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/*
* Copyright (C) 2013 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_queue.h"
#include "accounting/card_table-inl.h"
#include "base/mutex.h"
#include "collector/concurrent_copying.h"
#include "heap.h"
#include "mirror/class-inl.h"
#include "mirror/object-inl.h"
#include "mirror/reference-inl.h"
#include "object_callbacks.h"
namespace art HIDDEN {
namespace gc {
ReferenceQueue::ReferenceQueue(Mutex* lock) : lock_(lock), list_(nullptr) {
}
void ReferenceQueue::AtomicEnqueueIfNotEnqueued(Thread* self, ObjPtr<mirror::Reference> ref) {
DCHECK(ref != nullptr);
MutexLock mu(self, *lock_);
if (ref->IsUnprocessed()) {
EnqueueReference(ref);
}
}
void ReferenceQueue::EnqueueReference(ObjPtr<mirror::Reference> ref) {
DCHECK(ref != nullptr);
CHECK(ref->IsUnprocessed());
if (IsEmpty()) {
// 1 element cyclic queue, ie: Reference ref = ..; ref.pendingNext = ref;
list_ = ref.Ptr();
} else {
// The list is owned by the GC, everything that has been inserted must already be at least
// gray.
ObjPtr<mirror::Reference> head = list_->GetPendingNext<kWithoutReadBarrier>();
DCHECK(head != nullptr);
ref->SetPendingNext(head);
}
// Add the reference in the middle to preserve the cycle.
list_->SetPendingNext(ref);
}
ObjPtr<mirror::Reference> ReferenceQueue::DequeuePendingReference() {
DCHECK(!IsEmpty());
ObjPtr<mirror::Reference> ref = list_->GetPendingNext<kWithoutReadBarrier>();
DCHECK(ref != nullptr);
// Note: the following code is thread-safe because it is only called from ProcessReferences which
// is single threaded.
if (list_ == ref) {
list_ = nullptr;
} else {
ObjPtr<mirror::Reference> next = ref->GetPendingNext<kWithoutReadBarrier>();
list_->SetPendingNext(next);
}
ref->SetPendingNext(nullptr);
return ref;
}
// This must be called whenever DequeuePendingReference is called.
void ReferenceQueue::DisableReadBarrierForReference(ObjPtr<mirror::Reference> ref,
std::memory_order order) {
Heap* heap = Runtime::Current()->GetHeap();
if (kUseBakerReadBarrier && heap->CurrentCollectorType() == kCollectorTypeCC &&
heap->ConcurrentCopyingCollector()->IsActive()) {
// Change the gray ptr we left in ConcurrentCopying::ProcessMarkStackRef() to non-gray.
// We check IsActive() above because we don't want to do this when the zygote compaction
// collector (SemiSpace) is running.
CHECK(ref != nullptr);
collector::ConcurrentCopying* concurrent_copying = heap->ConcurrentCopyingCollector();
uint32_t rb_state = ref->GetReadBarrierState();
if (rb_state == ReadBarrier::GrayState()) {
ref->AtomicSetReadBarrierState(ReadBarrier::GrayState(), ReadBarrier::NonGrayState(), order);
CHECK_EQ(ref->GetReadBarrierState(), ReadBarrier::NonGrayState());
} else {
// In ConcurrentCopying::ProcessMarkStackRef() we may leave a non-gray reference in the queue
// and find it here, which is OK.
CHECK_EQ(rb_state, ReadBarrier::NonGrayState()) << "ref=" << ref << " rb_state=" << rb_state;
ObjPtr<mirror::Object> referent = ref->GetReferent<kWithoutReadBarrier>();
// The referent could be null if it's cleared by a mutator (Reference.clear()).
if (referent != nullptr) {
CHECK(concurrent_copying->IsInToSpace(referent.Ptr()))
<< "ref=" << ref << " rb_state=" << ref->GetReadBarrierState()
<< " referent=" << referent;
}
}
}
}
void ReferenceQueue::Dump(std::ostream& os) const {
ObjPtr<mirror::Reference> cur = list_;
os << "Reference starting at list_=" << list_ << "\n";
if (cur == nullptr) {
return;
}
do {
ObjPtr<mirror::Reference> pending_next = cur->GetPendingNext();
os << "Reference= " << cur << " PendingNext=" << pending_next;
if (cur->IsFinalizerReferenceInstance()) {
os << " Zombie=" << cur->AsFinalizerReference()->GetZombie();
}
os << "\n";
cur = pending_next;
} while (cur != list_);
}
size_t ReferenceQueue::GetLength() const {
size_t count = 0;
ObjPtr<mirror::Reference> cur = list_;
if (cur != nullptr) {
do {
++count;
cur = cur->GetPendingNext();
} while (cur != list_);
}
return count;
}
void ReferenceQueue::ClearWhiteReferences(ReferenceQueue* cleared_references,
collector::GarbageCollector* collector,
bool report_cleared) {
while (!IsEmpty()) {
ObjPtr<mirror::Reference> ref = DequeuePendingReference();
mirror::HeapReference<mirror::Object>* referent_addr = ref->GetReferentReferenceAddr();
// do_atomic_update is false because this happens during the reference processing phase where
// Reference.clear() would block.
if (!collector->IsNullOrMarkedHeapReference(referent_addr, /*do_atomic_update=*/false)) {
// Referent is white, clear it.
if (Runtime::Current()->IsActiveTransaction()) {
ref->ClearReferent<true>();
} else {
ref->ClearReferent<false>();
}
cleared_references->EnqueueReference(ref);
if (report_cleared) {
static bool already_reported = false;
if (!already_reported) {
// TODO: Maybe do this only if the queue is non-null?
LOG(WARNING)
<< "Cleared Reference was only reachable from finalizer (only reported once)";
already_reported = true;
}
}
}
// Delay disabling the read barrier until here so that the ClearReferent call above in
// transaction mode will trigger the read barrier.
DisableReadBarrierForReference(ref, std::memory_order_relaxed);
}
}
FinalizerStats ReferenceQueue::EnqueueFinalizerReferences(ReferenceQueue* cleared_references,
collector::GarbageCollector* collector) {
uint32_t num_refs(0), num_enqueued(0);
while (!IsEmpty()) {
ObjPtr<mirror::FinalizerReference> ref = DequeuePendingReference()->AsFinalizerReference();
++num_refs;
mirror::HeapReference<mirror::Object>* referent_addr = ref->GetReferentReferenceAddr();
// do_atomic_update is false because this happens during the reference processing phase where
// Reference.clear() would block.
if (!collector->IsNullOrMarkedHeapReference(referent_addr, /*do_atomic_update=*/false)) {
ObjPtr<mirror::Object> forward_address = collector->MarkObject(referent_addr->AsMirrorPtr());
// Move the updated referent to the zombie field.
if (Runtime::Current()->IsActiveTransaction()) {
ref->SetZombie<true>(forward_address);
ref->ClearReferent<true>();
} else {
ref->SetZombie<false>(forward_address);
ref->ClearReferent<false>();
}
cleared_references->EnqueueReference(ref);
++num_enqueued;
}
// Delay disabling the read barrier until here so that the ClearReferent call above in
// transaction mode will trigger the read barrier.
DisableReadBarrierForReference(ref->AsReference(), std::memory_order_relaxed);
}
return FinalizerStats(num_refs, num_enqueued);
}
uint32_t ReferenceQueue::ForwardSoftReferences(MarkObjectVisitor* visitor) {
uint32_t num_refs(0);
Thread* self = Thread::Current();
static constexpr int SR_BUF_SIZE = 32;
ObjPtr<mirror::Reference> buf[SR_BUF_SIZE];
int n_entries;
bool empty;
do {
{
// Acquire lock only a few times and hold it as briefly as possible.
MutexLock mu(self, *lock_);
empty = IsEmpty();
for (n_entries = 0; n_entries < SR_BUF_SIZE && !empty; ++n_entries) {
// Dequeuing the Reference here means it could possibly be enqueued again during this GC.
// That's unlikely and benign.
buf[n_entries] = DequeuePendingReference();
empty = IsEmpty();
}
}
for (int i = 0; i < n_entries; ++i) {
mirror::HeapReference<mirror::Object>* referent_addr = buf[i]->GetReferentReferenceAddr();
if (referent_addr->AsMirrorPtr() != nullptr) {
visitor->MarkHeapReference(referent_addr, /*do_atomic_update=*/ true);
++num_refs;
}
DisableReadBarrierForReference(buf[i]->AsReference(), std::memory_order_release);
}
} while (!empty);
return num_refs;
}
void ReferenceQueue::UpdateRoots(IsMarkedVisitor* visitor) {
if (list_ != nullptr) {
list_ = down_cast<mirror::Reference*>(visitor->IsMarked(list_));
}
}
} // namespace gc
} // namespace art
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