base/memory/singleton.cc

   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
   8 namespace base {
   9 namespace internal {
  10
  11 subtle::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