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/*
* Copyright (C) 2015 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.
*/
#ifndef ART_RUNTIME_GC_COLLECTOR_CONCURRENT_COPYING_INL_H_
#define ART_RUNTIME_GC_COLLECTOR_CONCURRENT_COPYING_INL_H_
#include "concurrent_copying.h"
#include "gc/accounting/atomic_stack.h"
#include "gc/accounting/space_bitmap-inl.h"
#include "gc/heap.h"
#include "gc/space/region_space-inl.h"
#include "gc/verification.h"
#include "lock_word.h"
#include "mirror/class.h"
#include "mirror/object-readbarrier-inl.h"
namespace art {
namespace gc {
namespace collector {
inline mirror::Object* ConcurrentCopying::MarkUnevacFromSpaceRegion(
Thread* const self,
mirror::Object* ref,
accounting::ContinuousSpaceBitmap* bitmap) {
if (use_generational_cc_ && !done_scanning_.load(std::memory_order_acquire)) {
// Everything in the unevac space should be marked for young generation CC,
// except for large objects.
DCHECK(!young_gen_ || region_space_bitmap_->Test(ref) || region_space_->IsLargeObject(ref))
<< ref << " "
<< ref->GetClass<kVerifyNone, kWithoutReadBarrier>()->PrettyClass();
// Since the mark bitmap is still filled in from last GC (or from marking phase of 2-phase CC,
// we can not use that or else the mutator may see references to the from space. Instead, use
// the baker pointer itself as the mark bit.
if (ref->AtomicSetReadBarrierState(ReadBarrier::NonGrayState(), ReadBarrier::GrayState())) {
// TODO: We don't actually need to scan this object later, we just need to clear the gray
// bit.
// TODO: We could also set the mark bit here for "free" since this case comes from the
// read barrier.
PushOntoMarkStack(self, ref);
}
DCHECK_EQ(ref->GetReadBarrierState(), ReadBarrier::GrayState());
return ref;
}
// For the Baker-style RB, in a rare case, we could incorrectly change the object from non-gray
// (black) to gray even though the object has already been marked through. This happens if a
// mutator thread gets preempted before the AtomicSetReadBarrierState below, GC marks through the
// object (changes it from non-gray (white) to gray and back to non-gray (black)), and the thread
// runs and incorrectly changes it from non-gray (black) to gray. If this happens, the object
// will get added to the mark stack again and get changed back to non-gray (black) after it is
// processed.
if (kUseBakerReadBarrier) {
// Test the bitmap first to avoid graying an object that has already been marked through most
// of the time.
if (bitmap->Test(ref)) {
return ref;
}
}
// This may or may not succeed, which is ok because the object may already be gray.
bool success = false;
if (kUseBakerReadBarrier) {
// GC will mark the bitmap when popping from mark stack. If only the GC is touching the bitmap
// we can avoid an expensive CAS.
// For the baker case, an object is marked if either the mark bit marked or the bitmap bit is
// set.
success = ref->AtomicSetReadBarrierState(/* expected_rb_state= */ ReadBarrier::NonGrayState(),
/* rb_state= */ ReadBarrier::GrayState());
} else {
success = !bitmap->AtomicTestAndSet(ref);
}
if (success) {
// Newly marked.
if (kUseBakerReadBarrier) {
DCHECK_EQ(ref->GetReadBarrierState(), ReadBarrier::GrayState());
}
PushOntoMarkStack(self, ref);
}
return ref;
}
template<bool kGrayImmuneObject>
inline mirror::Object* ConcurrentCopying::MarkImmuneSpace(Thread* const self,
mirror::Object* ref) {
if (kUseBakerReadBarrier) {
// The GC-running thread doesn't (need to) gray immune objects except when updating thread roots
// in the thread flip on behalf of suspended threads (when gc_grays_immune_objects_ is
// true). Also, a mutator doesn't (need to) gray an immune object after GC has updated all
// immune space objects (when updated_all_immune_objects_ is true).
if (kIsDebugBuild) {
if (self == thread_running_gc_) {
DCHECK(!kGrayImmuneObject ||
updated_all_immune_objects_.load(std::memory_order_relaxed) ||
gc_grays_immune_objects_);
} else {
DCHECK(kGrayImmuneObject);
}
}
if (!kGrayImmuneObject || updated_all_immune_objects_.load(std::memory_order_relaxed)) {
return ref;
}
// This may or may not succeed, which is ok because the object may already be gray.
bool success =
ref->AtomicSetReadBarrierState(/* expected_rb_state= */ ReadBarrier::NonGrayState(),
/* rb_state= */ ReadBarrier::GrayState());
if (success) {
MutexLock mu(self, immune_gray_stack_lock_);
immune_gray_stack_.push_back(ref);
}
}
return ref;
}
template<bool kGrayImmuneObject, bool kNoUnEvac, bool kFromGCThread>
inline mirror::Object* ConcurrentCopying::Mark(Thread* const self,
mirror::Object* from_ref,
mirror::Object* holder,
MemberOffset offset) {
// Cannot have `kNoUnEvac` when Generational CC collection is disabled.
DCHECK_IMPLIES(kNoUnEvac, use_generational_cc_);
if (from_ref == nullptr) {
return nullptr;
}
DCHECK(heap_->collector_type_ == kCollectorTypeCC);
if (kFromGCThread) {
DCHECK(is_active_);
DCHECK_EQ(self, thread_running_gc_);
} else if (UNLIKELY(kUseBakerReadBarrier && !is_active_)) {
// In the lock word forward address state, the read barrier bits
// in the lock word are part of the stored forwarding address and
// invalid. This is usually OK as the from-space copy of objects
// aren't accessed by mutators due to the to-space
// invariant. However, during the dex2oat image writing relocation
// and the zygote compaction, objects can be in the forward
// address state (to store the forward/relocation addresses) and
// they can still be accessed and the invalid read barrier bits
// are consulted. If they look like gray but aren't really, the
// read barriers slow path can trigger when it shouldn't. To guard
// against this, return here if the CC collector isn't running.
return from_ref;
}
DCHECK(region_space_ != nullptr) << "Read barrier slow path taken when CC isn't running?";
if (region_space_->HasAddress(from_ref)) {
space::RegionSpace::RegionType rtype = region_space_->GetRegionTypeUnsafe(from_ref);
switch (rtype) {
case space::RegionSpace::RegionType::kRegionTypeToSpace:
// It's already marked.
return from_ref;
case space::RegionSpace::RegionType::kRegionTypeFromSpace: {
mirror::Object* to_ref = GetFwdPtr(from_ref);
if (to_ref == nullptr) {
// It isn't marked yet. Mark it by copying it to the to-space.
to_ref = Copy(self, from_ref, holder, offset);
}
// The copy should either be in a to-space region, or in the
// non-moving space, if it could not fit in a to-space region.
DCHECK(region_space_->IsInToSpace(to_ref) || heap_->non_moving_space_->HasAddress(to_ref))
<< "from_ref=" << from_ref << " to_ref=" << to_ref;
return to_ref;
}
case space::RegionSpace::RegionType::kRegionTypeUnevacFromSpace:
if (kNoUnEvac && use_generational_cc_ && !region_space_->IsLargeObject(from_ref)) {
if (!kFromGCThread) {
DCHECK(IsMarkedInUnevacFromSpace(from_ref)) << "Returning unmarked object to mutator";
}
return from_ref;
}
return MarkUnevacFromSpaceRegion(self, from_ref, region_space_bitmap_);
default:
// The reference is in an unused region. Remove memory protection from
// the region space and log debugging information.
region_space_->Unprotect();
LOG(FATAL_WITHOUT_ABORT) << DumpHeapReference(holder, offset, from_ref);
region_space_->DumpNonFreeRegions(LOG_STREAM(FATAL_WITHOUT_ABORT));
heap_->GetVerification()->LogHeapCorruption(holder, offset, from_ref, /* fatal= */ true);
UNREACHABLE();
}
} else {
if (immune_spaces_.ContainsObject(from_ref)) {
return MarkImmuneSpace<kGrayImmuneObject>(self, from_ref);
} else {
return MarkNonMoving(self, from_ref, holder, offset);
}
}
}
inline mirror::Object* ConcurrentCopying::MarkFromReadBarrier(mirror::Object* from_ref) {
mirror::Object* ret;
Thread* const self = Thread::Current();
// We can get here before marking starts since we gray immune objects before the marking phase.
if (from_ref == nullptr || !self->GetIsGcMarking()) {
return from_ref;
}
// TODO: Consider removing this check when we are done investigating slow paths. b/30162165
if (UNLIKELY(mark_from_read_barrier_measurements_)) {
ret = MarkFromReadBarrierWithMeasurements(self, from_ref);
} else {
ret = Mark</*kGrayImmuneObject=*/true, /*kNoUnEvac=*/false, /*kFromGCThread=*/false>(self,
from_ref);
}
// Only set the mark bit for baker barrier.
if (kUseBakerReadBarrier && LIKELY(!rb_mark_bit_stack_full_ && ret->AtomicSetMarkBit(0, 1))) {
// If the mark stack is full, we may temporarily go to mark and back to unmarked. Seeing both
// values are OK since the only race is doing an unnecessary Mark.
if (!rb_mark_bit_stack_->AtomicPushBack(ret)) {
// Mark stack is full, set the bit back to zero.
CHECK(ret->AtomicSetMarkBit(1, 0));
// Set rb_mark_bit_stack_full_, this is racy but OK since AtomicPushBack is thread safe.
rb_mark_bit_stack_full_ = true;
}
}
return ret;
}
inline mirror::Object* ConcurrentCopying::GetFwdPtrUnchecked(mirror::Object* from_ref) {
LockWord lw = from_ref->GetLockWord(false);
if (lw.GetState() == LockWord::kForwardingAddress) {
mirror::Object* fwd_ptr = reinterpret_cast<mirror::Object*>(lw.ForwardingAddress());
DCHECK(fwd_ptr != nullptr);
return fwd_ptr;
} else {
return nullptr;
}
}
inline mirror::Object* ConcurrentCopying::GetFwdPtr(mirror::Object* from_ref) {
DCHECK(region_space_->IsInFromSpace(from_ref));
return GetFwdPtrUnchecked(from_ref);
}
inline bool ConcurrentCopying::IsMarkedInUnevacFromSpace(mirror::Object* from_ref) {
// Use load-acquire on the read barrier pointer to ensure that we never see a black (non-gray)
// read barrier state with an unmarked bit due to reordering.
DCHECK(region_space_->IsInUnevacFromSpace(from_ref));
if (kUseBakerReadBarrier && from_ref->GetReadBarrierStateAcquire() == ReadBarrier::GrayState()) {
return true;
} else if (!use_generational_cc_ || done_scanning_.load(std::memory_order_acquire)) {
// If the card table scanning is not finished yet, then only read-barrier
// state should be checked. Checking the mark bitmap is unreliable as there
// may be some objects - whose corresponding card is dirty - which are
// marked in the mark bitmap, but cannot be considered marked unless their
// read-barrier state is set to Gray.
//
// Why read read-barrier state before checking done_scanning_?
// If the read-barrier state was read *after* done_scanning_, then there
// exists a concurrency race due to which even after the object is marked,
// read-barrier state is checked *after* that, this function will return
// false. The following scenario may cause the race:
//
// 1. Mutator thread reads done_scanning_ and upon finding it false, gets
// suspended before reading the object's read-barrier state.
// 2. GC thread finishes card-table scan and then sets done_scanning_ to
// true.
// 3. GC thread grays the object, scans it, marks in the bitmap, and then
// changes its read-barrier state back to non-gray.
// 4. Mutator thread resumes, reads the object's read-barrier state and
// returns false.
return region_space_bitmap_->Test(from_ref);
}
return false;
}
} // namespace collector
} // namespace gc
} // namespace art
#endif // ART_RUNTIME_GC_COLLECTOR_CONCURRENT_COPYING_INL_H_