commit 3ed8ec10be0c79e7f7bbe73a754da0daf997b994
author Mathieu Chartier <mathieuc@google.com> Thu Apr 20 19:28:54 2017 -0700
committer Mathieu Chartier <mathieuc@google.com> Fri Apr 21 09:26:58 2017 -0700
tree 53e3f3e105d1d4f582a3f35cf874a1f58e1328eb
parent bdbc63b892c571101a301ce805ca136acd769218

Store java_lang_Object_ in the flip callback

There was a race where the GC thread would not have is_marking =
true, and call
WellKnownClasses::ToClass(WellKnownClasses::java_lang_Object). This
meant that the returned class was maybe in the from-space for the
no image case. The fix was to move this decoding into the flip
callback since this callback is called before flipping any thread
roots.

Bug: 37531237
Bug: 12687968
Test: test-art-host

Change-Id: I9a09249e9c6ea2b3b124e957a9e4b61017869306

runtime/gc/collector/concurrent_copying.cc

diff --git a/runtime/gc/collector/concurrent_copying.cc b/runtime/gc/collector/concurrent_copying.cc
index 792c191..bd81f47 100644
--- a/runtime/gc/collector/concurrent_copying.cc
+++ b/runtime/gc/collector/concurrent_copying.cc
@@ -374,6 +374,8 @@
         cc->VerifyGrayImmuneObjects();
       }
     }
+    cc->java_lang_Object_ = down_cast<mirror::Class*>(cc->Mark(
+        WellKnownClasses::ToClass(WellKnownClasses::java_lang_Object).Ptr()));
   }
 
  private:
@@ -2067,11 +2069,9 @@
   size_t data_offset = mirror::Array::DataOffset(component_size).SizeValue();
   if (data_offset > byte_size) {
     // An int array is too big. Use java.lang.Object.
-    ObjPtr<mirror::Class> java_lang_Object =
-        WellKnownClasses::ToClass(WellKnownClasses::java_lang_Object);
-    AssertToSpaceInvariant(nullptr, MemberOffset(0), java_lang_Object.Ptr());
-    CHECK_EQ(byte_size, (java_lang_Object->GetObjectSize<kVerifyNone, kWithoutReadBarrier>()));
-    dummy_obj->SetClass(java_lang_Object.Ptr());
+    AssertToSpaceInvariant(nullptr, MemberOffset(0), java_lang_Object_);
+    CHECK_EQ(byte_size, (java_lang_Object_->GetObjectSize<kVerifyNone, kWithoutReadBarrier>()));
+    dummy_obj->SetClass(java_lang_Object_);
     CHECK_EQ(byte_size, (dummy_obj->SizeOf<kVerifyNone>()));
   } else {
     // Use an int array.

runtime/gc/collector/concurrent_copying.h

diff --git a/runtime/gc/collector/concurrent_copying.h b/runtime/gc/collector/concurrent_copying.h
index a0da9fc..398a7e2 100644
--- a/runtime/gc/collector/concurrent_copying.h
+++ b/runtime/gc/collector/concurrent_copying.h
@@ -316,6 +316,11 @@
   Mutex immune_gray_stack_lock_ DEFAULT_MUTEX_ACQUIRED_AFTER;
   std::vector<mirror::Object*> immune_gray_stack_ GUARDED_BY(immune_gray_stack_lock_);
 
+  // Class of java.lang.Object. Filled in from WellKnownClasses in FlipCallback. Must
+  // be filled in before flipping thread roots so that FillDummyObject can run. Not
+  // ObjPtr since the GC may transition to suspended and runnable between phases.
+  mirror::Class* java_lang_Object_;
+
   class AssertToSpaceInvariantFieldVisitor;
   class AssertToSpaceInvariantObjectVisitor;
   class AssertToSpaceInvariantRefsVisitor;