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/*
* 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-inl.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) {
MutexLock mu(self, *Locks::reference_processor_lock_);
condition_.Broadcast(self);
}
ObjPtr<mirror::Object> ReferenceProcessor::GetReferent(Thread* self,
ObjPtr<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.
ObjPtr<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())) {
ObjPtr<mirror::Object> referent = reference->GetReferent<kWithoutReadBarrier>();
// 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 == 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.
// Use the cached referent instead of calling GetReferent since other threads could call
// Reference.clear() after we did the null check resulting in a null pointer being
// incorrectly passed to IsMarked. b/33569625
ObjPtr<mirror::Object> forwarded_ref = collector_->IsMarked(referent.Ptr());
if (forwarded_ref != nullptr) {
// Non null means that it is marked.
if (!preserving_references_ ||
(LIKELY(!reference->IsFinalizerReferenceInstance()) && reference->IsUnprocessed())) {
return forwarded_ref;
}
}
}
// Check and run the empty checkpoint before blocking so the empty checkpoint will work in the
// presence of threads blocking for weak ref access.
self->CheckEmptyCheckpointFromWeakRefAccess(Locks::reference_processor_lock_);
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);
}
}
if (kIsDebugBuild && collector->IsTransactionActive()) {
// In transaction mode, we shouldn't enqueue any Reference to the queues.
// See DelayReferenceReferent().
DCHECK(soft_reference_queue_.IsEmpty());
DCHECK(weak_reference_queue_.IsEmpty());
DCHECK(finalizer_reference_queue_.IsEmpty());
DCHECK(phantom_reference_queue_.IsEmpty());
}
// 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(ObjPtr<mirror::Class> klass,
ObjPtr<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();
// do_atomic_update needs to be true because this happens outside of the reference processing
// phase.
if (!collector->IsNullOrMarkedHeapReference(referent, /*do_atomic_update*/true)) {
if (UNLIKELY(collector->IsTransactionActive())) {
// In transaction mode, keep the referent alive and avoid any reference processing to avoid the
// issue of rolling back reference processing. do_atomic_update needs to be true because this
// happens outside of the reference processing phase.
if (!referent->IsNull()) {
collector->MarkHeapReference(referent, /*do_atomic_update*/ true);
}
return;
}
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 " << klass->PrettyClass() << " " << 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();
}
}
void ReferenceProcessor::ClearReferent(ObjPtr<mirror::Reference> ref) {
Thread* self = Thread::Current();
MutexLock mu(self, *Locks::reference_processor_lock_);
// Need to wait until reference processing is done since IsMarkedHeapReference does not have a
// CAS. If we do not wait, it can result in the GC un-clearing references due to race conditions.
// This also handles the race where the referent gets cleared after a null check but before
// IsMarkedHeapReference is called.
WaitUntilDoneProcessingReferences(self);
if (Runtime::Current()->IsActiveTransaction()) {
ref->ClearReferent<true>();
} else {
ref->ClearReferent<false>();
}
}
void ReferenceProcessor::WaitUntilDoneProcessingReferences(Thread* self) {
// Wait until we are done processing reference.
while ((!kUseReadBarrier && SlowPathEnabled()) ||
(kUseReadBarrier && !self->GetWeakRefAccessEnabled())) {
// Check and run the empty checkpoint before blocking so the empty checkpoint will work in the
// presence of threads blocking for weak ref access.
self->CheckEmptyCheckpointFromWeakRefAccess(Locks::reference_processor_lock_);
condition_.WaitHoldingLocks(self);
}
}
bool ReferenceProcessor::MakeCircularListIfUnenqueued(
ObjPtr<mirror::FinalizerReference> reference) {
Thread* self = Thread::Current();
MutexLock mu(self, *Locks::reference_processor_lock_);
WaitUntilDoneProcessingReferences(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