src/core/SkSharedMutex.cpp - skia - Git at Google

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */

 #include "SkSharedMutex.h"

 #include "SkTypes.h"
 #include "SkSemaphore.h"

 #if !defined(__has_feature)
     #define __has_feature(x) 0
 #endif

 #if __has_feature(thread_sanitizer)

     /* Report that a lock has been created at address "lock". */
     #define ANNOTATE_RWLOCK_CREATE(lock) \
         AnnotateRWLockCreate(__FILE__, __LINE__, lock)

     /* Report that the lock at address "lock" is about to be destroyed. */
     #define ANNOTATE_RWLOCK_DESTROY(lock) \
         AnnotateRWLockDestroy(__FILE__, __LINE__, lock)

     /* Report that the lock at address "lock" has been acquired.
        is_w=1 for writer lock, is_w=0 for reader lock. */
     #define ANNOTATE_RWLOCK_ACQUIRED(lock, is_w) \
         AnnotateRWLockAcquired(__FILE__, __LINE__, lock, is_w)

     /* Report that the lock at address "lock" is about to be released. */
     #define ANNOTATE_RWLOCK_RELEASED(lock, is_w) \
       AnnotateRWLockReleased(__FILE__, __LINE__, lock, is_w)

     #if defined(DYNAMIC_ANNOTATIONS_WANT_ATTRIBUTE_WEAK)
         #if defined(__GNUC__)
             #define DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK __attribute__((weak))
         #else
             /* TODO(glider): for Windows support we may want to change this macro in order
                to prepend __declspec(selectany) to the annotations' declarations. */
             #error weak annotations are not supported for your compiler
         #endif
     #else
         #define DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK
     #endif

     extern "C" {
     void AnnotateRWLockCreate(
         const char *file, int line,
         const volatile void *lock) DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK;
     void AnnotateRWLockDestroy(
         const char *file, int line,
         const volatile void *lock) DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK;
     void AnnotateRWLockAcquired(
         const char *file, int line,
         const volatile void *lock, long is_w) DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK;
     void AnnotateRWLockReleased(
         const char *file, int line,
         const volatile void *lock, long is_w) DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK;
     }

 #else

     #define ANNOTATE_RWLOCK_CREATE(lock)
     #define ANNOTATE_RWLOCK_DESTROY(lock)
     #define ANNOTATE_RWLOCK_ACQUIRED(lock, is_w)
     #define ANNOTATE_RWLOCK_RELEASED(lock, is_w)

 #endif

 #ifdef SK_DEBUG

     #include "SkThreadID.h"
     #include "SkTDArray.h"

     class SkSharedMutex::ThreadIDSet {
     public:
         // Returns true if threadID is in the set.
         bool find(SkThreadID threadID) const {
             for (auto& t : fThreadIDs) {
                 if (t == threadID) return true;
             }
             return false;
         }

         // Returns true if did not already exist.
         bool tryAdd(SkThreadID threadID) {
             for (auto& t : fThreadIDs) {
                 if (t == threadID) return false;
             }
             fThreadIDs.append(1, &threadID);
             return true;
         }
         // Returns true if already exists in Set.
         bool tryRemove(SkThreadID threadID) {
             for (int i = 0; i < fThreadIDs.count(); ++i) {
                 if (fThreadIDs[i] == threadID) {
                     fThreadIDs.remove(i);
                     return true;
                 }
             }
             return false;
         }

         void swap(ThreadIDSet& other) {
             fThreadIDs.swap(other.fThreadIDs);
         }

         int count() const {
             return fThreadIDs.count();
         }

     private:
         SkTDArray<SkThreadID> fThreadIDs;
     };

     SkSharedMutex::SkSharedMutex()
         : fCurrentShared(new ThreadIDSet)
         , fWaitingExclusive(new ThreadIDSet)
         , fWaitingShared(new ThreadIDSet){
         ANNOTATE_RWLOCK_CREATE(this);
     }

     SkSharedMutex::~SkSharedMutex() {  ANNOTATE_RWLOCK_DESTROY(this); }

     void SkSharedMutex::acquire() {
         SkThreadID threadID(SkGetThreadID());
         int currentSharedCount;
         int waitingExclusiveCount;
         {
             SkAutoMutexAcquire l(&fMu);

             SkASSERTF(!fCurrentShared->find(threadID),
                       "Thread %lx already has an shared lock\n", threadID);

             if (!fWaitingExclusive->tryAdd(threadID)) {
                 SkDEBUGFAILF("Thread %lx already has an exclusive lock\n", threadID);
             }

             currentSharedCount = fCurrentShared->count();
             waitingExclusiveCount = fWaitingExclusive->count();
         }

         if (currentSharedCount > 0 || waitingExclusiveCount > 1) {
             fExclusiveQueue.wait();
         }

         ANNOTATE_RWLOCK_ACQUIRED(this, 1);
     }

     // Implementation Detail:
     // The shared threads need two seperate queues to keep the threads that were added after the
     // exclusive lock separate from the threads added before.
     void SkSharedMutex::release() {
         ANNOTATE_RWLOCK_RELEASED(this, 1);
         SkThreadID threadID(SkGetThreadID());
         int sharedWaitingCount;
         int exclusiveWaitingCount;
         int sharedQueueSelect;
         {
             SkAutoMutexAcquire l(&fMu);
             SkASSERT(0 == fCurrentShared->count());
             if (!fWaitingExclusive->tryRemove(threadID)) {
                 SkDEBUGFAILF("Thread %lx did not have the lock held.\n", threadID);
             }
             exclusiveWaitingCount = fWaitingExclusive->count();
             sharedWaitingCount = fWaitingShared->count();
             fWaitingShared.swap(fCurrentShared);
             sharedQueueSelect = fSharedQueueSelect;
             if (sharedWaitingCount > 0) {
                 fSharedQueueSelect = 1 - fSharedQueueSelect;
             }
         }

         if (sharedWaitingCount > 0) {
             fSharedQueue[sharedQueueSelect].signal(sharedWaitingCount);
         } else if (exclusiveWaitingCount > 0) {
             fExclusiveQueue.signal();
         }
     }

     void SkSharedMutex::assertHeld() const {
         SkThreadID threadID(SkGetThreadID());
         SkAutoMutexAcquire l(&fMu);
         SkASSERT(0 == fCurrentShared->count());
         SkASSERT(fWaitingExclusive->find(threadID));
     }

     void SkSharedMutex::acquireShared() {
         SkThreadID threadID(SkGetThreadID());
         int exclusiveWaitingCount;
         int sharedQueueSelect;
         {
             SkAutoMutexAcquire l(&fMu);
             exclusiveWaitingCount = fWaitingExclusive->count();
             if (exclusiveWaitingCount > 0) {
                 if (!fWaitingShared->tryAdd(threadID)) {
                     SkDEBUGFAILF("Thread %lx was already waiting!\n", threadID);
                 }
             } else {
                 if (!fCurrentShared->tryAdd(threadID)) {
                     SkDEBUGFAILF("Thread %lx already holds a shared lock!\n", threadID);
                 }
             }
             sharedQueueSelect = fSharedQueueSelect;
         }

         if (exclusiveWaitingCount > 0) {
             fSharedQueue[sharedQueueSelect].wait();
         }

         ANNOTATE_RWLOCK_ACQUIRED(this, 0);
     }

     void SkSharedMutex::releaseShared() {
         ANNOTATE_RWLOCK_RELEASED(this, 0);
         SkThreadID threadID(SkGetThreadID());

         int currentSharedCount;
         int waitingExclusiveCount;
         {
             SkAutoMutexAcquire l(&fMu);
             if (!fCurrentShared->tryRemove(threadID)) {
                 SkDEBUGFAILF("Thread %lx does not hold a shared lock.\n", threadID);
             }
             currentSharedCount = fCurrentShared->count();
             waitingExclusiveCount = fWaitingExclusive->count();
         }

         if (0 == currentSharedCount && waitingExclusiveCount > 0) {
             fExclusiveQueue.signal();
         }
     }

     void SkSharedMutex::assertHeldShared() const {
         SkThreadID threadID(SkGetThreadID());
         SkAutoMutexAcquire l(&fMu);
         SkASSERT(fCurrentShared->find(threadID));
     }

 #else

     // The fQueueCounts fields holds many counts in an int32_t in order to make managing them atomic.
     // These three counts must be the same size, so each gets 10 bits. The 10 bits represent
     // the log of the count which is 1024.
     //
     // The three counts held in fQueueCounts are:
     // * Shared - the number of shared lock holders currently running.
     // * WaitingExclusive - the number of threads waiting for an exclusive lock.
     // * WaitingShared - the number of threads waiting to run while waiting for an exclusive thread
     //   to finish.
     static const int kLogThreadCount = 10;

     enum {
         kSharedOffset          = (0 * kLogThreadCount),
         kWaitingExlusiveOffset = (1 * kLogThreadCount),
         kWaitingSharedOffset   = (2 * kLogThreadCount),
         kSharedMask            = ((1 << kLogThreadCount) - 1) << kSharedOffset,
         kWaitingExclusiveMask  = ((1 << kLogThreadCount) - 1) << kWaitingExlusiveOffset,
         kWaitingSharedMask     = ((1 << kLogThreadCount) - 1) << kWaitingSharedOffset,
     };

     SkSharedMutex::SkSharedMutex() : fQueueCounts(0) { ANNOTATE_RWLOCK_CREATE(this); }
     SkSharedMutex::~SkSharedMutex() {  ANNOTATE_RWLOCK_DESTROY(this); }
     void SkSharedMutex::acquire() {
         // Increment the count of exclusive queue waiters.
         int32_t oldQueueCounts = fQueueCounts.fetch_add(1 << kWaitingExlusiveOffset,
                                                         std::memory_order_acquire);

         // If there are no other exclusive waiters and no shared threads are running then run
         // else wait.
         if ((oldQueueCounts & kWaitingExclusiveMask) > 0 || (oldQueueCounts & kSharedMask) > 0) {
             fExclusiveQueue.wait();
         }
         ANNOTATE_RWLOCK_ACQUIRED(this, 1);
     }

     void SkSharedMutex::release() {
         ANNOTATE_RWLOCK_RELEASED(this, 1);

         int32_t oldQueueCounts = fQueueCounts.load(std::memory_order_relaxed);
         int32_t waitingShared;
         int32_t newQueueCounts;
         do {
             newQueueCounts = oldQueueCounts;

             // Decrement exclusive waiters.
             newQueueCounts -= 1 << kWaitingExlusiveOffset;

             // The number of threads waiting to acquire a shared lock.
             waitingShared = (oldQueueCounts & kWaitingSharedMask) >> kWaitingSharedOffset;

             // If there are any move the counts of all the shared waiters to actual shared. They are
             // going to run next.
             if (waitingShared > 0) {

                 // Set waiting shared to zero.
                 newQueueCounts &= ~kWaitingSharedMask;

                 // Because this is the exclusive release, then there are zero readers. So, the bits
                 // for shared locks should be zero. Since those bits are zero, we can just |= in the
                 // waitingShared count instead of clearing with an &= and then |= the count.
                 newQueueCounts |= waitingShared << kSharedOffset;
             }

         } while (!fQueueCounts.compare_exchange_strong(oldQueueCounts, newQueueCounts,
                                                        std::memory_order_release,
                                                        std::memory_order_relaxed));

         if (waitingShared > 0) {
             // Run all the shared.
             fSharedQueue.signal(waitingShared);
         } else if ((newQueueCounts & kWaitingExclusiveMask) > 0) {
             // Run a single exclusive waiter.
             fExclusiveQueue.signal();
         }
     }

     void SkSharedMutex::acquireShared() {
         int32_t oldQueueCounts = fQueueCounts.load(std::memory_order_relaxed);
         int32_t newQueueCounts;
         do {
             newQueueCounts = oldQueueCounts;
             // If there are waiting exclusives then this shared lock waits else it runs.
             if ((newQueueCounts & kWaitingExclusiveMask) > 0) {
                 newQueueCounts += 1 << kWaitingSharedOffset;
             } else {
                 newQueueCounts += 1 << kSharedOffset;
             }
         } while (!fQueueCounts.compare_exchange_strong(oldQueueCounts, newQueueCounts,
                                                        std::memory_order_acquire,
                                                        std::memory_order_relaxed));

         // If there are waiting exclusives, then this shared waits until after it runs.
         if ((newQueueCounts & kWaitingExclusiveMask) > 0) {
             fSharedQueue.wait();
         }
         ANNOTATE_RWLOCK_ACQUIRED(this, 0);

     }

     void SkSharedMutex::releaseShared() {
         ANNOTATE_RWLOCK_RELEASED(this, 0);

         // Decrement the shared count.
         int32_t oldQueueCounts = fQueueCounts.fetch_sub(1 << kSharedOffset,
                                                         std::memory_order_release);

         // If shared count is going to zero (because the old count == 1) and there are exclusive
         // waiters, then run a single exclusive waiter.
         if (((oldQueueCounts & kSharedMask) >> kSharedOffset) == 1
             && (oldQueueCounts & kWaitingExclusiveMask) > 0) {
             fExclusiveQueue.signal();
         }
     }

 #endif
	/*
	* Copyright 2015 Google Inc.
	*
	* Use of this source code is governed by a BSD-style license that can be
	* found in the LICENSE file.
	*/

	#include "SkSharedMutex.h"

	#include "SkTypes.h"
	#include "SkSemaphore.h"

	#if !defined(__has_feature)
	#define __has_feature(x) 0
	#endif

	#if __has_feature(thread_sanitizer)

	/* Report that a lock has been created at address "lock". */
	#define ANNOTATE_RWLOCK_CREATE(lock) \
	AnnotateRWLockCreate(__FILE__, __LINE__, lock)

	/* Report that the lock at address "lock" is about to be destroyed. */
	#define ANNOTATE_RWLOCK_DESTROY(lock) \
	AnnotateRWLockDestroy(__FILE__, __LINE__, lock)

	/* Report that the lock at address "lock" has been acquired.
	is_w=1 for writer lock, is_w=0 for reader lock. */
	#define ANNOTATE_RWLOCK_ACQUIRED(lock, is_w) \
	AnnotateRWLockAcquired(__FILE__, __LINE__, lock, is_w)

	/* Report that the lock at address "lock" is about to be released. */
	#define ANNOTATE_RWLOCK_RELEASED(lock, is_w) \
	AnnotateRWLockReleased(__FILE__, __LINE__, lock, is_w)

	#if defined(DYNAMIC_ANNOTATIONS_WANT_ATTRIBUTE_WEAK)
	#if defined(__GNUC__)
	#define DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK __attribute__((weak))
	#else
	/* TODO(glider): for Windows support we may want to change this macro in order
	to prepend __declspec(selectany) to the annotations' declarations. */
	#error weak annotations are not supported for your compiler
	#endif
	#else
	#define DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK
	#endif

	extern "C" {
	void AnnotateRWLockCreate(
	const char *file, int line,
	const volatile void *lock) DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK;
	void AnnotateRWLockDestroy(
	const char *file, int line,
	const volatile void *lock) DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK;
	void AnnotateRWLockAcquired(
	const char *file, int line,
	const volatile void *lock, long is_w) DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK;
	void AnnotateRWLockReleased(
	const char *file, int line,
	const volatile void *lock, long is_w) DYNAMIC_ANNOTATIONS_ATTRIBUTE_WEAK;
	}

	#else

	#define ANNOTATE_RWLOCK_CREATE(lock)
	#define ANNOTATE_RWLOCK_DESTROY(lock)
	#define ANNOTATE_RWLOCK_ACQUIRED(lock, is_w)
	#define ANNOTATE_RWLOCK_RELEASED(lock, is_w)

	#endif

	#ifdef SK_DEBUG

	#include "SkThreadID.h"
	#include "SkTDArray.h"

	class SkSharedMutex::ThreadIDSet {
	public:
	// Returns true if threadID is in the set.
	bool find(SkThreadID threadID) const {
	for (auto& t : fThreadIDs) {
	if (t == threadID) return true;
	}
	return false;
	}

	// Returns true if did not already exist.
	bool tryAdd(SkThreadID threadID) {
	for (auto& t : fThreadIDs) {
	if (t == threadID) return false;
	}
	fThreadIDs.append(1, &threadID);
	return true;
	}
	// Returns true if already exists in Set.
	bool tryRemove(SkThreadID threadID) {
	for (int i = 0; i < fThreadIDs.count(); ++i) {
	if (fThreadIDs[i] == threadID) {
	fThreadIDs.remove(i);
	return true;
	}
	}
	return false;
	}

	void swap(ThreadIDSet& other) {
	fThreadIDs.swap(other.fThreadIDs);
	}

	int count() const {
	return fThreadIDs.count();
	}

	private:
	SkTDArray<SkThreadID> fThreadIDs;
	};

	SkSharedMutex::SkSharedMutex()
	: fCurrentShared(new ThreadIDSet)
	, fWaitingExclusive(new ThreadIDSet)
	, fWaitingShared(new ThreadIDSet){
	ANNOTATE_RWLOCK_CREATE(this);
	}

	SkSharedMutex::~SkSharedMutex() { ANNOTATE_RWLOCK_DESTROY(this); }

	void SkSharedMutex::acquire() {
	SkThreadID threadID(SkGetThreadID());
	int currentSharedCount;
	int waitingExclusiveCount;
	{
	SkAutoMutexAcquire l(&fMu);

	SkASSERTF(!fCurrentShared->find(threadID),
	"Thread %lx already has an shared lock\n", threadID);

	if (!fWaitingExclusive->tryAdd(threadID)) {
	SkDEBUGFAILF("Thread %lx already has an exclusive lock\n", threadID);
	}

	currentSharedCount = fCurrentShared->count();
	waitingExclusiveCount = fWaitingExclusive->count();
	}

	if (currentSharedCount > 0 \|\| waitingExclusiveCount > 1) {
	fExclusiveQueue.wait();
	}

	ANNOTATE_RWLOCK_ACQUIRED(this, 1);
	}

	// Implementation Detail:
	// The shared threads need two seperate queues to keep the threads that were added after the
	// exclusive lock separate from the threads added before.
	void SkSharedMutex::release() {
	ANNOTATE_RWLOCK_RELEASED(this, 1);
	SkThreadID threadID(SkGetThreadID());
	int sharedWaitingCount;
	int exclusiveWaitingCount;
	int sharedQueueSelect;
	{
	SkAutoMutexAcquire l(&fMu);
	SkASSERT(0 == fCurrentShared->count());
	if (!fWaitingExclusive->tryRemove(threadID)) {
	SkDEBUGFAILF("Thread %lx did not have the lock held.\n", threadID);
	}
	exclusiveWaitingCount = fWaitingExclusive->count();
	sharedWaitingCount = fWaitingShared->count();
	fWaitingShared.swap(fCurrentShared);
	sharedQueueSelect = fSharedQueueSelect;
	if (sharedWaitingCount > 0) {
	fSharedQueueSelect = 1 - fSharedQueueSelect;
	}
	}

	if (sharedWaitingCount > 0) {
	fSharedQueue[sharedQueueSelect].signal(sharedWaitingCount);
	} else if (exclusiveWaitingCount > 0) {
	fExclusiveQueue.signal();
	}
	}

	void SkSharedMutex::assertHeld() const {
	SkThreadID threadID(SkGetThreadID());
	SkAutoMutexAcquire l(&fMu);
	SkASSERT(0 == fCurrentShared->count());
	SkASSERT(fWaitingExclusive->find(threadID));
	}

	void SkSharedMutex::acquireShared() {
	SkThreadID threadID(SkGetThreadID());
	int exclusiveWaitingCount;
	int sharedQueueSelect;
	{
	SkAutoMutexAcquire l(&fMu);
	exclusiveWaitingCount = fWaitingExclusive->count();
	if (exclusiveWaitingCount > 0) {
	if (!fWaitingShared->tryAdd(threadID)) {
	SkDEBUGFAILF("Thread %lx was already waiting!\n", threadID);
	}
	} else {
	if (!fCurrentShared->tryAdd(threadID)) {
	SkDEBUGFAILF("Thread %lx already holds a shared lock!\n", threadID);
	}
	}
	sharedQueueSelect = fSharedQueueSelect;
	}

	if (exclusiveWaitingCount > 0) {
	fSharedQueue[sharedQueueSelect].wait();
	}

	ANNOTATE_RWLOCK_ACQUIRED(this, 0);
	}

	void SkSharedMutex::releaseShared() {
	ANNOTATE_RWLOCK_RELEASED(this, 0);
	SkThreadID threadID(SkGetThreadID());

	int currentSharedCount;
	int waitingExclusiveCount;
	{
	SkAutoMutexAcquire l(&fMu);
	if (!fCurrentShared->tryRemove(threadID)) {
	SkDEBUGFAILF("Thread %lx does not hold a shared lock.\n", threadID);
	}
	currentSharedCount = fCurrentShared->count();
	waitingExclusiveCount = fWaitingExclusive->count();
	}

	if (0 == currentSharedCount && waitingExclusiveCount > 0) {
	fExclusiveQueue.signal();
	}
	}

	void SkSharedMutex::assertHeldShared() const {
	SkThreadID threadID(SkGetThreadID());
	SkAutoMutexAcquire l(&fMu);
	SkASSERT(fCurrentShared->find(threadID));
	}

	#else

	// The fQueueCounts fields holds many counts in an int32_t in order to make managing them atomic.
	// These three counts must be the same size, so each gets 10 bits. The 10 bits represent
	// the log of the count which is 1024.
	//
	// The three counts held in fQueueCounts are:
	// * Shared - the number of shared lock holders currently running.
	// * WaitingExclusive - the number of threads waiting for an exclusive lock.
	// * WaitingShared - the number of threads waiting to run while waiting for an exclusive thread
	// to finish.
	static const int kLogThreadCount = 10;

	enum {
	kSharedOffset = (0 * kLogThreadCount),
	kWaitingExlusiveOffset = (1 * kLogThreadCount),
	kWaitingSharedOffset = (2 * kLogThreadCount),
	kSharedMask = ((1 << kLogThreadCount) - 1) << kSharedOffset,
	kWaitingExclusiveMask = ((1 << kLogThreadCount) - 1) << kWaitingExlusiveOffset,
	kWaitingSharedMask = ((1 << kLogThreadCount) - 1) << kWaitingSharedOffset,
	};

	SkSharedMutex::SkSharedMutex() : fQueueCounts(0) { ANNOTATE_RWLOCK_CREATE(this); }
	SkSharedMutex::~SkSharedMutex() { ANNOTATE_RWLOCK_DESTROY(this); }
	void SkSharedMutex::acquire() {
	// Increment the count of exclusive queue waiters.
	int32_t oldQueueCounts = fQueueCounts.fetch_add(1 << kWaitingExlusiveOffset,
	std::memory_order_acquire);

	// If there are no other exclusive waiters and no shared threads are running then run
	// else wait.
	if ((oldQueueCounts & kWaitingExclusiveMask) > 0 \|\| (oldQueueCounts & kSharedMask) > 0) {
	fExclusiveQueue.wait();
	}
	ANNOTATE_RWLOCK_ACQUIRED(this, 1);
	}

	void SkSharedMutex::release() {
	ANNOTATE_RWLOCK_RELEASED(this, 1);

	int32_t oldQueueCounts = fQueueCounts.load(std::memory_order_relaxed);
	int32_t waitingShared;
	int32_t newQueueCounts;
	do {
	newQueueCounts = oldQueueCounts;

	// Decrement exclusive waiters.
	newQueueCounts -= 1 << kWaitingExlusiveOffset;

	// The number of threads waiting to acquire a shared lock.
	waitingShared = (oldQueueCounts & kWaitingSharedMask) >> kWaitingSharedOffset;

	// If there are any move the counts of all the shared waiters to actual shared. They are
	// going to run next.
	if (waitingShared > 0) {

	// Set waiting shared to zero.
	newQueueCounts &= ~kWaitingSharedMask;

	// Because this is the exclusive release, then there are zero readers. So, the bits
	// for shared locks should be zero. Since those bits are zero, we can just \|= in the
	// waitingShared count instead of clearing with an &= and then \|= the count.
	newQueueCounts \|= waitingShared << kSharedOffset;
	}

	} while (!fQueueCounts.compare_exchange_strong(oldQueueCounts, newQueueCounts,
	std::memory_order_release,
	std::memory_order_relaxed));

	if (waitingShared > 0) {
	// Run all the shared.
	fSharedQueue.signal(waitingShared);
	} else if ((newQueueCounts & kWaitingExclusiveMask) > 0) {
	// Run a single exclusive waiter.
	fExclusiveQueue.signal();
	}
	}

	void SkSharedMutex::acquireShared() {
	int32_t oldQueueCounts = fQueueCounts.load(std::memory_order_relaxed);
	int32_t newQueueCounts;
	do {
	newQueueCounts = oldQueueCounts;
	// If there are waiting exclusives then this shared lock waits else it runs.
	if ((newQueueCounts & kWaitingExclusiveMask) > 0) {
	newQueueCounts += 1 << kWaitingSharedOffset;
	} else {
	newQueueCounts += 1 << kSharedOffset;
	}
	} while (!fQueueCounts.compare_exchange_strong(oldQueueCounts, newQueueCounts,
	std::memory_order_acquire,
	std::memory_order_relaxed));

	// If there are waiting exclusives, then this shared waits until after it runs.
	if ((newQueueCounts & kWaitingExclusiveMask) > 0) {
	fSharedQueue.wait();
	}
	ANNOTATE_RWLOCK_ACQUIRED(this, 0);

	}

	void SkSharedMutex::releaseShared() {
	ANNOTATE_RWLOCK_RELEASED(this, 0);

	// Decrement the shared count.
	int32_t oldQueueCounts = fQueueCounts.fetch_sub(1 << kSharedOffset,
	std::memory_order_release);

	// If shared count is going to zero (because the old count == 1) and there are exclusive
	// waiters, then run a single exclusive waiter.
	if (((oldQueueCounts & kSharedMask) >> kSharedOffset) == 1
	&& (oldQueueCounts & kWaitingExclusiveMask) > 0) {
	fExclusiveQueue.signal();
	}
	}

	#endif