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.
NeedsLazyInstance(subtle::AtomicWord * state)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
CompleteLazyInstance(subtle::AtomicWord * state,subtle::AtomicWord new_instance,void * lazy_instance,void (* dtor)(void *))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
60