libs/ui/FenceTime.cpp - android_frameworks_native - Gitiles

 /*
 * Copyright 2016 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 <ui/FenceTime.h>

 #define LOG_TAG "FenceTime"

 #include <cutils/compiler.h>  // For CC_[UN]LIKELY
 #include <utils/Log.h>
 #include <inttypes.h>
 #include <stdlib.h>

 #include <memory>

 namespace android {

 // ============================================================================
 // FenceTime
 // ============================================================================

 const auto FenceTime::NO_FENCE = std::make_shared<FenceTime>(Fence::NO_FENCE);

 void* FenceTime::operator new(size_t byteCount) noexcept {
     void *p = nullptr;
     if (posix_memalign(&p, alignof(FenceTime), byteCount)) {
         return nullptr;
     }
     return p;
 }

 void FenceTime::operator delete(void *p) {
     free(p);
 }

 FenceTime::FenceTime(const sp<Fence>& fence)
   : mState(((fence.get() != nullptr) && fence->isValid()) ?
             State::VALID : State::INVALID),
     mFence(fence),
     mSignalTime(mState == State::INVALID ?
             Fence::SIGNAL_TIME_INVALID : Fence::SIGNAL_TIME_PENDING) {
 }

 FenceTime::FenceTime(sp<Fence>&& fence)
   : mState(((fence.get() != nullptr) && fence->isValid()) ?
             State::VALID : State::INVALID),
     mFence(std::move(fence)),
     mSignalTime(mState == State::INVALID ?
             Fence::SIGNAL_TIME_INVALID : Fence::SIGNAL_TIME_PENDING) {
 }

 FenceTime::FenceTime(nsecs_t signalTime)
   : mState(Fence::isValidTimestamp(signalTime) ? State::VALID : State::INVALID),
     mFence(nullptr),
     mSignalTime(signalTime) {
     if (CC_UNLIKELY(mSignalTime == Fence::SIGNAL_TIME_PENDING)) {
         ALOGE("Pending signal time not allowed after signal.");
         mSignalTime = Fence::SIGNAL_TIME_INVALID;
     }
 }

 void FenceTime::applyTrustedSnapshot(const Snapshot& src) {
     if (CC_UNLIKELY(src.state != Snapshot::State::SIGNAL_TIME)) {
         // Applying Snapshot::State::FENCE, could change the valid state of the
         // FenceTime, which is not allowed. Callers should create a new
         // FenceTime from the snapshot instead.
         ALOGE("applyTrustedSnapshot: Unexpected fence.");
         return;
     }

     if (src.state == Snapshot::State::EMPTY) {
         return;
     }

     nsecs_t signalTime = mSignalTime.load(std::memory_order_relaxed);
     if (signalTime != Fence::SIGNAL_TIME_PENDING) {
         // We should always get the same signalTime here that we did in
         // getSignalTime(). This check races with getSignalTime(), but it is
         // only a sanity check so that's okay.
         if (CC_UNLIKELY(signalTime != src.signalTime)) {
             ALOGE("FenceTime::applyTrustedSnapshot: signalTime mismatch. "
                     "(%" PRId64 " (old) != %" PRId64 " (new))",
                     signalTime, src.signalTime);
         }
         return;
     }

     std::lock_guard<std::mutex> lock(mMutex);
     mFence.clear();
     mSignalTime.store(src.signalTime, std::memory_order_relaxed);
 }

 bool FenceTime::isValid() const {
     // We store the valid state in the constructors and return it here.
     // This lets release code remember the valid state even after the
     // underlying fence is destroyed.
     return mState != State::INVALID;
 }

 nsecs_t FenceTime::getSignalTime() {
     // See if we already have a cached value we can return.
     nsecs_t signalTime = mSignalTime.load(std::memory_order_relaxed);
     if (signalTime != Fence::SIGNAL_TIME_PENDING) {
         return signalTime;
     }

     // Hold a reference to the fence on the stack in case the class'
     // reference is removed by another thread. This prevents the
     // fence from being destroyed until the end of this method, where
     // we conveniently do not have the lock held.
     sp<Fence> fence;
     {
         // With the lock acquired this time, see if we have the cached
         // value or if we need to poll the fence.
         std::lock_guard<std::mutex> lock(mMutex);
         if (!mFence.get()) {
             // Another thread set the signal time just before we added the
             // reference to mFence.
             return mSignalTime.load(std::memory_order_relaxed);
         }
         fence = mFence;
     }

     // Make the system call without the lock held.
     signalTime = fence->getSignalTime();

     // Allow tests to override SIGNAL_TIME_INVALID behavior, since tests
     // use invalid underlying Fences without real file descriptors.
     if (CC_UNLIKELY(mState == State::FORCED_VALID_FOR_TEST)) {
         if (signalTime == Fence::SIGNAL_TIME_INVALID) {
             signalTime = Fence::SIGNAL_TIME_PENDING;
         }
     }

     // Make the signal time visible to everyone if it is no longer pending
     // and remove the class' reference to the fence.
     if (signalTime != Fence::SIGNAL_TIME_PENDING) {
         std::lock_guard<std::mutex> lock(mMutex);
         mFence.clear();
         mSignalTime.store(signalTime, std::memory_order_relaxed);
     }

     return signalTime;
 }

 nsecs_t FenceTime::getCachedSignalTime() const {
     // memory_order_acquire since we don't have a lock fallback path
     // that will do an acquire.
     return mSignalTime.load(std::memory_order_acquire);
 }

 FenceTime::Snapshot FenceTime::getSnapshot() const {
     // Quick check without the lock.
     nsecs_t signalTime = mSignalTime.load(std::memory_order_relaxed);
     if (signalTime != Fence::SIGNAL_TIME_PENDING) {
         return Snapshot(signalTime);
     }

     // Do the full check with the lock.
     std::lock_guard<std::mutex> lock(mMutex);
     signalTime = mSignalTime.load(std::memory_order_relaxed);
     if (signalTime != Fence::SIGNAL_TIME_PENDING) {
         return Snapshot(signalTime);
     }
     return Snapshot(mFence);
 }

 // For tests only. If forceValidForTest is true, then getSignalTime will
 // never return SIGNAL_TIME_INVALID and isValid will always return true.
 FenceTime::FenceTime(const sp<Fence>& fence, bool forceValidForTest)
   : mState(forceValidForTest ?
             State::FORCED_VALID_FOR_TEST : State::INVALID),
     mFence(fence),
     mSignalTime(mState == State::INVALID ?
             Fence::SIGNAL_TIME_INVALID : Fence::SIGNAL_TIME_PENDING) {
 }

 void FenceTime::signalForTest(nsecs_t signalTime) {
     // To be realistic, this should really set a hidden value that
     // gets picked up in the next call to getSignalTime, but this should
     // be good enough.
     std::lock_guard<std::mutex> lock(mMutex);
     mFence.clear();
     mSignalTime.store(signalTime, std::memory_order_relaxed);
 }

 // ============================================================================
 // FenceTime::Snapshot
 // ============================================================================
 FenceTime::Snapshot::Snapshot(const sp<Fence>& srcFence)
     : state(State::FENCE), fence(srcFence) {
 }

 FenceTime::Snapshot::Snapshot(nsecs_t srcSignalTime)
     : state(State::SIGNAL_TIME), signalTime(srcSignalTime) {
 }

 size_t FenceTime::Snapshot::getFlattenedSize() const {
     constexpr size_t min = sizeof(state);
     switch (state) {
         case State::EMPTY:
             return min;
         case State::FENCE:
             return min + fence->getFlattenedSize();
         case State::SIGNAL_TIME:
             return min + sizeof(signalTime);
     }
     return 0;
 }

 size_t FenceTime::Snapshot::getFdCount() const {
     return state == State::FENCE ? fence->getFdCount() : 0u;
 }

 status_t FenceTime::Snapshot::flatten(
         void*& buffer, size_t& size, int*& fds, size_t& count) const {
     if (size < getFlattenedSize()) {
         return NO_MEMORY;
     }

     FlattenableUtils::write(buffer, size, state);
     switch (state) {
         case State::EMPTY:
             return NO_ERROR;
         case State::FENCE:
             return fence->flatten(buffer, size, fds, count);
         case State::SIGNAL_TIME:
             FlattenableUtils::write(buffer, size, signalTime);
             return NO_ERROR;
     }

     return NO_ERROR;
 }

 status_t FenceTime::Snapshot::unflatten(
         void const*& buffer, size_t& size, int const*& fds, size_t& count) {
     if (size < sizeof(state)) {
         return NO_MEMORY;
     }

     FlattenableUtils::read(buffer, size, state);
     switch (state) {
         case State::EMPTY:
             return NO_ERROR;
         case State::FENCE:
             fence = new Fence;
             return fence->unflatten(buffer, size, fds, count);
         case State::SIGNAL_TIME:
             if (size < sizeof(signalTime)) {
                 return NO_MEMORY;
             }
             FlattenableUtils::read(buffer, size, signalTime);
             return NO_ERROR;
     }

     return NO_ERROR;
 }

 // ============================================================================
 // FenceTimeline
 // ============================================================================
 void FenceTimeline::push(const std::shared_ptr<FenceTime>& fence) {
     std::lock_guard<std::mutex> lock(mMutex);
     while (mQueue.size() >= MAX_ENTRIES) {
         // This is a sanity check to make sure the queue doesn't grow unbounded.
         // MAX_ENTRIES should be big enough not to trigger this path.
         // In case this path is taken though, users of FenceTime must make sure
         // not to rely solely on FenceTimeline to get the final timestamp and
         // should eventually call Fence::getSignalTime on their own.
         std::shared_ptr<FenceTime> front = mQueue.front().lock();
         if (front) {
             // Make a last ditch effort to get the signalTime here since
             // we are removing it from the timeline.
             front->getSignalTime();
         }
         mQueue.pop();
     }
     mQueue.push(fence);
 }

 void FenceTimeline::updateSignalTimes() {
     while (!mQueue.empty()) {
         std::lock_guard<std::mutex> lock(mMutex);
         std::shared_ptr<FenceTime> fence = mQueue.front().lock();
         if (!fence) {
             // The shared_ptr no longer exists and no one cares about the
             // timestamp anymore.
             mQueue.pop();
             continue;
         } else if (fence->getSignalTime() != Fence::SIGNAL_TIME_PENDING) {
             // The fence has signaled and we've removed the sp<Fence> ref.
             mQueue.pop();
             continue;
         } else {
             // The fence didn't signal yet. Break since the later ones
             // shouldn't have signaled either.
             break;
         }
     }
 }

 // ============================================================================
 // FenceToFenceTimeMap
 // ============================================================================
 std::shared_ptr<FenceTime> FenceToFenceTimeMap::createFenceTimeForTest(
         const sp<Fence>& fence) {
     std::lock_guard<std::mutex> lock(mMutex);
     // Always garbage collecting isn't efficient, but this is only for testing.
     garbageCollectLocked();
     std::shared_ptr<FenceTime> fenceTime(new FenceTime(fence, true));
     mMap[fence.get()].push_back(fenceTime);
     return fenceTime;
 }

 void FenceToFenceTimeMap::signalAllForTest(
         const sp<Fence>& fence, nsecs_t signalTime) {
     bool signaled = false;

     std::lock_guard<std::mutex> lock(mMutex);
     auto it = mMap.find(fence.get());
     if (it != mMap.end()) {
         for (auto& weakFenceTime : it->second) {
             std::shared_ptr<FenceTime> fenceTime = weakFenceTime.lock();
             if (!fenceTime) {
                 continue;
             }
             ALOGE_IF(!fenceTime->isValid(),
                     "signalAllForTest: Signaling invalid fence.");
             fenceTime->signalForTest(signalTime);
             signaled = true;
         }
     }

     ALOGE_IF(!signaled, "signalAllForTest: Nothing to signal.");
 }

 void FenceToFenceTimeMap::garbageCollectLocked() {
     for (auto& it : mMap) {
         // Erase all expired weak pointers from the vector.
         auto& vect = it.second;
         vect.erase(
                 std::remove_if(vect.begin(), vect.end(),
                         [](const std::weak_ptr<FenceTime>& ft) {
                             return ft.expired();
                         }),
                 vect.end());

         // Also erase the map entry if the vector is now empty.
         if (vect.empty()) {
             mMap.erase(it.first);
         }
     }
 }

 } // namespace android
	/*
	* Copyright 2016 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 <ui/FenceTime.h>

	#define LOG_TAG "FenceTime"

	#include <cutils/compiler.h> // For CC_[UN]LIKELY
	#include <utils/Log.h>
	#include <inttypes.h>
	#include <stdlib.h>

	#include <memory>

	namespace android {

	// ============================================================================
	// FenceTime
	// ============================================================================

	const auto FenceTime::NO_FENCE = std::make_shared<FenceTime>(Fence::NO_FENCE);

	void* FenceTime::operator new(size_t byteCount) noexcept {
	void *p = nullptr;
	if (posix_memalign(&p, alignof(FenceTime), byteCount)) {
	return nullptr;
	}
	return p;
	}

	void FenceTime::operator delete(void *p) {
	free(p);
	}

	FenceTime::FenceTime(const sp<Fence>& fence)
	: mState(((fence.get() != nullptr) && fence->isValid()) ?
	State::VALID : State::INVALID),
	mFence(fence),
	mSignalTime(mState == State::INVALID ?
	Fence::SIGNAL_TIME_INVALID : Fence::SIGNAL_TIME_PENDING) {
	}

	FenceTime::FenceTime(sp<Fence>&& fence)
	: mState(((fence.get() != nullptr) && fence->isValid()) ?
	State::VALID : State::INVALID),
	mFence(std::move(fence)),
	mSignalTime(mState == State::INVALID ?
	Fence::SIGNAL_TIME_INVALID : Fence::SIGNAL_TIME_PENDING) {
	}

	FenceTime::FenceTime(nsecs_t signalTime)
	: mState(Fence::isValidTimestamp(signalTime) ? State::VALID : State::INVALID),
	mFence(nullptr),
	mSignalTime(signalTime) {
	if (CC_UNLIKELY(mSignalTime == Fence::SIGNAL_TIME_PENDING)) {
	ALOGE("Pending signal time not allowed after signal.");
	mSignalTime = Fence::SIGNAL_TIME_INVALID;
	}
	}

	void FenceTime::applyTrustedSnapshot(const Snapshot& src) {
	if (CC_UNLIKELY(src.state != Snapshot::State::SIGNAL_TIME)) {
	// Applying Snapshot::State::FENCE, could change the valid state of the
	// FenceTime, which is not allowed. Callers should create a new
	// FenceTime from the snapshot instead.
	ALOGE("applyTrustedSnapshot: Unexpected fence.");
	return;
	}

	if (src.state == Snapshot::State::EMPTY) {
	return;
	}

	nsecs_t signalTime = mSignalTime.load(std::memory_order_relaxed);
	if (signalTime != Fence::SIGNAL_TIME_PENDING) {
	// We should always get the same signalTime here that we did in
	// getSignalTime(). This check races with getSignalTime(), but it is
	// only a sanity check so that's okay.
	if (CC_UNLIKELY(signalTime != src.signalTime)) {
	ALOGE("FenceTime::applyTrustedSnapshot: signalTime mismatch. "
	"(%" PRId64 " (old) != %" PRId64 " (new))",
	signalTime, src.signalTime);
	}
	return;
	}

	std::lock_guard<std::mutex> lock(mMutex);
	mFence.clear();
	mSignalTime.store(src.signalTime, std::memory_order_relaxed);
	}

	bool FenceTime::isValid() const {
	// We store the valid state in the constructors and return it here.
	// This lets release code remember the valid state even after the
	// underlying fence is destroyed.
	return mState != State::INVALID;
	}

	nsecs_t FenceTime::getSignalTime() {
	// See if we already have a cached value we can return.
	nsecs_t signalTime = mSignalTime.load(std::memory_order_relaxed);
	if (signalTime != Fence::SIGNAL_TIME_PENDING) {
	return signalTime;
	}

	// Hold a reference to the fence on the stack in case the class'
	// reference is removed by another thread. This prevents the
	// fence from being destroyed until the end of this method, where
	// we conveniently do not have the lock held.
	sp<Fence> fence;
	{
	// With the lock acquired this time, see if we have the cached
	// value or if we need to poll the fence.
	std::lock_guard<std::mutex> lock(mMutex);
	if (!mFence.get()) {
	// Another thread set the signal time just before we added the
	// reference to mFence.
	return mSignalTime.load(std::memory_order_relaxed);
	}
	fence = mFence;
	}

	// Make the system call without the lock held.
	signalTime = fence->getSignalTime();

	// Allow tests to override SIGNAL_TIME_INVALID behavior, since tests
	// use invalid underlying Fences without real file descriptors.
	if (CC_UNLIKELY(mState == State::FORCED_VALID_FOR_TEST)) {
	if (signalTime == Fence::SIGNAL_TIME_INVALID) {
	signalTime = Fence::SIGNAL_TIME_PENDING;
	}
	}

	// Make the signal time visible to everyone if it is no longer pending
	// and remove the class' reference to the fence.
	if (signalTime != Fence::SIGNAL_TIME_PENDING) {
	std::lock_guard<std::mutex> lock(mMutex);
	mFence.clear();
	mSignalTime.store(signalTime, std::memory_order_relaxed);
	}

	return signalTime;
	}

	nsecs_t FenceTime::getCachedSignalTime() const {
	// memory_order_acquire since we don't have a lock fallback path
	// that will do an acquire.
	return mSignalTime.load(std::memory_order_acquire);
	}

	FenceTime::Snapshot FenceTime::getSnapshot() const {
	// Quick check without the lock.
	nsecs_t signalTime = mSignalTime.load(std::memory_order_relaxed);
	if (signalTime != Fence::SIGNAL_TIME_PENDING) {
	return Snapshot(signalTime);
	}

	// Do the full check with the lock.
	std::lock_guard<std::mutex> lock(mMutex);
	signalTime = mSignalTime.load(std::memory_order_relaxed);
	if (signalTime != Fence::SIGNAL_TIME_PENDING) {
	return Snapshot(signalTime);
	}
	return Snapshot(mFence);
	}

	// For tests only. If forceValidForTest is true, then getSignalTime will
	// never return SIGNAL_TIME_INVALID and isValid will always return true.
	FenceTime::FenceTime(const sp<Fence>& fence, bool forceValidForTest)
	: mState(forceValidForTest ?
	State::FORCED_VALID_FOR_TEST : State::INVALID),
	mFence(fence),
	mSignalTime(mState == State::INVALID ?
	Fence::SIGNAL_TIME_INVALID : Fence::SIGNAL_TIME_PENDING) {
	}

	void FenceTime::signalForTest(nsecs_t signalTime) {
	// To be realistic, this should really set a hidden value that
	// gets picked up in the next call to getSignalTime, but this should
	// be good enough.
	std::lock_guard<std::mutex> lock(mMutex);
	mFence.clear();
	mSignalTime.store(signalTime, std::memory_order_relaxed);
	}

	// ============================================================================
	// FenceTime::Snapshot
	// ============================================================================
	FenceTime::Snapshot::Snapshot(const sp<Fence>& srcFence)
	: state(State::FENCE), fence(srcFence) {
	}

	FenceTime::Snapshot::Snapshot(nsecs_t srcSignalTime)
	: state(State::SIGNAL_TIME), signalTime(srcSignalTime) {
	}

	size_t FenceTime::Snapshot::getFlattenedSize() const {
	constexpr size_t min = sizeof(state);
	switch (state) {
	case State::EMPTY:
	return min;
	case State::FENCE:
	return min + fence->getFlattenedSize();
	case State::SIGNAL_TIME:
	return min + sizeof(signalTime);
	}
	return 0;
	}

	size_t FenceTime::Snapshot::getFdCount() const {
	return state == State::FENCE ? fence->getFdCount() : 0u;
	}

	status_t FenceTime::Snapshot::flatten(
	void& buffer, size_t& size, int& fds, size_t& count) const {
	if (size < getFlattenedSize()) {
	return NO_MEMORY;
	}

	FlattenableUtils::write(buffer, size, state);
	switch (state) {
	case State::EMPTY:
	return NO_ERROR;
	case State::FENCE:
	return fence->flatten(buffer, size, fds, count);
	case State::SIGNAL_TIME:
	FlattenableUtils::write(buffer, size, signalTime);
	return NO_ERROR;
	}

	return NO_ERROR;
	}

	status_t FenceTime::Snapshot::unflatten(
	void const& buffer, size_t& size, int const& fds, size_t& count) {
	if (size < sizeof(state)) {
	return NO_MEMORY;
	}

	FlattenableUtils::read(buffer, size, state);
	switch (state) {
	case State::EMPTY:
	return NO_ERROR;
	case State::FENCE:
	fence = new Fence;
	return fence->unflatten(buffer, size, fds, count);
	case State::SIGNAL_TIME:
	if (size < sizeof(signalTime)) {
	return NO_MEMORY;
	}
	FlattenableUtils::read(buffer, size, signalTime);
	return NO_ERROR;
	}

	return NO_ERROR;
	}

	// ============================================================================
	// FenceTimeline
	// ============================================================================
	void FenceTimeline::push(const std::shared_ptr<FenceTime>& fence) {
	std::lock_guard<std::mutex> lock(mMutex);
	while (mQueue.size() >= MAX_ENTRIES) {
	// This is a sanity check to make sure the queue doesn't grow unbounded.
	// MAX_ENTRIES should be big enough not to trigger this path.
	// In case this path is taken though, users of FenceTime must make sure
	// not to rely solely on FenceTimeline to get the final timestamp and
	// should eventually call Fence::getSignalTime on their own.
	std::shared_ptr<FenceTime> front = mQueue.front().lock();
	if (front) {
	// Make a last ditch effort to get the signalTime here since
	// we are removing it from the timeline.
	front->getSignalTime();
	}
	mQueue.pop();
	}
	mQueue.push(fence);
	}

	void FenceTimeline::updateSignalTimes() {
	while (!mQueue.empty()) {
	std::lock_guard<std::mutex> lock(mMutex);
	std::shared_ptr<FenceTime> fence = mQueue.front().lock();
	if (!fence) {
	// The shared_ptr no longer exists and no one cares about the
	// timestamp anymore.
	mQueue.pop();
	continue;
	} else if (fence->getSignalTime() != Fence::SIGNAL_TIME_PENDING) {
	// The fence has signaled and we've removed the sp<Fence> ref.
	mQueue.pop();
	continue;
	} else {
	// The fence didn't signal yet. Break since the later ones
	// shouldn't have signaled either.
	break;
	}
	}
	}

	// ============================================================================
	// FenceToFenceTimeMap
	// ============================================================================
	std::shared_ptr<FenceTime> FenceToFenceTimeMap::createFenceTimeForTest(
	const sp<Fence>& fence) {
	std::lock_guard<std::mutex> lock(mMutex);
	// Always garbage collecting isn't efficient, but this is only for testing.
	garbageCollectLocked();
	std::shared_ptr<FenceTime> fenceTime(new FenceTime(fence, true));
	mMap[fence.get()].push_back(fenceTime);
	return fenceTime;
	}

	void FenceToFenceTimeMap::signalAllForTest(
	const sp<Fence>& fence, nsecs_t signalTime) {
	bool signaled = false;

	std::lock_guard<std::mutex> lock(mMutex);
	auto it = mMap.find(fence.get());
	if (it != mMap.end()) {
	for (auto& weakFenceTime : it->second) {
	std::shared_ptr<FenceTime> fenceTime = weakFenceTime.lock();
	if (!fenceTime) {
	continue;
	}
	ALOGE_IF(!fenceTime->isValid(),
	"signalAllForTest: Signaling invalid fence.");
	fenceTime->signalForTest(signalTime);
	signaled = true;
	}
	}

	ALOGE_IF(!signaled, "signalAllForTest: Nothing to signal.");
	}

	void FenceToFenceTimeMap::garbageCollectLocked() {
	for (auto& it : mMap) {
	// Erase all expired weak pointers from the vector.
	auto& vect = it.second;
	vect.erase(
	std::remove_if(vect.begin(), vect.end(),
	[](const std::weak_ptr<FenceTime>& ft) {
	return ft.expired();
	}),
	vect.end());

	// Also erase the map entry if the vector is now empty.
	if (vect.empty()) {
	mMap.erase(it.first);
	}
	}
	}

	} // namespace android