base/lazy_instance.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/lazy_instance.h"
   6
   7 #include "base/at_exit.h"
   8 #include "base/atomicops.h"
   9 #include "base/basictypes.h"
  10 #include "base/threading/platform_thread.h"
  11 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
  12
  13 namespace base {
  14 namespace internal {
  15
  16 // TODO(joth): This function could be shared with Singleton, in place of its
  17 // WaitForInstance() call.
  18 bool NeedsLazyInstance(subtle::AtomicWord* state) {
  19   // Try to create the instance, if we're the first, will go from 0 to
  20   // kLazyInstanceStateCreating, otherwise we've already been beaten here.
  21   // The memory access has no memory ordering as state 0 and
  22   // kLazyInstanceStateCreating have no associated data (memory barriers are
  23   // all about ordering of memory accesses to *associated* data).
  24   if (subtle::NoBarrier_CompareAndSwap(state, 0,
  25                                        kLazyInstanceStateCreating) == 0)
  26     // Caller must create instance
  27     return true;
  28
  29   // It's either in the process of being created, or already created. Spin.
  30   // The load has acquire memory ordering as a thread which sees
  31   // state_ == STATE_CREATED needs to acquire visibility over
  32   // the associated data (buf_). Pairing Release_Store is in
  33   // CompleteLazyInstance().
  34   while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) {
  35     PlatformThread::YieldCurrentThread();
  36   }
  37   // Someone else created the instance.
  38   return false;
  39 }
  40
  41 void CompleteLazyInstance(subtle::AtomicWord* state,
  42                           subtle::AtomicWord new_instance,
  43                           void* lazy_instance,
  44                           void (*dtor)(void*)) {
  45   // See the comment to the corresponding HAPPENS_AFTER in Pointer().
  46   ANNOTATE_HAPPENS_BEFORE(state);
  47
  48   // Instance is created, go from CREATING to CREATED.
  49   // Releases visibility over private_buf_ to readers. Pairing Acquire_Load's
  50   // are in NeedsInstance() and Pointer().
  51   subtle::Release_Store(state, new_instance);
  52
  53   // Make sure that the lazily instantiated object will get destroyed at exit.
  54   if (dtor)
  55     AtExitManager::RegisterCallback(dtor, lazy_instance);
  56 }
  57
  58 }  // namespace internal
  59 }  // namespace base