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
13namespace base {
14namespace internal {
15
16// TODO(joth): This function could be shared with Singleton, in place of its
17// WaitForInstance() call.
18bool 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
41void 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
60