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// 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 {
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 base::internal
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