1 // Copyright 2011 The Chromium Authors 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 // This is a low level implementation of atomic semantics for reference 6 // counting. Please use base/memory/ref_counted.h directly instead. 7 8 #ifndef PARTITION_ALLOC_PARTITION_ALLOC_BASE_ATOMIC_REF_COUNT_H_ 9 #define PARTITION_ALLOC_PARTITION_ALLOC_BASE_ATOMIC_REF_COUNT_H_ 10 11 #include <atomic> 12 13 namespace partition_alloc::internal::base { 14 15 class AtomicRefCount { 16 public: AtomicRefCount()17 constexpr AtomicRefCount() : ref_count_(0) {} AtomicRefCount(int initial_value)18 explicit constexpr AtomicRefCount(int initial_value) 19 : ref_count_(initial_value) {} 20 21 // Increment a reference count. 22 // Returns the previous value of the count. Increment()23 int Increment() { return Increment(1); } 24 25 // Increment a reference count by "increment", which must exceed 0. 26 // Returns the previous value of the count. Increment(int increment)27 int Increment(int increment) { 28 return ref_count_.fetch_add(increment, std::memory_order_relaxed); 29 } 30 31 // Decrement a reference count, and return whether the result is non-zero. 32 // Insert barriers to ensure that state written before the reference count 33 // became zero will be visible to a thread that has just made the count zero. Decrement()34 bool Decrement() { 35 // TODO(jbroman): Technically this doesn't need to be an acquire operation 36 // unless the result is 1 (i.e., the ref count did indeed reach zero). 37 // However, there are toolchain issues that make that not work as well at 38 // present (notably TSAN doesn't like it). 39 return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1; 40 } 41 42 // Return whether the reference count is one. If the reference count is used 43 // in the conventional way, a reference count of 1 implies that the current 44 // thread owns the reference and no other thread shares it. This call 45 // performs the test for a reference count of one, and performs the memory 46 // barrier needed for the owning thread to act on the object, knowing that it 47 // has exclusive access to the object. IsOne()48 bool IsOne() const { return ref_count_.load(std::memory_order_acquire) == 1; } 49 50 // Return whether the reference count is zero. With conventional object 51 // referencing counting, the object will be destroyed, so the reference count 52 // should never be zero. Hence this is generally used for a debug check. IsZero()53 bool IsZero() const { 54 return ref_count_.load(std::memory_order_acquire) == 0; 55 } 56 57 // Returns the current reference count (with no barriers). This is subtle, and 58 // should be used only for debugging. SubtleRefCountForDebug()59 int SubtleRefCountForDebug() const { 60 return ref_count_.load(std::memory_order_relaxed); 61 } 62 63 private: 64 std::atomic_int ref_count_; 65 }; 66 67 } // namespace partition_alloc::internal::base 68 69 #endif // PARTITION_ALLOC_PARTITION_ALLOC_BASE_ATOMIC_REF_COUNT_H_ 70