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64 lines
2.6 KiB
64 lines
2.6 KiB
// 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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