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65 lines
2.6 KiB
C++
65 lines
2.6 KiB
C++
// Copyright 2018 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_helpers.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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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, kLazyInstanceStateCreating) ==
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0) {
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// Caller must create instance
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return true;
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}
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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::TimeTicks start = base::TimeTicks::Now();
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do {
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const base::TimeDelta elapsed = base::TimeTicks::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 (or reset it if
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// |new_instance| is null). Releases visibility over |private_buf_| to
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// readers. Pairing Acquire_Load is in NeedsLazyInstance().
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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 (new_instance && 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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