singleton.cc revision a1401311d1ab56c4ed0a474bd38c108f75cb0cd9 
1// Copyright (c) 2011 The Chromium 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#include "base/memory/singleton.h"
6#include "base/threading/platform_thread.h"
7
8namespace base {
9namespace internal {
10
11subtle::AtomicWord WaitForInstance(subtle::AtomicWord* instance) {
12  // Handle the race. Another thread beat us and either:
13  // - Has the object in BeingCreated state
14  // - Already has the object created...
15  // We know value != NULL.  It could be kBeingCreatedMarker, or a valid ptr.
16  // Unless your constructor can be very time consuming, it is very unlikely
17  // to hit this race.  When it does, we just spin and yield the thread until
18  // the object has been created.
19  subtle::AtomicWord value;
20  while (true) {
21    // The load has acquire memory ordering as the thread which reads the
22    // instance pointer must acquire visibility over the associated data.
23    // The pairing Release_Store operation is in Singleton::get().
24    value = subtle::Acquire_Load(instance);
25    if (value != kBeingCreatedMarker)
26      break;
27    PlatformThread::YieldCurrentThread();
28  }
29  return value;
30}
31
32}  // namespace internal
33}  // namespace base
34
35