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
11 namespace base {
12 namespace internal {
13
14 // TODO(joth): This function could be shared with Singleton, in place of its
15 // WaitForInstance() call.
NeedsLazyInstance(subtle::AtomicWord * state)16 bool 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
CompleteLazyInstance(subtle::AtomicWord * state,subtle::AtomicWord new_instance,void * lazy_instance,void (* dtor)(void *))39 void 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