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