false;
+  Thread* suspended_thread = nullptr;
   Thread* const self = Thread::Current();
   CHECK_NE(thread_id, kInvalidThreadId);
   VLOG(threads) << "SuspendThreadByThreadId starting";
-  Thread* thread;
-  WrappedSuspend1Barrier wrapped_barrier{};
-  for (int iter_count = 1;; ++iter_count) {
+  while (true) {
     {
-      // Note: this will transition to runnable and potentially suspend.
+      // Note: this will transition to runnable and potentially suspend. We ensure only one thread
+      // is requesting another suspend, to avoid deadlock, by requiring this function be called
+      // holding Locks::thread_list_suspend_thread_lock_. Its important this thread suspend rather
+      // than request thread suspension, to avoid potential cycles in threads requesting each other
+      // suspend.
       ScopedObjectAccess soa(self);
       MutexLock thread_list_mu(self, *Locks::thread_list_lock_);
-      thread = FindThreadByThreadId(thread_id);
+      Thread* thread = nullptr;
+      for (const auto& it : list_) {
+        if (it->GetThreadId() == thread_id) {
+          thread = it;
+          break;
+        }
+      }
       if (thread == nullptr) {
+        CHECK(suspended_thread == nullptr) << "Suspended thread " << suspended_thread
+            << " no longer in thread list";
         // There's a race in inflating a lock and the owner giving up ownership and then dying.
         ThreadSuspendByThreadIdWarning(::android::base::WARNING,
                                        "No such thread id for suspend",
                                        thread_id);
         return nullptr;
       }
-      DCHECK(Contains(thread));
-      CHECK_NE(thread, self) << "Attempt to suspend the current thread for the debugger";
       VLOG(threads) << "SuspendThreadByThreadId found thread: " << *thread;
+      DCHECK(Contains(thread));
       {
         MutexLock suspend_count_mu(self, *Locks::thread_suspend_count_lock_);
-        if (LIKELY(self->GetSuspendCount() == 0)) {
-          thread->IncrementSuspendCount(self, nullptr, &wrapped_barrier, reason);
-          if (thread->IsSuspended()) {
-            // See the discussion in mutator_gc_coord.md and SuspendAllInternal for the race here.
-            thread->RemoveFirstSuspend1Barrier();
-            if (!thread->HasActiveSuspendBarrier()) {
-              thread->AtomicClearFlag(ThreadFlag::kActiveSuspendBarrier);
-            }
-            is_suspended = true;
+        if (suspended_thread == nullptr) {
+          if (self->GetSuspendCount() > 0) {
+            // We hold the suspend count lock but another thread is trying to suspend us. Its not
+            // safe to try to suspend another thread in case we get a cycle. Start the loop again
+            // which will allow this thread to be suspended.
+            continue;
           }
-          DCHECK_GT(thread->GetSuspendCount(), 0);
-          break;
+          bool updated = thread->ModifySuspendCount(self, +1, nullptr, reason);
+          DCHECK(updated);
+          suspended_thread = thread;
+        } else {
+          CHECK_EQ(suspended_thread, thread);
+          // If the caller isn't requesting suspension, a suspension should have already occurred.
+          CHECK_GT(thread->GetSuspendCount(), 0);
         }
-        // Else we hold the suspend count lock but another thread is trying to suspend us,
-        // making it unsafe to try to suspend another thread in case we get a cycle.
-        // Start the loop again, which will allow this thread to be suspended.
+        // IsSuspended on the current thread will fail as the current thread is changed into
+        // Runnable above. As the suspend count is now raised if this is the current thread
+        // it will self suspend on transition to Runnable, making it hard to work with. It's simpler
+        // to just explicitly handle the current thread in the callers to this code.
+        CHECK_NE(thread, self) << "Attempt to suspend the current thread for the debugger";
+        // If thread is suspended (perhaps it was already not Runnable but didn't have a suspend
+        // count, or else we've waited and it has self suspended) or is the current thread, we're
+        // done.
+        if (thread->IsSuspended()) {
+          if (ATraceEnabled()) {
+            std::string name;
+            thread->GetThreadName(name);
+            ATraceBegin(StringPrintf("SuspendThreadByThreadId suspended %s id=%d",
+                                      name.c_str(), thread_id).c_str());
+          }
+          VLOG(threads) << "SuspendThreadByThreadId thread suspended: " << *thread;
+          return thread;
+        }
+        const uint64_t total_delay = NanoTime() - start_time;
+        if (total_delay >= thread_suspend_timeout_ns_) {
+          ThreadSuspendByThreadIdWarning(::android::base::WARNING,
+                                         "Thread suspension timed out",
+                                         thread_id);
+          if (suspended_thread != nullptr) {
+            bool updated = thread->ModifySuspendCount(soa.Self(), -1, nullptr, reason);
+            DCHECK(updated);
+          }
+          *timed_out = true;
+          return nullptr;
+        } else if (sleep_us == 0 &&
+            total_delay > static_cast<uint64_t>(kThreadSuspendMaxYieldUs) * 1000) {
+          // We have spun for kThreadSuspendMaxYieldUs time, switch to sleeps to prevent
+          // excessive CPU usage.
+          sleep_us = kThreadSuspendMaxYieldUs / 2;
+        }
       }
+      // Release locks and come out of runnable state.
     }
-    // All locks are released, and we should quickly exit the suspend-unfriendly state. Retry.
-    if (iter_count >= kMaxSuspendRetries) {
-      LOG(FATAL) << "Too many suspend retries";
-    }
-    usleep(kThreadSuspendSleepUs);
-  }
-  // Now wait for target to decrement suspend barrier.
-  if (is_suspended || WaitForSuspendBarrier(&wrapped_barrier.barrier_)) {
-    // wrapped_barrier.barrier_ has been decremented and will no longer be accessed.
-    VLOG(threads) << "SuspendThreadByThreadId thread suspended: " << *thread;
-    if (ATraceEnabled()) {
-      std::string name;
-      thread->GetThreadName(name);
-      ATraceBegin(
-          StringPrintf("SuspendThreadByPeer suspended %s for id=%d", name.c_str(), thread_id)
-              .c_str());
-    }
-    DCHECK(thread->IsSuspended());
-    return thread;
-  } else {
-    // thread still has a pointer to wrapped_barrier. Returning and continuing would be unsafe
-    // without additional cleanup.
-    ThreadSuspendByThreadIdWarning(
-        ::android::base::FATAL, "SuspendThreadByThreadId timed out", thread_id);
-    UNREACHABLE();
+    VLOG(threads) << "SuspendThreadByThreadId waiting to allow thread chance to suspend";
+    ThreadSuspendSleep(sleep_us);
+    sleep_us = std::min(sleep_us * 2, kThreadSuspendMaxSleepUs);
   }
 }
 
@@ -1175,7 +1213,8 @@
       // daemons.
       CHECK(thread->IsDaemon()) << *thread;
       if (thread != self) {
-        thread->IncrementSuspendCount(self);
+        bool updated = thread->ModifySuspendCount(self, +1, nullptr, SuspendReason::kInternal);
+        DCHECK(updated);
         ++daemons_left;
       }
       // We are shutting down the runtime, set the JNI functions of all the JNIEnvs to be
@@ -1275,10 +1314,11 @@
   // SuspendAll requests.
   MutexLock mu(self, *Locks::thread_list_lock_);
   MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
-  if (suspend_all_count_ == 1) {
-    self->IncrementSuspendCount(self);
-  } else {
-    DCHECK_EQ(suspend_all_count_, 0);
+  // Modify suspend count in increments of 1 to maintain invariants in ModifySuspendCount. While
+  // this isn't particularly efficient the suspend counts are most commonly 0 or 1.
+  for (int delta = suspend_all_count_; delta > 0; delta--) {
+    bool updated = self->ModifySuspendCount(self, +1, nullptr, SuspendReason::kInternal);
+    DCHECK(updated);
   }
   CHECK(!Contains(self));
   list_.push_back(self);
@@ -1329,15 +1369,12 @@
         self->GetThreadName(thread_name);
         std::ostringstream os;
         DumpNativeStack(os, GetTid(), "  native: ", nullptr);
-        LOG(FATAL) << "Request to unregister unattached thread " << thread_name << "\n" << os.str();
-        UNREACHABLE();
+        LOG(ERROR) << "Request to unregister unattached thread " << thread_name << "\n" << os.str();
+        break;
       } else {
         MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
-        Thread::StateAndFlags state_and_flags = self->GetStateAndFlags(std::memory_order_acquire);
-        if (!state_and_flags.IsFlagSet(ThreadFlag::kRunningFlipFunction) &&
-            !state_and_flags.IsFlagSet(ThreadFlag::kSuspendRequest)) {
+        if (!self->IsSuspended()) {
           list_.remove(self);
-          self->SignalExitFlags();
           break;
         }
       }
@@ -1345,7 +1382,7 @@
     // In the case where we are not suspended yet, sleep to leave other threads time to execute.
     // This is important if there are realtime threads. b/111277984
     usleep(1);
-    // We failed to remove the thread due to a suspend request or the like, loop and try again.
+    // We failed to remove the thread due to a suspend request, loop and try again.
   }
   delete self;
 
@@ -1392,11 +1429,13 @@
     MutexLock mu(self, *Locks::thread_list_lock_);
     MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
     for (Thread* thread : list_) {
-      thread->IncrementSuspendCount(self);
+      bool suspended = thread->ModifySuspendCount(self, +1, nullptr, SuspendReason::kInternal);
+      DCHECK(suspended);
       if (thread == self || thread->IsSuspended()) {
         threads_to_visit.push_back(thread);
       } else {
-        thread->DecrementSuspendCount(self);
+        bool resumed = thread->ModifySuspendCount(self, -1, nullptr, SuspendReason::kInternal);
+        DCHECK(resumed);
       }
     }
   }
@@ -1411,9 +1450,9 @@
   {
     MutexLock mu2(self, *Locks::thread_suspend_count_lock_);
     for (Thread* thread : threads_to_visit) {
-      thread->DecrementSuspendCount(self);
+      bool updated = thread->ModifySuspendCount(self, -1, nullptr, SuspendReason::kInternal);
+      DCHECK(updated);
     }
-    Thread::resume_cond_->Broadcast(self);
   }
 }
 
diff --git a/runtime/thread_list.h b/runtime/thread_list.h
index 55baed8..db06611 100644
--- a/runtime/thread_list.h
+++ b/runtime/thread_list.h
@@ -49,11 +49,7 @@
   static constexpr uint32_t kInvalidThreadId = 0;
   static constexpr uint32_t kMainThreadId = 1;
   static constexpr uint64_t kDefaultThreadSuspendTimeout =
-      kIsDebugBuild ? 2'000'000'000ull : 4'000'000'000ull;
-  // We fail more aggressively in debug builds to catch potential issues early.
-  // The number of times we may retry when we find ourselves in a suspend-unfriendly state.
-  static constexpr int kMaxSuspendRetries = kIsDebugBuild ? 500 : 5000;
-  static constexpr useconds_t kThreadSuspendSleepUs = 100;
+      kIsDebugBuild ? 50'000'000'000ull : 10'000'000'000ull;
 
   explicit ThreadList(uint64_t thread_suspend_timeout_ns);
   ~ThreadList();
@@ -74,7 +70,7 @@
   bool Resume(Thread* thread, SuspendReason reason = SuspendReason::kInternal)
       REQUIRES(!Locks::thread_suspend_count_lock_) WARN_UNUSED;
 
-  // Suspends all other threads and gets exclusive access to the mutator lock.
+  // Suspends all threads and gets exclusive access to the mutator lock.
   // If long_suspend is true, then other threads who try to suspend will never timeout.
   // long_suspend is currenly used for hprof since large heaps take a long time.
   void SuspendAll(const char* cause, bool long_suspend = false)
@@ -85,7 +81,10 @@
 
   // Suspend a thread using a peer, typically used by the debugger. Returns the thread on success,
   // else null. The peer is used to identify the thread to avoid races with the thread terminating.
-  Thread* SuspendThreadByPeer(jobject peer, SuspendReason reason)
+  // If the suspension times out then *timeout is set to true.
+  Thread* SuspendThreadByPeer(jobject peer,
+                              SuspendReason reason,
+                              bool* timed_out)
       REQUIRES(!Locks::mutator_lock_,
                !Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_);
@@ -93,8 +92,8 @@
   // Suspend a thread using its thread id, typically used by lock/monitor inflation. Returns the
   // thread on success else null. The thread id is used to identify the thread to avoid races with
   // the thread terminating. Note that as thread ids are recycled this may not suspend the expected
-  // thread, that may be terminating.
-  Thread* SuspendThreadByThreadId(uint32_t thread_id, SuspendReason reason)
+  // thread, that may be terminating. If the suspension times out then *timeout is set to true.
+  Thread* SuspendThreadByThreadId(uint32_t thread_id, SuspendReason reason, bool* timed_out)
       REQUIRES(!Locks::mutator_lock_,
                !Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_);
@@ -114,24 +113,11 @@
   // Running threads are not suspended but run the checkpoint inside of the suspend check. The
   // return value includes already suspended threads for b/24191051. Runs or requests the
   // callback, if non-null, inside the thread_list_lock critical section after determining the
-  // runnable/suspended states of the threads. Does not wait for completion of the checkpoint
-  // function in running threads. If the caller holds the mutator lock, then all instances of the
-  // checkpoint function are run with the mutator lock. If the caller does not hold the mutator
-  // lock (see mutator_gc_coord.md) then, since the checkpoint code may not acquire or release the
-  // mutator lock, the checkpoint will have no way to access Java data.
-  // TODO: Is it possible to just require the mutator lock here?
-  size_t RunCheckpoint(Closure* checkpoint_function,
-                       Closure* callback = nullptr,
-                       bool allow_lock_checking = true)
+  // runnable/suspended states of the threads. Does not wait for completion of the callbacks in
+  // running threads.
+  size_t RunCheckpoint(Closure* checkpoint_function, Closure* callback = nullptr)
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
 
-  // Convenience version of the above to disable lock checking inside Run function. Hopefully this
-  // and the third parameter above will eventually disappear.
-  size_t RunCheckpointUnchecked(Closure* checkpoint_function, Closure* callback = nullptr)
-      REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_) {
-    return RunCheckpoint(checkpoint_function, callback, false);
-  }
-
   // Run an empty checkpoint on threads. Wait until threads pass the next suspend point or are
   // suspended. This is used to ensure that the threads finish or aren't in the middle of an
   // in-flight mutator heap access (eg. a read barrier.) Runnable threads will respond by
@@ -140,17 +126,12 @@
   void RunEmptyCheckpoint()
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
 
-  // Used to flip thread roots from from-space refs to to-space refs. Used only by the concurrent
-  // moving collectors during a GC, and hence cannot be called from multiple threads concurrently.
-  //
-  // Briefly suspends all threads to atomically install a checkpoint-like thread_flip_visitor
-  // function to be run on each thread. Run flip_callback while threads are suspended.
-  // Thread_flip_visitors are run by each thread before it becomes runnable, or by us. We do not
-  // return until all thread_flip_visitors have been run.
-  void FlipThreadRoots(Closure* thread_flip_visitor,
-                       Closure* flip_callback,
-                       gc::collector::GarbageCollector* collector,
-                       gc::GcPauseListener* pause_listener)
+  // Flip thread roots from from-space refs to to-space refs. Used by
+  // the concurrent moving collectors.
+  size_t FlipThreadRoots(Closure* thread_flip_visitor,
+                         Closure* flip_callback,
+                         gc::collector::GarbageCollector* collector,
+                         gc::GcPauseListener* pause_listener)
       REQUIRES(!Locks::mutator_lock_,
                !Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_);
@@ -211,29 +192,26 @@
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_,
                !Locks::mutator_lock_);
 
-  // Wait for suspend barrier to reach zero. Return false on timeout.
-  bool WaitForSuspendBarrier(AtomicInteger* barrier);
-
  private:
   uint32_t AllocThreadId(Thread* self);
   void ReleaseThreadId(Thread* self, uint32_t id) REQUIRES(!Locks::allocated_thread_ids_lock_);
 
+  size_t RunCheckpoint(Closure* checkpoint_function, bool includeSuspended)
+      REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
+
   void DumpUnattachedThreads(std::ostream& os, bool dump_native_stack)
       REQUIRES(!Locks::thread_list_lock_);
 
   void SuspendAllDaemonThreadsForShutdown()
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
 
-  void ResumeAllInternal(Thread* self)
-      REQUIRES(Locks::thread_list_lock_, Locks::thread_suspend_count_lock_)
-          UNLOCK_FUNCTION(Locks::mutator_lock_);
+  void SuspendAllInternal(Thread* self,
+                          Thread* ignore1,
+                          Thread* ignore2 = nullptr,
+                          SuspendReason reason = SuspendReason::kInternal)
+      REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
 
-  void SuspendAllInternal(Thread* self, SuspendReason reason = SuspendReason::kInternal)
-      REQUIRES(!Locks::thread_list_lock_,
-               !Locks::thread_suspend_count_lock_,
-               !Locks::mutator_lock_);
-
-  void AssertOtherThreadsAreSuspended(Thread* self)
+  void AssertThreadsAreSuspended(Thread* self, Thread* ignore1, Thread* ignore2 = nullptr)
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
 
   std::bitset<kMaxThreadId> allocated_ids_ GUARDED_BY(Locks::allocated_thread_ids_lock_);
@@ -241,10 +219,7 @@
   // The actual list of all threads.
   std::list<Thread*> list_ GUARDED_BY(Locks::thread_list_lock_);
 
-  // Ongoing suspend all requests, used to ensure threads added to list_ respect SuspendAll, and
-  // to ensure that only one SuspendAll ot FlipThreadRoots call is active at a time.  The value is
-  // always either 0 or 1. Thread_suspend_count_lock must be held continuously while these two
-  // functions modify suspend counts of all other threads and modify suspend_all_count_ .
+  // Ongoing suspend all requests, used to ensure threads added to list_ respect SuspendAll.
   int suspend_all_count_ GUARDED_BY(Locks::thread_suspend_count_lock_);
 
   // Number of threads unregistering, ~ThreadList blocks until this hits 0.
@@ -252,7 +227,7 @@
 
   // Thread suspend time histogram. Only modified when all the threads are suspended, so guarding
   // by mutator lock ensures no thread can read when another thread is modifying it.
-  Histogram<uint64_t> suspend_all_histogram_ GUARDED_BY(Locks::mutator_lock_);
+  Histogram<uint64_t> suspend_all_historam_ GUARDED_BY(Locks::mutator_lock_);
 
   // Whether or not the current thread suspension is long.
   bool long_suspend_;
@@ -268,9 +243,6 @@
 
   friend class Thread;
 
-  friend class Mutex;
-  friend class BaseMutex;
-
   DISALLOW_COPY_AND_ASSIGN(ThreadList);
 };
 
diff --git a/runtime/thread_test.cc b/runtime/thread_test.cc
deleted file mode 100644
index c4577bc..0000000
--- a/runtime/thread_test.cc
+++ /dev/null
@@ -1,99 +0,0 @@
-/*
- * Copyright (C) 2023 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-#include "thread.h"
-
-#include "android-base/logging.h"
-#include "base/locks.h"
-#include "base/mutex.h"
-#include "common_runtime_test.h"
-#include "thread-current-inl.h"
-#include "thread-inl.h"
-
-namespace art {
-
-class ThreadTest : public CommonRuntimeTest {};
-
-// Ensure that basic list operations on ThreadExitFlags work. These are rarely
-// exercised in practice, since normally only one flag is registered at a time.
-
-TEST_F(ThreadTest, ThreadExitFlagTest) {
-  Thread* self = Thread::Current();
-  ThreadExitFlag tefs[3];
-  {
-    MutexLock mu(self, *Locks::thread_list_lock_);
-    self->NotifyOnThreadExit(&tefs[2]);
-    ASSERT_TRUE(self->IsRegistered(&tefs[2]));
-    ASSERT_FALSE(tefs[2].HasExited());
-    ASSERT_FALSE(self->IsRegistered(&tefs[1]));
-    self->NotifyOnThreadExit(&tefs[1]);
-    self->NotifyOnThreadExit(&tefs[0]);
-    ASSERT_TRUE(self->IsRegistered(&tefs[0]));
-    ASSERT_TRUE(self->IsRegistered(&tefs[1]));
-    ASSERT_TRUE(self->IsRegistered(&tefs[2]));
-    self->UnregisterThreadExitFlag(&tefs[1]);
-    ASSERT_TRUE(self->IsRegistered(&tefs[0]));
-    ASSERT_FALSE(self->IsRegistered(&tefs[1]));
-    ASSERT_TRUE(self->IsRegistered(&tefs[2]));
-    self->UnregisterThreadExitFlag(&tefs[2]);
-    ASSERT_TRUE(self->IsRegistered(&tefs[0]));
-    ASSERT_FALSE(self->IsRegistered(&tefs[1]));
-    ASSERT_FALSE(self->IsRegistered(&tefs[2]));
-  }
-  Thread::DCheckUnregisteredEverywhere(&tefs[1], &tefs[2]);
-  {
-    MutexLock mu(self, *Locks::thread_list_lock_);
-    self->UnregisterThreadExitFlag(&tefs[0]);
-    ASSERT_FALSE(self->IsRegistered(&tefs[0]));
-    ASSERT_FALSE(self->IsRegistered(&tefs[1]));
-    ASSERT_FALSE(self->IsRegistered(&tefs[2]));
-  }
-  Thread::DCheckUnregisteredEverywhere(&tefs[0], &tefs[2]);
-}
-
-TEST_F(ThreadTest, ThreadExitSignalTest) {
-  Thread* self = Thread::Current();
-  ThreadExitFlag tefs[3];
-  {
-    MutexLock mu(self, *Locks::thread_list_lock_);
-    self->NotifyOnThreadExit(&tefs[2]);
-    ASSERT_TRUE(self->IsRegistered(&tefs[2]));
-    ASSERT_FALSE(self->IsRegistered(&tefs[1]));
-    self->NotifyOnThreadExit(&tefs[1]);
-    ASSERT_TRUE(self->IsRegistered(&tefs[1]));
-    self->SignalExitFlags();
-    ASSERT_TRUE(tefs[1].HasExited());
-    ASSERT_TRUE(tefs[2].HasExited());
-  }
-  Thread::DCheckUnregisteredEverywhere(&tefs[1], &tefs[2]);
-  {
-    MutexLock mu(self, *Locks::thread_list_lock_);
-    self->NotifyOnThreadExit(&tefs[0]);
-    tefs[2].~ThreadExitFlag();  //  Destroy and reinitialize.
-    new (&tefs[2]) ThreadExitFlag();
-    self->NotifyOnThreadExit(&tefs[2]);
-    ASSERT_FALSE(tefs[0].HasExited());
-    ASSERT_TRUE(tefs[1].HasExited());
-    ASSERT_FALSE(tefs[2].HasExited());
-    self->SignalExitFlags();
-    ASSERT_TRUE(tefs[0].HasExited());
-    ASSERT_TRUE(tefs[1].HasExited());
-    ASSERT_TRUE(tefs[2].HasExited());
-  }
-  Thread::DCheckUnregisteredEverywhere(&tefs[0], &tefs[2]);
-}
-
-}  // namespace art
diff --git a/test/129-ThreadGetId/expected-stdout.txt b/test/129-ThreadGetId/expected-stdout.txt
index 4455320..aadf90d 100644
--- a/test/129-ThreadGetId/expected-stdout.txt
+++ b/test/129-ThreadGetId/expected-stdout.txt
@@ -1,8 +1,2 @@
-Thread finished
-Thread finished
-Thread finished
-Thread finished
-Thread finished
-All joined
 HeapTaskDaemon depth 0
 Finishing
diff --git a/test/129-ThreadGetId/src/Main.java b/test/129-ThreadGetId/src/Main.java
index 3b4b076..50e8c09 100644
--- a/test/129-ThreadGetId/src/Main.java
+++ b/test/129-ThreadGetId/src/Main.java
@@ -16,71 +16,23 @@
 
 import java.lang.reflect.Field;
 import java.util.Map;
-import java.util.concurrent.atomic.AtomicInteger;
 
 public class Main implements Runnable {
     static final int NUMBER_OF_THREADS = 5;
-    static volatile int ops_per_thread = 1000;
-    static AtomicInteger operations_completed = new AtomicInteger(0);
-    static int[] progress = new int[NUMBER_OF_THREADS];
-    static AtomicInteger totalStackFrames = new AtomicInteger(0);
-    static final boolean printStats = false;  // True causes test to fail.
-    int index;
-
-    Main(int i) {
-        index = i;
-    }
+    static final int TOTAL_OPERATIONS = 900;
 
     public static void main(String[] args) throws Exception {
         final Thread[] threads = new Thread[NUMBER_OF_THREADS];
-        Thread watchdog = new Thread() {
-            public void run() {
-                try {
-                    if (printStats) {
-                        System.out.println("ops_per_thread = " + ops_per_thread);
-                    }
-                    Thread.sleep(10_000);
-                    if (printStats) {
-                        System.out.println("Ops completed after 10 seconds: " +
-                                operations_completed.get());
-                    }
-                    if (operations_completed.get() < NUMBER_OF_THREADS * ops_per_thread / 2) {
-                        // We're in some sort of "go slow" mode, probably gcstress. Finish early.
-                        ops_per_thread /= 10;
-                    }
-                    if (printStats) {
-                        System.out.println("ops_per_thread = " + ops_per_thread);
-                    }
-                    Thread.sleep(200_000);
-                    System.out.print("Watchdog timed out: ");
-                    for (int i = 0; i < NUMBER_OF_THREADS; ++i) {
-                        System.out.print(progress[i] + ", ");
-                    }
-                    System.out.println("");
-                    System.err.println("Watchdog thread timed out");
-                    System.exit(1);
-                } catch (InterruptedException e) {}
-            }
-        };
-        watchdog.start();
-        long start_millis = System.currentTimeMillis();
         for (int t = 0; t < threads.length; t++) {
-            threads[t] = new Thread(new Main(t));
+            threads[t] = new Thread(new Main());
             threads[t].start();
         }
         for (Thread t : threads) {
             t.join();
         }
-        if (printStats) {
-            long elapsed_millis = System.currentTimeMillis() - start_millis;
-            System.out.println("Captured " + totalStackFrames + " stack frames in " +
-                    elapsed_millis + "msecs");
-        }
-        System.out.println("All joined");
         // Do this test after the other part to leave some time for the heap task daemon to start
         // up.
         test_getStackTraces();
-        watchdog.interrupt();
         System.out.println("Finishing");
     }
 
@@ -94,9 +46,9 @@
             Thread[] array = new Thread[activeCount];
             systemThreadGroup.enumerate(array);
             for (Thread thread : array) {
-                if (thread.getName().equals("HeapTaskDaemon") &&
-                    thread.getState() != Thread.State.NEW) {
-                    return thread;
+               if (thread.getName().equals("HeapTaskDaemon") &&
+                   thread.getState() != Thread.State.NEW) {
+                  return thread;
                 }
             }
             // Yield to eventually get the daemon started.
@@ -131,16 +83,12 @@
             if (thread.getId() <= 0) {
                 System.out.println("thread's ID is not positive: " + thread.getName());
             }
-            totalStackFrames.addAndGet(stMap.get(thread).length);
         }
     }
 
     public void run() {
-        for (int i = 1; i <= ops_per_thread; ++i) {
+        for (int i = 0; i < TOTAL_OPERATIONS; ++i) {
             test_getId();
-            operations_completed.addAndGet(1);
-            progress[index] = i;
         }
-        System.out.println("Thread finished");
     }
 }
diff --git a/test/2011-stack-walk-concurrent-instrument/stack_walk_concurrent.cc b/test/2011-stack-walk-concurrent-instrument/stack_walk_concurrent.cc
index 9ae1bed..ae1d830 100644
--- a/test/2011-stack-walk-concurrent-instrument/stack_walk_concurrent.cc
+++ b/test/2011-stack-walk-concurrent-instrument/stack_walk_concurrent.cc
@@ -81,8 +81,10 @@
                                                                    jobject target) {
   while (!instrument_waiting) {
   }
+  bool timed_out = false;
   Thread* other = Runtime::Current()->GetThreadList()->SuspendThreadByPeer(
-      target, SuspendReason::kInternal);
+      target, SuspendReason::kInternal, &timed_out);
+  CHECK(!timed_out);
   CHECK(other != nullptr);
   ScopedSuspendAll ssa(__FUNCTION__);
   Runtime::Current()->GetInstrumentation()->InstrumentThreadStack(other,