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// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/lazy_instance.h"
#include "base/at_exit.h"
#include "base/atomicops.h"
#include "base/basictypes.h"
#include "base/threading/platform_thread.h"
#include "base/third_party/dynamic_annotations/dynamic_annotations.h"
namespace base {
bool LazyInstanceHelper::NeedsInstance() {
// Try to create the instance, if we're the first, will go from EMPTY
// to CREATING, otherwise we've already been beaten here.
// The memory access has no memory ordering as STATE_EMPTY and STATE_CREATING
// has no associated data (memory barriers are all about ordering
// of memory accesses to *associated* data).
if (base::subtle::NoBarrier_CompareAndSwap(
&state_, STATE_EMPTY, STATE_CREATING) == STATE_EMPTY)
// Caller must create instance
return true;
// It's either in the process of being created, or already created. Spin.
// The load has acquire memory ordering as a thread which sees
// state_ == STATE_CREATED needs to acquire visibility over
// the associated data (buf_). Pairing Release_Store is in
// CompleteInstance().
while (base::subtle::Acquire_Load(&state_) != STATE_CREATED)
PlatformThread::YieldCurrentThread();
// Someone else created the instance.
return false;
}
void LazyInstanceHelper::CompleteInstance(void* instance, void (*dtor)(void*)) {
// See the comment to the corresponding HAPPENS_AFTER in Pointer().
ANNOTATE_HAPPENS_BEFORE(&state_);
// Instance is created, go from CREATING to CREATED.
// Releases visibility over buf_ to readers. Pairing Acquire_Load's are in
// NeedsInstance() and Pointer().
base::subtle::Release_Store(&state_, STATE_CREATED);
// Make sure that the lazily instantiated object will get destroyed at exit.
if (dtor)
base::AtExitManager::RegisterCallback(dtor, instance);
}
} // namespace base
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