1 // Copyright (c) 2014 Marshall A. Greenblatt. Portions copyright (c) 2011 2 // Google Inc. All rights reserved. 3 // 4 // Redistribution and use in source and binary forms, with or without 5 // modification, are permitted provided that the following conditions are 6 // met: 7 // 8 // * Redistributions of source code must retain the above copyright 9 // notice, this list of conditions and the following disclaimer. 10 // * Redistributions in binary form must reproduce the above 11 // copyright notice, this list of conditions and the following disclaimer 12 // in the documentation and/or other materials provided with the 13 // distribution. 14 // * Neither the name of Google Inc. nor the name Chromium Embedded 15 // Framework nor the names of its contributors may be used to endorse 16 // or promote products derived from this software without specific prior 17 // written permission. 18 // 19 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 20 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 21 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 22 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 23 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 24 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 25 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 26 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 27 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 28 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 29 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 30 31 // This is a low level implementation of atomic semantics for reference 32 // counting. Please use cef_ref_counted.h directly instead. 33 // 34 // The Chromium implementation includes annotations to avoid some false 35 // positives when using data race detection tools. Annotations are not 36 // currently supported by the CEF implementation. 37 38 #ifndef CEF_INCLUDE_BASE_CEF_ATOMIC_REF_COUNT_H_ 39 #define CEF_INCLUDE_BASE_CEF_ATOMIC_REF_COUNT_H_ 40 #pragma once 41 42 #if defined(USING_CHROMIUM_INCLUDES) 43 // When building CEF include the Chromium header directly. 44 #include "base/atomic_ref_count.h" 45 46 #else // !USING_CHROMIUM_INCLUDES 47 // The following is substantially similar to the Chromium implementation. 48 // If the Chromium implementation diverges the below implementation should be 49 // updated to match. 50 51 #include <atomic> 52 53 namespace base { 54 55 class AtomicRefCount { 56 public: AtomicRefCount()57 constexpr AtomicRefCount() : ref_count_(0) {} AtomicRefCount(int initial_value)58 explicit constexpr AtomicRefCount(int initial_value) 59 : ref_count_(initial_value) {} 60 61 // Increment a reference count. 62 // Returns the previous value of the count. Increment()63 int Increment() { return Increment(1); } 64 65 // Increment a reference count by "increment", which must exceed 0. 66 // Returns the previous value of the count. Increment(int increment)67 int Increment(int increment) { 68 return ref_count_.fetch_add(increment, std::memory_order_relaxed); 69 } 70 71 // Decrement a reference count, and return whether the result is non-zero. 72 // Insert barriers to ensure that state written before the reference count 73 // became zero will be visible to a thread that has just made the count zero. Decrement()74 bool Decrement() { 75 // TODO(jbroman): Technically this doesn't need to be an acquire operation 76 // unless the result is 1 (i.e., the ref count did indeed reach zero). 77 // However, there are toolchain issues that make that not work as well at 78 // present (notably TSAN doesn't like it). 79 return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1; 80 } 81 82 // Return whether the reference count is one. If the reference count is used 83 // in the conventional way, a refrerence count of 1 implies that the current 84 // thread owns the reference and no other thread shares it. This call 85 // performs the test for a reference count of one, and performs the memory 86 // barrier needed for the owning thread to act on the object, knowing that it 87 // has exclusive access to the object. IsOne()88 bool IsOne() const { return ref_count_.load(std::memory_order_acquire) == 1; } 89 90 // Return whether the reference count is zero. With conventional object 91 // referencing counting, the object will be destroyed, so the reference count 92 // should never be zero. Hence this is generally used for a debug check. IsZero()93 bool IsZero() const { 94 return ref_count_.load(std::memory_order_acquire) == 0; 95 } 96 97 // Returns the current reference count (with no barriers). This is subtle, and 98 // should be used only for debugging. SubtleRefCountForDebug()99 int SubtleRefCountForDebug() const { 100 return ref_count_.load(std::memory_order_relaxed); 101 } 102 103 private: 104 std::atomic_int ref_count_; 105 }; 106 107 } // namespace base 108 109 #endif // !USING_CHROMIUM_INCLUDES 110 111 #endif // CEF_INCLUDE_BASE_CEF_ATOMIC_REF_COUNT_H_ 112