| // 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 { |
| namespace internal { |
| |
| bool NeedsLazyInstance(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 |
| 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 (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. |
| return false; |
| } |
| |
| void CompleteLazyInstance(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 internal |
| } // namespace base |