| // Copyright 2018 The Chromium Authors |
| // 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<atomic> |
| |
| #include"base/at_exit.h" |
| #include"base/threading/platform_thread.h" |
| |
| namespace base::internal{ |
| |
| boolNeedsLazyInstance(std::atomic<uintptr_t>& 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). |
| uintptr_t expected=0; |
| if(state.compare_exchange_strong(expected, kLazyInstanceStateCreating, |
| std::memory_order_relaxed, |
| std::memory_order_relaxed)){ |
| // Caller must create instance |
| returntrue; |
| } |
| |
| // 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(state.load(std::memory_order_acquire)== 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<Milliseconds(1)){ |
| PlatformThread::YieldCurrentThread(); |
| }else{ |
| PlatformThread::Sleep(Milliseconds(1)); |
| } |
| }while(state.load(std::memory_order_acquire)== |
| kLazyInstanceStateCreating); |
| } |
| // Someone else created the instance. |
| returnfalse; |
| } |
| |
| voidCompleteLazyInstance(std::atomic<uintptr_t>& state, |
| uintptr_t 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(). |
| state.store(new_instance, std::memory_order_release); |
| |
| // Make sure that the lazily instantiated object will get destroyed at exit. |
| if(new_instance&& destructor){ |
| AtExitManager::RegisterCallback(destructor, destructor_arg); |
| } |
| } |
| |
| }// namespace base::internal |
