runtime/gc/reference_processor.cc - platform/art - Git at Google

 /*
  * Copyright (C) 2014 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 "reference_processor.h"

 #include "base/time_utils.h"
 #include "collector/garbage_collector.h"
 #include "mirror/class-inl.h"
 #include "mirror/object-inl.h"
 #include "mirror/reference-inl.h"
 #include "reference_processor-inl.h"
 #include "reflection.h"
 #include "ScopedLocalRef.h"
 #include "scoped_thread_state_change.h"
 #include "task_processor.h"
 #include "utils.h"
 #include "well_known_classes.h"

 namespace art {
 namespace gc {

 static constexpr bool kAsyncReferenceQueueAdd = false;

 ReferenceProcessor::ReferenceProcessor()
     : collector_(nullptr),
       preserving_references_(false),
       condition_("reference processor condition", *Locks::reference_processor_lock_) ,
       soft_reference_queue_(Locks::reference_queue_soft_references_lock_),
       weak_reference_queue_(Locks::reference_queue_weak_references_lock_),
       finalizer_reference_queue_(Locks::reference_queue_finalizer_references_lock_),
       phantom_reference_queue_(Locks::reference_queue_phantom_references_lock_),
       cleared_references_(Locks::reference_queue_cleared_references_lock_) {
 }

 void ReferenceProcessor::EnableSlowPath() {
   mirror::Reference::GetJavaLangRefReference()->SetSlowPath(true);
 }

 void ReferenceProcessor::DisableSlowPath(Thread* self) {
   mirror::Reference::GetJavaLangRefReference()->SetSlowPath(false);
   condition_.Broadcast(self);
 }

 void ReferenceProcessor::BroadcastForSlowPath(Thread* self) {
   CHECK(kUseReadBarrier);
   MutexLock mu(self, *Locks::reference_processor_lock_);
   condition_.Broadcast(self);
 }

 mirror::Object* ReferenceProcessor::GetReferent(Thread* self, mirror::Reference* reference) {
   if (!kUseReadBarrier || self->GetWeakRefAccessEnabled()) {
     // Under read barrier / concurrent copying collector, it's not safe to call GetReferent() when
     // weak ref access is disabled as the call includes a read barrier which may push a ref onto the
     // mark stack and interfere with termination of marking.
     mirror::Object* const referent = reference->GetReferent();
     // If the referent is null then it is already cleared, we can just return null since there is no
     // scenario where it becomes non-null during the reference processing phase.
     if (UNLIKELY(!SlowPathEnabled()) || referent == nullptr) {
       return referent;
     }
   }
   MutexLock mu(self, *Locks::reference_processor_lock_);
   while ((!kUseReadBarrier && SlowPathEnabled()) ||
          (kUseReadBarrier && !self->GetWeakRefAccessEnabled())) {
     mirror::HeapReference<mirror::Object>* const referent_addr =
         reference->GetReferentReferenceAddr();
     // If the referent became cleared, return it. Don't need barrier since thread roots can't get
     // updated until after we leave the function due to holding the mutator lock.
     if (referent_addr->AsMirrorPtr() == nullptr) {
       return nullptr;
     }
     // Try to see if the referent is already marked by using the is_marked_callback. We can return
     // it to the mutator as long as the GC is not preserving references.
     if (LIKELY(collector_ != nullptr)) {
       // If it's null it means not marked, but it could become marked if the referent is reachable
       // by finalizer referents. So we cannot return in this case and must block. Otherwise, we
       // can return it to the mutator as long as the GC is not preserving references, in which
       // case only black nodes can be safely returned. If the GC is preserving references, the
       // mutator could take a white field from a grey or white node and move it somewhere else
       // in the heap causing corruption since this field would get swept.
       if (collector_->IsMarkedHeapReference(referent_addr)) {
         if (!preserving_references_ ||
            (LIKELY(!reference->IsFinalizerReferenceInstance()) && reference->IsUnprocessed())) {
           return referent_addr->AsMirrorPtr();
         }
       }
     }
     condition_.WaitHoldingLocks(self);
   }
   return reference->GetReferent();
 }

 void ReferenceProcessor::StartPreservingReferences(Thread* self) {
   MutexLock mu(self, *Locks::reference_processor_lock_);
   preserving_references_ = true;
 }

 void ReferenceProcessor::StopPreservingReferences(Thread* self) {
   MutexLock mu(self, *Locks::reference_processor_lock_);
   preserving_references_ = false;
   // We are done preserving references, some people who are blocked may see a marked referent.
   condition_.Broadcast(self);
 }

 // Process reference class instances and schedule finalizations.
 void ReferenceProcessor::ProcessReferences(bool concurrent, TimingLogger* timings,
                                            bool clear_soft_references,
                                            collector::GarbageCollector* collector) {
   TimingLogger::ScopedTiming t(concurrent ? __FUNCTION__ : "(Paused)ProcessReferences", timings);
   Thread* self = Thread::Current();
   {
     MutexLock mu(self, *Locks::reference_processor_lock_);
     collector_ = collector;
     if (!kUseReadBarrier) {
       CHECK_EQ(SlowPathEnabled(), concurrent) << "Slow path must be enabled iff concurrent";
     } else {
       // Weak ref access is enabled at Zygote compaction by SemiSpace (concurrent == false).
       CHECK_EQ(!self->GetWeakRefAccessEnabled(), concurrent);
     }
   }
   // Unless required to clear soft references with white references, preserve some white referents.
   if (!clear_soft_references) {
     TimingLogger::ScopedTiming split(concurrent ? "ForwardSoftReferences" :
         "(Paused)ForwardSoftReferences", timings);
     if (concurrent) {
       StartPreservingReferences(self);
     }
     // TODO: Add smarter logic for preserving soft references. The behavior should be a conditional
     // mark if the SoftReference is supposed to be preserved.
     soft_reference_queue_.ForwardSoftReferences(collector);
     collector->ProcessMarkStack();
     if (concurrent) {
       StopPreservingReferences(self);
     }
   }
   // Clear all remaining soft and weak references with white referents.
   soft_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
   weak_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
   {
     TimingLogger::ScopedTiming t2(concurrent ? "EnqueueFinalizerReferences" :
         "(Paused)EnqueueFinalizerReferences", timings);
     if (concurrent) {
       StartPreservingReferences(self);
     }
     // Preserve all white objects with finalize methods and schedule them for finalization.
     finalizer_reference_queue_.EnqueueFinalizerReferences(&cleared_references_, collector);
     collector->ProcessMarkStack();
     if (concurrent) {
       StopPreservingReferences(self);
     }
   }
   // Clear all finalizer referent reachable soft and weak references with white referents.
   soft_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
   weak_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
   // Clear all phantom references with white referents.
   phantom_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
   // At this point all reference queues other than the cleared references should be empty.
   DCHECK(soft_reference_queue_.IsEmpty());
   DCHECK(weak_reference_queue_.IsEmpty());
   DCHECK(finalizer_reference_queue_.IsEmpty());
   DCHECK(phantom_reference_queue_.IsEmpty());
   {
     MutexLock mu(self, *Locks::reference_processor_lock_);
     // Need to always do this since the next GC may be concurrent. Doing this for only concurrent
     // could result in a stale is_marked_callback_ being called before the reference processing
     // starts since there is a small window of time where slow_path_enabled_ is enabled but the
     // callback isn't yet set.
     collector_ = nullptr;
     if (!kUseReadBarrier && concurrent) {
       // Done processing, disable the slow path and broadcast to the waiters.
       DisableSlowPath(self);
     }
   }
 }

 // Process the "referent" field in a java.lang.ref.Reference.  If the referent has not yet been
 // marked, put it on the appropriate list in the heap for later processing.
 void ReferenceProcessor::DelayReferenceReferent(mirror::Class* klass, mirror::Reference* ref,
                                                 collector::GarbageCollector* collector) {
   // klass can be the class of the old object if the visitor already updated the class of ref.
   DCHECK(klass != nullptr);
   DCHECK(klass->IsTypeOfReferenceClass());
   mirror::HeapReference<mirror::Object>* referent = ref->GetReferentReferenceAddr();
   if (referent->AsMirrorPtr() != nullptr && !collector->IsMarkedHeapReference(referent)) {
     Thread* self = Thread::Current();
     // TODO: Remove these locks, and use atomic stacks for storing references?
     // We need to check that the references haven't already been enqueued since we can end up
     // scanning the same reference multiple times due to dirty cards.
     if (klass->IsSoftReferenceClass()) {
       soft_reference_queue_.AtomicEnqueueIfNotEnqueued(self, ref);
     } else if (klass->IsWeakReferenceClass()) {
       weak_reference_queue_.AtomicEnqueueIfNotEnqueued(self, ref);
     } else if (klass->IsFinalizerReferenceClass()) {
       finalizer_reference_queue_.AtomicEnqueueIfNotEnqueued(self, ref);
     } else if (klass->IsPhantomReferenceClass()) {
       phantom_reference_queue_.AtomicEnqueueIfNotEnqueued(self, ref);
     } else {
       LOG(FATAL) << "Invalid reference type " << PrettyClass(klass) << " " << std::hex
                  << klass->GetAccessFlags();
     }
   }
 }

 void ReferenceProcessor::UpdateRoots(IsMarkedVisitor* visitor) {
   cleared_references_.UpdateRoots(visitor);
 }

 class ClearedReferenceTask : public HeapTask {
  public:
   explicit ClearedReferenceTask(jobject cleared_references)
       : HeapTask(NanoTime()), cleared_references_(cleared_references) {
   }
   virtual void Run(Thread* thread) {
     ScopedObjectAccess soa(thread);
     jvalue args[1];
     args[0].l = cleared_references_;
     InvokeWithJValues(soa, nullptr, WellKnownClasses::java_lang_ref_ReferenceQueue_add, args);
     soa.Env()->DeleteGlobalRef(cleared_references_);
   }

  private:
   const jobject cleared_references_;
 };

 void ReferenceProcessor::EnqueueClearedReferences(Thread* self) {
   Locks::mutator_lock_->AssertNotHeld(self);
   // When a runtime isn't started there are no reference queues to care about so ignore.
   if (!cleared_references_.IsEmpty()) {
     if (LIKELY(Runtime::Current()->IsStarted())) {
       jobject cleared_references;
       {
         ReaderMutexLock mu(self, *Locks::mutator_lock_);
         cleared_references = self->GetJniEnv()->vm->AddGlobalRef(
             self, cleared_references_.GetList());
       }
       if (kAsyncReferenceQueueAdd) {
         // TODO: This can cause RunFinalization to terminate before newly freed objects are
         // finalized since they may not be enqueued by the time RunFinalization starts.
         Runtime::Current()->GetHeap()->GetTaskProcessor()->AddTask(
             self, new ClearedReferenceTask(cleared_references));
       } else {
         ClearedReferenceTask task(cleared_references);
         task.Run(self);
       }
     }
     cleared_references_.Clear();
   }
 }

 bool ReferenceProcessor::MakeCircularListIfUnenqueued(mirror::FinalizerReference* reference) {
   Thread* self = Thread::Current();
   MutexLock mu(self, *Locks::reference_processor_lock_);
   // Wait untul we are done processing reference.
   while ((!kUseReadBarrier && SlowPathEnabled()) ||
          (kUseReadBarrier && !self->GetWeakRefAccessEnabled())) {
     condition_.WaitHoldingLocks(self);
   }
   // At this point, since the sentinel of the reference is live, it is guaranteed to not be
   // enqueued if we just finished processing references. Otherwise, we may be doing the main GC
   // phase. Since we are holding the reference processor lock, it guarantees that reference
   // processing can't begin. The GC could have just enqueued the reference one one of the internal
   // GC queues, but since we hold the lock finalizer_reference_queue_ lock it also prevents this
   // race.
   MutexLock mu2(self, *Locks::reference_queue_finalizer_references_lock_);
   if (reference->IsUnprocessed()) {
     CHECK(reference->IsFinalizerReferenceInstance());
     reference->SetPendingNext(reference);
     return true;
   }
   return false;
 }

 }  // namespace gc
 }  // namespace art
	/*
	* Copyright (C) 2014 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 "reference_processor.h"

	#include "base/time_utils.h"
	#include "collector/garbage_collector.h"
	#include "mirror/class-inl.h"
	#include "mirror/object-inl.h"
	#include "mirror/reference-inl.h"
	#include "reference_processor-inl.h"
	#include "reflection.h"
	#include "ScopedLocalRef.h"
	#include "scoped_thread_state_change.h"
	#include "task_processor.h"
	#include "utils.h"
	#include "well_known_classes.h"

	namespace art {
	namespace gc {

	static constexpr bool kAsyncReferenceQueueAdd = false;

	ReferenceProcessor::ReferenceProcessor()
	: collector_(nullptr),
	preserving_references_(false),
	condition_("reference processor condition", *Locks::reference_processor_lock_) ,
	soft_reference_queue_(Locks::reference_queue_soft_references_lock_),
	weak_reference_queue_(Locks::reference_queue_weak_references_lock_),
	finalizer_reference_queue_(Locks::reference_queue_finalizer_references_lock_),
	phantom_reference_queue_(Locks::reference_queue_phantom_references_lock_),
	cleared_references_(Locks::reference_queue_cleared_references_lock_) {
	}

	void ReferenceProcessor::EnableSlowPath() {
	mirror::Reference::GetJavaLangRefReference()->SetSlowPath(true);
	}

	void ReferenceProcessor::DisableSlowPath(Thread* self) {
	mirror::Reference::GetJavaLangRefReference()->SetSlowPath(false);
	condition_.Broadcast(self);
	}

	void ReferenceProcessor::BroadcastForSlowPath(Thread* self) {
	CHECK(kUseReadBarrier);
	MutexLock mu(self, *Locks::reference_processor_lock_);
	condition_.Broadcast(self);
	}

	mirror::Object* ReferenceProcessor::GetReferent(Thread* self, mirror::Reference* reference) {
	if (!kUseReadBarrier \|\| self->GetWeakRefAccessEnabled()) {
	// Under read barrier / concurrent copying collector, it's not safe to call GetReferent() when
	// weak ref access is disabled as the call includes a read barrier which may push a ref onto the
	// mark stack and interfere with termination of marking.
	mirror::Object* const referent = reference->GetReferent();
	// If the referent is null then it is already cleared, we can just return null since there is no
	// scenario where it becomes non-null during the reference processing phase.
	if (UNLIKELY(!SlowPathEnabled()) \|\| referent == nullptr) {
	return referent;
	}
	}
	MutexLock mu(self, *Locks::reference_processor_lock_);
	while ((!kUseReadBarrier && SlowPathEnabled()) \|\|
	(kUseReadBarrier && !self->GetWeakRefAccessEnabled())) {
	mirror::HeapReference<mirror::Object>* const referent_addr =
	reference->GetReferentReferenceAddr();
	// If the referent became cleared, return it. Don't need barrier since thread roots can't get
	// updated until after we leave the function due to holding the mutator lock.
	if (referent_addr->AsMirrorPtr() == nullptr) {
	return nullptr;
	}
	// Try to see if the referent is already marked by using the is_marked_callback. We can return
	// it to the mutator as long as the GC is not preserving references.
	if (LIKELY(collector_ != nullptr)) {
	// If it's null it means not marked, but it could become marked if the referent is reachable
	// by finalizer referents. So we cannot return in this case and must block. Otherwise, we
	// can return it to the mutator as long as the GC is not preserving references, in which
	// case only black nodes can be safely returned. If the GC is preserving references, the
	// mutator could take a white field from a grey or white node and move it somewhere else
	// in the heap causing corruption since this field would get swept.
	if (collector_->IsMarkedHeapReference(referent_addr)) {
	if (!preserving_references_ \|\|
	(LIKELY(!reference->IsFinalizerReferenceInstance()) && reference->IsUnprocessed())) {
	return referent_addr->AsMirrorPtr();
	}
	}
	}
	condition_.WaitHoldingLocks(self);
	}
	return reference->GetReferent();
	}

	void ReferenceProcessor::StartPreservingReferences(Thread* self) {
	MutexLock mu(self, *Locks::reference_processor_lock_);
	preserving_references_ = true;
	}

	void ReferenceProcessor::StopPreservingReferences(Thread* self) {
	MutexLock mu(self, *Locks::reference_processor_lock_);
	preserving_references_ = false;
	// We are done preserving references, some people who are blocked may see a marked referent.
	condition_.Broadcast(self);
	}

	// Process reference class instances and schedule finalizations.
	void ReferenceProcessor::ProcessReferences(bool concurrent, TimingLogger* timings,
	bool clear_soft_references,
	collector::GarbageCollector* collector) {
	TimingLogger::ScopedTiming t(concurrent ? __FUNCTION__ : "(Paused)ProcessReferences", timings);
	Thread* self = Thread::Current();
	{
	MutexLock mu(self, *Locks::reference_processor_lock_);
	collector_ = collector;
	if (!kUseReadBarrier) {
	CHECK_EQ(SlowPathEnabled(), concurrent) << "Slow path must be enabled iff concurrent";
	} else {
	// Weak ref access is enabled at Zygote compaction by SemiSpace (concurrent == false).
	CHECK_EQ(!self->GetWeakRefAccessEnabled(), concurrent);
	}
	}
	// Unless required to clear soft references with white references, preserve some white referents.
	if (!clear_soft_references) {
	TimingLogger::ScopedTiming split(concurrent ? "ForwardSoftReferences" :
	"(Paused)ForwardSoftReferences", timings);
	if (concurrent) {
	StartPreservingReferences(self);
	}
	// TODO: Add smarter logic for preserving soft references. The behavior should be a conditional
	// mark if the SoftReference is supposed to be preserved.
	soft_reference_queue_.ForwardSoftReferences(collector);
	collector->ProcessMarkStack();
	if (concurrent) {
	StopPreservingReferences(self);
	}
	}
	// Clear all remaining soft and weak references with white referents.
	soft_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
	weak_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
	{
	TimingLogger::ScopedTiming t2(concurrent ? "EnqueueFinalizerReferences" :
	"(Paused)EnqueueFinalizerReferences", timings);
	if (concurrent) {
	StartPreservingReferences(self);
	}
	// Preserve all white objects with finalize methods and schedule them for finalization.
	finalizer_reference_queue_.EnqueueFinalizerReferences(&cleared_references_, collector);
	collector->ProcessMarkStack();
	if (concurrent) {
	StopPreservingReferences(self);
	}
	}
	// Clear all finalizer referent reachable soft and weak references with white referents.
	soft_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
	weak_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
	// Clear all phantom references with white referents.
	phantom_reference_queue_.ClearWhiteReferences(&cleared_references_, collector);
	// At this point all reference queues other than the cleared references should be empty.
	DCHECK(soft_reference_queue_.IsEmpty());
	DCHECK(weak_reference_queue_.IsEmpty());
	DCHECK(finalizer_reference_queue_.IsEmpty());
	DCHECK(phantom_reference_queue_.IsEmpty());
	{
	MutexLock mu(self, *Locks::reference_processor_lock_);
	// Need to always do this since the next GC may be concurrent. Doing this for only concurrent
	// could result in a stale is_marked_callback_ being called before the reference processing
	// starts since there is a small window of time where slow_path_enabled_ is enabled but the
	// callback isn't yet set.
	collector_ = nullptr;
	if (!kUseReadBarrier && concurrent) {
	// Done processing, disable the slow path and broadcast to the waiters.
	DisableSlowPath(self);
	}
	}
	}

	// Process the "referent" field in a java.lang.ref.Reference. If the referent has not yet been
	// marked, put it on the appropriate list in the heap for later processing.
	void ReferenceProcessor::DelayReferenceReferent(mirror::Class* klass, mirror::Reference* ref,
	collector::GarbageCollector* collector) {
	// klass can be the class of the old object if the visitor already updated the class of ref.
	DCHECK(klass != nullptr);
	DCHECK(klass->IsTypeOfReferenceClass());
	mirror::HeapReference<mirror::Object>* referent = ref->GetReferentReferenceAddr();
	if (referent->AsMirrorPtr() != nullptr && !collector->IsMarkedHeapReference(referent)) {
	Thread* self = Thread::Current();
	// TODO: Remove these locks, and use atomic stacks for storing references?
	// We need to check that the references haven't already been enqueued since we can end up
	// scanning the same reference multiple times due to dirty cards.
	if (klass->IsSoftReferenceClass()) {
	soft_reference_queue_.AtomicEnqueueIfNotEnqueued(self, ref);
	} else if (klass->IsWeakReferenceClass()) {
	weak_reference_queue_.AtomicEnqueueIfNotEnqueued(self, ref);
	} else if (klass->IsFinalizerReferenceClass()) {
	finalizer_reference_queue_.AtomicEnqueueIfNotEnqueued(self, ref);
	} else if (klass->IsPhantomReferenceClass()) {
	phantom_reference_queue_.AtomicEnqueueIfNotEnqueued(self, ref);
	} else {
	LOG(FATAL) << "Invalid reference type " << PrettyClass(klass) << " " << std::hex
	<< klass->GetAccessFlags();
	}
	}
	}

	void ReferenceProcessor::UpdateRoots(IsMarkedVisitor* visitor) {
	cleared_references_.UpdateRoots(visitor);
	}

	class ClearedReferenceTask : public HeapTask {
	public:
	explicit ClearedReferenceTask(jobject cleared_references)
	: HeapTask(NanoTime()), cleared_references_(cleared_references) {
	}
	virtual void Run(Thread* thread) {
	ScopedObjectAccess soa(thread);
	jvalue args[1];
	args[0].l = cleared_references_;
	InvokeWithJValues(soa, nullptr, WellKnownClasses::java_lang_ref_ReferenceQueue_add, args);
	soa.Env()->DeleteGlobalRef(cleared_references_);
	}

	private:
	const jobject cleared_references_;
	};

	void ReferenceProcessor::EnqueueClearedReferences(Thread* self) {
	Locks::mutator_lock_->AssertNotHeld(self);
	// When a runtime isn't started there are no reference queues to care about so ignore.
	if (!cleared_references_.IsEmpty()) {
	if (LIKELY(Runtime::Current()->IsStarted())) {
	jobject cleared_references;
	{
	ReaderMutexLock mu(self, *Locks::mutator_lock_);
	cleared_references = self->GetJniEnv()->vm->AddGlobalRef(
	self, cleared_references_.GetList());
	}
	if (kAsyncReferenceQueueAdd) {
	// TODO: This can cause RunFinalization to terminate before newly freed objects are
	// finalized since they may not be enqueued by the time RunFinalization starts.
	Runtime::Current()->GetHeap()->GetTaskProcessor()->AddTask(
	self, new ClearedReferenceTask(cleared_references));
	} else {
	ClearedReferenceTask task(cleared_references);
	task.Run(self);
	}
	}
	cleared_references_.Clear();
	}
	}

	bool ReferenceProcessor::MakeCircularListIfUnenqueued(mirror::FinalizerReference* reference) {
	Thread* self = Thread::Current();
	MutexLock mu(self, *Locks::reference_processor_lock_);
	// Wait untul we are done processing reference.
	while ((!kUseReadBarrier && SlowPathEnabled()) \|\|
	(kUseReadBarrier && !self->GetWeakRefAccessEnabled())) {
	condition_.WaitHoldingLocks(self);
	}
	// At this point, since the sentinel of the reference is live, it is guaranteed to not be
	// enqueued if we just finished processing references. Otherwise, we may be doing the main GC
	// phase. Since we are holding the reference processor lock, it guarantees that reference
	// processing can't begin. The GC could have just enqueued the reference one one of the internal
	// GC queues, but since we hold the lock finalizer_reference_queue_ lock it also prevents this
	// race.
	MutexLock mu2(self, *Locks::reference_queue_finalizer_references_lock_);
	if (reference->IsUnprocessed()) {
	CHECK(reference->IsFinalizerReferenceInstance());
	reference->SetPendingNext(reference);
	return true;
	}
	return false;
	}

	} // namespace gc
	} // namespace art