| // Copyright 2018 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #ifndef THIRD_PARTY_BLINK_RENDERER_PLATFORM_HEAP_IMPL_ATOMIC_ENTRY_FLAG_H_ |
| #define THIRD_PARTY_BLINK_RENDERER_PLATFORM_HEAP_IMPL_ATOMIC_ENTRY_FLAG_H_ |
| |
| #include <atomic> |
| |
| #include "third_party/blink/renderer/platform/wtf/allocator/allocator.h" |
| |
| namespace blink { |
| |
| // A flag which provides a fast check whether a scope may be entered on the |
| // current thread, without needing to access thread-local storage or mutex. |
| // |
| // Can have false positives (i.e., spuriously report that it might be entered), |
| // so it is expected that this will be used in tandem with a precise check that |
| // the scope is in fact entered on that thread. |
| // |
| // Example: |
| // g_frobnicating_flag.MightBeEntered() && |
| // ThreadLocalFrobnicator().IsFrobnicating() |
| // |
| // Relaxed atomic operations are sufficient, since: |
| // - all accesses remain atomic |
| // - each thread must observe its own operations in order |
| // - no thread ever exits the flag more times than it enters (if used correctly) |
| // And so if a thread observes zero, it must be because it has observed an equal |
| // number of exits as entries. |
| class AtomicEntryFlag { |
| DISALLOW_NEW(); |
| |
| public: |
| inline void Enter() { entries_.fetch_add(1, std::memory_order_relaxed); } |
| inline void Exit() { entries_.fetch_sub(1, std::memory_order_relaxed); } |
| |
| // Returns false only if the current thread is not between a call to Enter and |
| // a call to Exit. Returns true if this thread or another thread may currently |
| // be in the scope guarded by this flag. |
| inline bool MightBeEntered() const { |
| return entries_.load(std::memory_order_relaxed) != 0; |
| } |
| |
| private: |
| std::atomic_int entries_{0}; |
| }; |
| |
| } // namespace blink |
| |
| #endif // THIRD_PARTY_BLINK_RENDERER_PLATFORM_HEAP_IMPL_ATOMIC_ENTRY_FLAG_H_ |