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/*
* Copyright (C) 2015-2019 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.
*/
#pragma once
#include <wtf/Atomics.h>
#include <wtf/Compiler.h>
#include <wtf/Locker.h>
#include <wtf/Noncopyable.h>
namespace TestWebKitAPI {
struct LockInspector;
};
namespace WTF {
// A WordLock is a fully adaptive mutex that uses sizeof(void*) storage. It has a fast path that is
// similar to a spinlock, and a slow path that is similar to std::mutex. In most cases, you should use
// Lock instead. WordLock sits lower in the stack and is used to implement Lock, so Lock is the main
// client of WordLock.
// NOTE: This is also a great lock to use if you are very low in the stack. For example,
// PrintStream uses this so that ParkingLot and Lock can use PrintStream. This means that if you
// try to use dataLog to debug this code, you will have a bad time.
class WordLock final {
WTF_MAKE_NONCOPYABLE(WordLock);
WTF_MAKE_FAST_ALLOCATED;
public:
constexpr WordLock() = default;
void lock()
{
if (LIKELY(m_word.compareExchangeWeak(0, isLockedBit, std::memory_order_acquire))) {
// WordLock acquired!
return;
}
lockSlow();
}
void unlock()
{
if (LIKELY(m_word.compareExchangeWeak(isLockedBit, 0, std::memory_order_release))) {
// WordLock released, and nobody was waiting!
return;
}
unlockSlow();
}
bool isHeld() const
{
return m_word.load(std::memory_order_acquire) & isLockedBit;
}
bool isLocked() const
{
return isHeld();
}
protected:
friend struct TestWebKitAPI::LockInspector;
static constexpr uintptr_t isLockedBit = 1;
static constexpr uintptr_t isQueueLockedBit = 2;
static constexpr uintptr_t queueHeadMask = 3;
WTF_EXPORT_PRIVATE void lockSlow();
WTF_EXPORT_PRIVATE void unlockSlow();
// Method used for testing only.
bool isFullyReset() const
{
return !m_word.load();
}
Atomic<uintptr_t> m_word { 0 };
};
using WordLockHolder = Locker<WordLock>;
} // namespace WTF
using WTF::WordLock;
using WTF::WordLockHolder;