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/*
* Copyright 2015 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkSharedLock_DEFINED
#define SkSharedLock_DEFINED
#include "SkAtomics.h"
#include "SkMacros.h"
#include "SkSemaphore.h"
#include "SkTypes.h"
#ifdef SK_DEBUG
#include "SkMutex.h"
#include <memory>
#endif // SK_DEBUG
// There are two shared lock implementations one debug the other is high performance. They implement
// an interface similar to pthread's rwlocks.
// This is a shared lock implementation similar to pthreads rwlocks. The high performance
// implementation is cribbed from Preshing's article:
// http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
//
// This lock does not obey strict queue ordering. It will always alternate between readers and
// a single writer.
class SkSharedMutex {
public:
SkSharedMutex();
~SkSharedMutex();
// Acquire lock for exclusive use.
void acquire();
// Release lock for exclusive use.
void release();
// Fail if exclusive is not held.
void assertHeld() const;
// Acquire lock for shared use.
void acquireShared();
// Release lock for shared use.
void releaseShared();
// Fail if shared lock not held.
void assertHeldShared() const;
private:
#ifdef SK_DEBUG
class ThreadIDSet;
std::unique_ptr<ThreadIDSet> fCurrentShared;
std::unique_ptr<ThreadIDSet> fWaitingExclusive;
std::unique_ptr<ThreadIDSet> fWaitingShared;
int fSharedQueueSelect{0};
mutable SkMutex fMu;
SkSemaphore fSharedQueue[2];
SkSemaphore fExclusiveQueue;
#else
SkAtomic<int32_t> fQueueCounts;
SkSemaphore fSharedQueue;
SkSemaphore fExclusiveQueue;
#endif // SK_DEBUG
};
#ifndef SK_DEBUG
inline void SkSharedMutex::assertHeld() const {};
inline void SkSharedMutex::assertHeldShared() const {};
#endif // SK_DEBUG
class SkAutoSharedMutexShared {
public:
SkAutoSharedMutexShared(SkSharedMutex& lock) : fLock(lock) { lock.acquireShared(); }
~SkAutoSharedMutexShared() { fLock.releaseShared(); }
private:
SkSharedMutex& fLock;
};
#define SkAutoSharedMutexShared(...) SK_REQUIRE_LOCAL_VAR(SkAutoSharedMutexShared)
#endif // SkSharedLock_DEFINED
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