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/*
* Copyright (C) 2013 Google 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:
*
* * Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* * 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.
* * Neither the name of Google Inc. nor the names of its
* contributors may be used to endorse or promote products derived from
* this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "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 THE COPYRIGHT
* OWNER 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_SpinLock_h
#define WTF_SpinLock_h
#include "wtf/Compiler.h"
#include "wtf/WTFExport.h"
#include <atomic>
#include <memory>
#include <mutex>
// DESCRIPTION
// Spinlock is a simple spinlock class based on the standard CPU primitive of
// atomic increment and decrement of an int at a given memory address. These are
// intended only for very short duration locks and assume a system with multiple
// cores. For any potentially longer wait you should be using a real lock.
namespace WTF {
class SpinLock {
public:
using Guard = std::lock_guard<SpinLock>;
ALWAYS_INLINE void lock()
{
static_assert(sizeof(m_lock) == sizeof(int), "int and m_lock are different sizes");
if (LIKELY(!m_lock.exchange(true, std::memory_order_acq_rel)))
return;
lockSlow();
}
ALWAYS_INLINE void unlock()
{
m_lock.store(false, std::memory_order_release);
}
private:
// This is called if the initial attempt to acquire the lock fails. It's
// slower, but has a much better scheduling and power consumption behavior.
WTF_EXPORT void lockSlow();
std::atomic_int m_lock;
};
} // namespace WTF
using WTF::SpinLock;
#endif // WTF_SpinLock_h
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