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// Copyright 2018 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 UTIL_AUTO_RESET_EVENT_H_
#define UTIL_AUTO_RESET_EVENT_H_
#include <atomic>
#include "base/logging.h"
#include "util/semaphore.h"
// From http://preshing.com/20150316/semaphores-are-surprisingly-versatile/,
// but using V8's Semaphore.
class AutoResetEvent {
private:
// status_ == 1: Event object is signaled.
// status_ == 0: Event object is reset and no threads are waiting.
// status_ == -N: Event object is reset and N threads are waiting.
std::atomic<int> status_;
Semaphore semaphore_;
public:
AutoResetEvent() : status_(0), semaphore_(0) {}
void Signal() {
int old_status = status_.load(std::memory_order_relaxed);
// Increment status_ atomically via CAS loop.
for (;;) {
DCHECK_LE(old_status, 1);
int new_status = old_status < 1 ? old_status + 1 : 1;
if (status_.compare_exchange_weak(old_status, new_status,
std::memory_order_release,
std::memory_order_relaxed)) {
break;
}
// The compare-exchange failed, likely because another thread changed
// status_. old_status has been updated. Retry the CAS loop.
}
if (old_status < 0)
semaphore_.Signal(); // Release one waiting thread.
}
void Wait() {
int old_status = status_.fetch_sub(1, std::memory_order_acquire);
DCHECK_LE(old_status, 1);
if (old_status < 1) {
semaphore_.Wait();
}
}
};
#endif // UTIL_AUTO_RESET_EVENT_H_
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