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- // 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 <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
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