1 /* 2 * Copyright 2017 The WebRTC Project Authors. All rights reserved. 3 * 4 * Use of this source code is governed by a BSD-style license 5 * that can be found in the LICENSE file in the root of the source 6 * tree. An additional intellectual property rights grant can be found 7 * in the file PATENTS. All contributing project authors may 8 * be found in the AUTHORS file in the root of the source tree. 9 */ 10 #ifndef RTC_BASE_REF_COUNTER_H_ 11 #define RTC_BASE_REF_COUNTER_H_ 12 13 #include <atomic> 14 15 #include "rtc_base/ref_count.h" 16 17 namespace webrtc { 18 namespace webrtc_impl { 19 20 class RefCounter { 21 public: RefCounter(int ref_count)22 explicit RefCounter(int ref_count) : ref_count_(ref_count) {} 23 RefCounter() = delete; 24 IncRef()25 void IncRef() { 26 // Relaxed memory order: The current thread is allowed to act on the 27 // resource protected by the reference counter both before and after the 28 // atomic op, so this function doesn't prevent memory access reordering. 29 ref_count_.fetch_add(1, std::memory_order_relaxed); 30 } 31 32 // Returns kDroppedLastRef if this call dropped the last reference; the caller 33 // should therefore free the resource protected by the reference counter. 34 // Otherwise, returns kOtherRefsRemained (note that in case of multithreading, 35 // some other caller may have dropped the last reference by the time this call 36 // returns; all we know is that we didn't do it). DecRef()37 rtc::RefCountReleaseStatus DecRef() { 38 // Use release-acquire barrier to ensure all actions on the protected 39 // resource are finished before the resource can be freed. 40 // When ref_count_after_subtract > 0, this function require 41 // std::memory_order_release part of the barrier. 42 // When ref_count_after_subtract == 0, this function require 43 // std::memory_order_acquire part of the barrier. 44 // In addition std::memory_order_release is used for synchronization with 45 // the HasOneRef function to make sure all actions on the protected resource 46 // are finished before the resource is assumed to have exclusive access. 47 int ref_count_after_subtract = 48 ref_count_.fetch_sub(1, std::memory_order_acq_rel) - 1; 49 return ref_count_after_subtract == 0 50 ? rtc::RefCountReleaseStatus::kDroppedLastRef 51 : rtc::RefCountReleaseStatus::kOtherRefsRemained; 52 } 53 54 // Return whether the reference count is one. If the reference count is used 55 // in the conventional way, a reference count of 1 implies that the current 56 // thread owns the reference and no other thread shares it. This call performs 57 // the test for a reference count of one, and performs the memory barrier 58 // needed for the owning thread to act on the resource protected by the 59 // reference counter, knowing that it has exclusive access. HasOneRef()60 bool HasOneRef() const { 61 // To ensure resource protected by the reference counter has exclusive 62 // access, all changes to the resource before it was released by other 63 // threads must be visible by current thread. That is provided by release 64 // (in DecRef) and acquire (in this function) ordering. 65 return ref_count_.load(std::memory_order_acquire) == 1; 66 } 67 68 private: 69 std::atomic<int> ref_count_; 70 }; 71 72 } // namespace webrtc_impl 73 } // namespace webrtc 74 75 #endif // RTC_BASE_REF_COUNTER_H_ 76