| // Copyright 2018 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_helpers.h" |
| |
| #include "base/at_exit.h" |
| #include "base/atomicops.h" |
| #include "base/threading/platform_thread.h" |
| |
| namespace base { |
| namespace internal { |
| |
| 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(). |
| if (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) { |
| const base::TimeTicks start = base::TimeTicks::Now(); |
| do { |
| const base::TimeDelta elapsed = base::TimeTicks::Now() - start; |
| // Spin with YieldCurrentThread for at most one ms - this ensures maximum |
| // responsiveness. After that spin with Sleep(1ms) so that we don't burn |
| // excessive CPU time - this also avoids infinite loops due to priority |
| // inversions (https://crbug.com/797129). |
| if (elapsed < TimeDelta::FromMilliseconds(1)) |
| PlatformThread::YieldCurrentThread(); |
| else |
| PlatformThread::Sleep(TimeDelta::FromMilliseconds(1)); |
| } while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating); |
| } |
| // 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 (or reset it if |
| // |new_instance| is null). Releases visibility over |private_buf_| to |
| // readers. Pairing Acquire_Load is in NeedsLazyInstance(). |
| subtle::Release_Store(state, new_instance); |
| |
| // Make sure that the lazily instantiated object will get destroyed at exit. |
| if (new_instance && destructor) |
| AtExitManager::RegisterCallback(destructor, destructor_arg); |
| } |
| |
| } // namespace internal |
| } // namespace base |