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// Copyright (c) 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_SYNCHRONIZATION_SPIN_LOCK_H
#define BASE_SYNCHRONIZATION_SPIN_LOCK_H
#include <atomic>
#include <memory>
#include <mutex>
#include "base/base_export.h"
#include "base/compiler_specific.h"
// 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 use a real lock, such as
// |base::Lock|.
//
// |SpinLock|s MUST be globals. Using them as (e.g.) struct/class members will
// result in an uninitialized lock, which is dangerously incorrect.
namespace base {
namespace subtle {
class SpinLock {
public:
using Guard = std::lock_guard<SpinLock>;
ALWAYS_INLINE void lock() {
static_assert(sizeof(lock_) == sizeof(int),
"int and lock_ are different sizes");
if (LIKELY(!lock_.exchange(true, std::memory_order_acquire)))
return;
LockSlow();
}
ALWAYS_INLINE void unlock() { 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.
BASE_EXPORT void LockSlow();
std::atomic_int lock_;
};
} // namespace subtle
} // namespace base
#endif // BASE_SYNCHRONIZATION_SPIN_LOCK_H
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