Remove race on cached thread-name deletion

Manage the thread name using an RCU-like scheme, in which the name
is an atomic pointer to an immutable string, and readers explicitly
maintain a count of active readers. Writers wait until there are
no readers before deallocating a name that was just replaced.

Currently this potentially blocks writers, but readers should be very
fast, and relatively infrequent, so I can't imagine this is a real
issue.

This also replaces JVM_SetNativeThreadName with a stub. This part of
the CL came from Nicolas' aosp/1706702, which this is otherwise
intended to replace.

Test: Build and boot AOSP.
Bug: 37970289
Change-Id: I7e1d13d24adc10b4009d60ee68bc9e837ce78b34

1 files changed, 6 insertions, 32 deletions

diff --git a/openjdkjvm/OpenjdkJvm.cc b/openjdkjvm/OpenjdkJvm.cc
index a9fa0fc6c7..63503f459e 100644
--- a/openjdkjvm/OpenjdkJvm.cc
+++ b/openjdkjvm/OpenjdkJvm.cc
@@ -406,38 +406,12 @@ JNIEXPORT jboolean JVM_HoldsLock(JNIEnv* env, jclass unused ATTRIBUTE_UNUSED, jo
   return soa.Self()->HoldsLock(object);
 }
 
-JNIEXPORT void JVM_SetNativeThreadName(JNIEnv* env, jobject jthread, jstring java_name) {
-  ScopedUtfChars name(env, java_name);
-  {
-    art::ScopedObjectAccess soa(env);
-    if (soa.Decode<art::mirror::Object>(jthread) == soa.Self()->GetPeer()) {
-      soa.Self()->SetThreadName(name.c_str());
-      return;
-    }
-  }
-  // Suspend thread to avoid it from killing itself while we set its name. We don't just hold the
-  // thread list lock to avoid this, as setting the thread name causes mutator to lock/unlock
-  // in the DDMS send code.
-  art::ThreadList* thread_list = art::Runtime::Current()->GetThreadList();
-  bool timed_out;
-  // Take suspend thread lock to avoid races with threads trying to suspend this one.
-  art::Thread* thread;
-  {
-    thread = thread_list->SuspendThreadByPeer(jthread,
-                                              art::SuspendReason::kInternal,
-                                              &timed_out);
-  }
-  if (thread != nullptr) {
-    {
-      art::ScopedObjectAccess soa(env);
-      thread->SetThreadName(name.c_str());
-    }
-    bool resumed = thread_list->Resume(thread, art::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.";
-  }
+JNIEXPORT __attribute__((noreturn)) void JVM_SetNativeThreadName(
+    JNIEnv* env ATTRIBUTE_UNUSED,
+    jobject jthread ATTRIBUTE_UNUSED,
+    jstring java_name ATTRIBUTE_UNUSED) {
+  UNIMPLEMENTED(FATAL) << "JVM_SetNativeThreadName is not implemented";
+  UNREACHABLE();
 }
 
 JNIEXPORT __attribute__((noreturn)) jint JVM_IHashCode(JNIEnv* env ATTRIBUTE_UNUSED,