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