// 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 (*destructor)(void*),

                          void* destructor_arg) {

  // 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 (destructor)

    AtExitManager::RegisterCallback(destructor, destructor_arg);

}

}  // namespace internal

}  // namespace base