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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/threading/platform_thread.h"
namespace base {
namespace internal {
// TODO(joth): This function could be shared with Singleton, in place of its
// WaitForInstance() call.
bool NeedsLazyInstance(subtle::AtomicWord* state) {
// Try to create the instance, if we're the first, will go from 0 to
// kLazyInstanceStateCreating, otherwise we've already been beaten here.
// The memory access has no memory ordering as state 0 and
// kLazyInstanceStateCreating have no associated data (memory barriers are
// all about ordering of memory accesses to *associated* data).
if (subtle::NoBarrier_CompareAndSwap(state, 0,
kLazyInstanceStateCreating) == 0)
// 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
// CompleteLazyInstance().
while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) {
PlatformThread::YieldCurrentThread();
}
// Someone else created the instance.
return false;
}
void CompleteLazyInstance(subtle::AtomicWord* state,
subtle::AtomicWord new_instance,
void* lazy_instance,
void (*dtor)(void*)) {
// Instance is created, go from CREATING to CREATED.
// Releases visibility over private_buf_ to readers. Pairing Acquire_Load's
// are in NeedsInstance() and Pointer().
subtle::Release_Store(state, new_instance);
// Make sure that the lazily instantiated object will get destroyed at exit.
if (dtor)
AtExitManager::RegisterCallback(dtor, lazy_instance);
}
} // namespace internal
} // namespace base
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