base/lazy_instance.cc

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