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/*
* Copyright (C) 2015 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#ifndef WTF_Lock_h
#define WTF_Lock_h
#include <wtf/Atomics.h>
#include <wtf/Compiler.h>
#include <wtf/Locker.h>
#include <wtf/Noncopyable.h>
namespace TestWebKitAPI {
struct LockInspector;
};
namespace WTF {
// This is a fully adaptive mutex that only requires 1 byte of storage. It has fast paths that are
// competetive to a spinlock (uncontended locking is inlined and is just a CAS, microcontention is
// handled by spinning and yielding), and a slow path that is competetive to std::mutex (if a lock
// cannot be acquired in a short period of time, the thread is put to sleep until the lock is available
// again). It uses less memory than a std::mutex.
// This is a struct without a constructor or destructor so that it can be statically initialized.
// Use Lock in instance variables.
struct LockBase {
void lock()
{
if (LIKELY(m_byte.compareExchangeWeak(0, isHeldBit, std::memory_order_acquire))) {
// Lock acquired!
return;
}
lockSlow();
}
bool tryLock()
{
for (;;) {
uint8_t currentByteValue = m_byte.load();
if (currentByteValue & isHeldBit)
return false;
if (m_byte.compareExchangeWeak(currentByteValue, currentByteValue | isHeldBit))
return true;
}
}
// Need this version for std::unique_lock.
bool try_lock()
{
return tryLock();
}
void unlock()
{
if (LIKELY(m_byte.compareExchangeWeak(isHeldBit, 0, std::memory_order_release))) {
// Lock released and nobody was waiting!
return;
}
unlockSlow();
}
bool isHeld() const
{
return m_byte.load(std::memory_order_acquire) & isHeldBit;
}
bool isLocked() const
{
return isHeld();
}
protected:
friend struct TestWebKitAPI::LockInspector;
static const uint8_t isHeldBit = 1;
static const uint8_t hasParkedBit = 2;
WTF_EXPORT_PRIVATE void lockSlow();
WTF_EXPORT_PRIVATE void unlockSlow();
// Method used for testing only.
bool isFullyReset() const
{
return !m_byte.load();
}
Atomic<uint8_t> m_byte;
};
class Lock : public LockBase {
WTF_MAKE_NONCOPYABLE(Lock);
WTF_MAKE_FAST_ALLOCATED;
public:
Lock()
{
m_byte.store(0, std::memory_order_relaxed);
}
};
typedef LockBase StaticLock;
typedef Locker<LockBase> LockHolder;
} // namespace WTF
using WTF::Lock;
using WTF::LockHolder;
using WTF::StaticLock;
#endif // WTF_Lock_h
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