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/*
* Copyright 2017 The WebRTC Project Authors. All rights reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#ifndef RTC_BASE_REF_COUNTER_H_
#define RTC_BASE_REF_COUNTER_H_
#include <atomic>
#include "rtc_base/ref_count.h"
namespace webrtc {
namespace webrtc_impl {
class RefCounter {
public:
explicit RefCounter(int ref_count) : ref_count_(ref_count) {}
RefCounter() = delete;
void IncRef() {
// Relaxed memory order: The current thread is allowed to act on the
// resource protected by the reference counter both before and after the
// atomic op, so this function doesn't prevent memory access reordering.
ref_count_.fetch_add(1, std::memory_order_relaxed);
}
// Returns kDroppedLastRef if this call dropped the last reference; the caller
// should therefore free the resource protected by the reference counter.
// Otherwise, returns kOtherRefsRemained (note that in case of multithreading,
// some other caller may have dropped the last reference by the time this call
// returns; all we know is that we didn't do it).
rtc::RefCountReleaseStatus DecRef() {
// Use release-acquire barrier to ensure all actions on the protected
// resource are finished before the resource can be freed.
// When ref_count_after_subtract > 0, this function require
// std::memory_order_release part of the barrier.
// When ref_count_after_subtract == 0, this function require
// std::memory_order_acquire part of the barrier.
// In addition std::memory_order_release is used for synchronization with
// the HasOneRef function to make sure all actions on the protected resource
// are finished before the resource is assumed to have exclusive access.
int ref_count_after_subtract =
ref_count_.fetch_sub(1, std::memory_order_acq_rel) - 1;
return ref_count_after_subtract == 0
? rtc::RefCountReleaseStatus::kDroppedLastRef
: rtc::RefCountReleaseStatus::kOtherRefsRemained;
}
// Return whether the reference count is one. If the reference count is used
// in the conventional way, a reference count of 1 implies that the current
// thread owns the reference and no other thread shares it. This call performs
// the test for a reference count of one, and performs the memory barrier
// needed for the owning thread to act on the resource protected by the
// reference counter, knowing that it has exclusive access.
bool HasOneRef() const {
// To ensure resource protected by the reference counter has exclusive
// access, all changes to the resource before it was released by other
// threads must be visible by current thread. That is provided by release
// (in DecRef) and acquire (in this function) ordering.
return ref_count_.load(std::memory_order_acquire) == 1;
}
private:
std::atomic<int> ref_count_;
};
} // namespace webrtc_impl
} // namespace webrtc
#endif // RTC_BASE_REF_COUNTER_H_
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