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66 lines
2.7 KiB
C++
66 lines
2.7 KiB
C++
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// Copyright (c) 2011 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/lazy_instance.h"
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#include "base/at_exit.h"
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#include "base/atomicops.h"
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#include "base/threading/platform_thread.h"
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namespace base {
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namespace internal {
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// TODO(joth): This function could be shared with Singleton, in place of its
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// WaitForInstance() call.
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bool NeedsLazyInstance(subtle::AtomicWord* state) {
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// Try to create the instance, if we're the first, will go from 0 to
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// kLazyInstanceStateCreating, otherwise we've already been beaten here.
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// The memory access has no memory ordering as state 0 and
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// kLazyInstanceStateCreating have no associated data (memory barriers are
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// all about ordering of memory accesses to *associated* data).
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if (subtle::NoBarrier_CompareAndSwap(state, 0,
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kLazyInstanceStateCreating) == 0)
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// Caller must create instance
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return true;
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// It's either in the process of being created, or already created. Spin.
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// The load has acquire memory ordering as a thread which sees
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// state_ == STATE_CREATED needs to acquire visibility over
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// the associated data (buf_). Pairing Release_Store is in
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// CompleteLazyInstance().
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if (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) {
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const base::Time start = base::Time::Now();
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do {
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const base::TimeDelta elapsed = base::Time::Now() - start;
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// Spin with YieldCurrentThread for at most one ms - this ensures maximum
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// responsiveness. After that spin with Sleep(1ms) so that we don't burn
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// excessive CPU time - this also avoids infinite loops due to priority
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// inversions (https://crbug.com/797129).
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if (elapsed < TimeDelta::FromMilliseconds(1))
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PlatformThread::YieldCurrentThread();
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else
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PlatformThread::Sleep(TimeDelta::FromMilliseconds(1));
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} while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating);
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}
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// Someone else created the instance.
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return false;
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}
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void CompleteLazyInstance(subtle::AtomicWord* state,
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subtle::AtomicWord new_instance,
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void (*destructor)(void*),
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void* destructor_arg) {
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// Instance is created, go from CREATING to CREATED.
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// Releases visibility over private_buf_ to readers. Pairing Acquire_Load's
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// are in NeedsInstance() and Pointer().
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subtle::Release_Store(state, new_instance);
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// Make sure that the lazily instantiated object will get destroyed at exit.
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if (destructor)
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AtExitManager::RegisterCallback(destructor, destructor_arg);
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}
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} // namespace internal
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} // namespace base
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