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atomic-entry-flag.h
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1 // Copyright 2020 the V8 project authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4 
5 #ifndef INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
6 #define INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
7 
8 #include <atomic>
9 
10 namespace cppgc {
11 namespace internal {
12 
13 // A flag which provides a fast check whether a scope may be entered on the
14 // current thread, without needing to access thread-local storage or mutex. Can
15 // have false positives (i.e., spuriously report that it might be entered), so
16 // it is expected that this will be used in tandem with a precise check that the
17 // scope is in fact entered on that thread.
18 //
19 // Example:
20 // g_frobnicating_flag.MightBeEntered() &&
21 // ThreadLocalFrobnicator().IsFrobnicating()
22 //
23 // Relaxed atomic operations are sufficient, since:
24 // - all accesses remain atomic
25 // - each thread must observe its own operations in order
26 // - no thread ever exits the flag more times than it enters (if used correctly)
27 // And so if a thread observes zero, it must be because it has observed an equal
28 // number of exits as entries.
29 class AtomicEntryFlag final {
30  public:
31  void Enter() { entries_.fetch_add(1, std::memory_order_relaxed); }
32  void Exit() { entries_.fetch_sub(1, std::memory_order_relaxed); }
33 
34  // Returns false only if the current thread is not between a call to Enter
35  // and a call to Exit. Returns true if this thread or another thread may
36  // currently be in the scope guarded by this flag.
37  bool MightBeEntered() const {
38  return entries_.load(std::memory_order_relaxed) != 0;
39  }
40 
41  private:
42  std::atomic_int entries_{0};
43 };
44 
45 } // namespace internal
46 } // namespace cppgc
47 
48 #endif // INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_