lazy_instance.cc revision 1320f92c476a1ad9d19dba2a48c72b75566198e9 
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
12namespace base {
13namespace internal {
14
15// TODO(joth): This function could be shared with Singleton, in place of its
16// WaitForInstance() call.
17bool NeedsLazyInstance(subtle::AtomicWord* state) {
18  // Try to create the instance, if we're the first, will go from 0 to
19  // kLazyInstanceStateCreating, otherwise we've already been beaten here.
20  // The memory access has no memory ordering as state 0 and
21  // kLazyInstanceStateCreating have no associated data (memory barriers are
22  // all about ordering of memory accesses to *associated* data).
23  if (subtle::NoBarrier_CompareAndSwap(state, 0,
24                                       kLazyInstanceStateCreating) == 0)
25    // Caller must create instance
26    return true;
27
28  // It's either in the process of being created, or already created. Spin.
29  // The load has acquire memory ordering as a thread which sees
30  // state_ == STATE_CREATED needs to acquire visibility over
31  // the associated data (buf_). Pairing Release_Store is in
32  // CompleteLazyInstance().
33  while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) {
34    PlatformThread::YieldCurrentThread();
35  }
36  // Someone else created the instance.
37  return false;
38}
39
40void CompleteLazyInstance(subtle::AtomicWord* state,
41                          subtle::AtomicWord new_instance,
42                          void* lazy_instance,
43                          void (*dtor)(void*)) {
44  // Instance is created, go from CREATING to CREATED.
45  // Releases visibility over private_buf_ to readers. Pairing Acquire_Load's
46  // are in NeedsInstance() and Pointer().
47  subtle::Release_Store(state, new_instance);
48
49  // Make sure that the lazily instantiated object will get destroyed at exit.
50  if (dtor)
51    AtExitManager::RegisterCallback(dtor, lazy_instance);
52}
53
54}  // namespace internal
55}  // namespace base
56