third_party/WebKit/Source/wtf/SpinLock.h

/*
 * Copyright (C) 2013 Google Inc. All rights reserved.
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 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
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 *
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 * 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
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 * 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
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#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_acquire)))
      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