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/*
* Copyright (C) 2008 The Android Open Source Project
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* * Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#include <pthread.h>
#include <stdatomic.h>
#include "private/bionic_futex.h"
#define ONCE_INITIALIZATION_NOT_YET_STARTED 0
#define ONCE_INITIALIZATION_UNDERWAY 1
#define ONCE_INITIALIZATION_COMPLETE 2
/* NOTE: this implementation doesn't support a init function that throws a C++ exception
* or calls fork()
*/
int pthread_once(pthread_once_t* once_control, void (*init_routine)(void)) {
static_assert(sizeof(atomic_int) == sizeof(pthread_once_t),
"pthread_once_t should actually be atomic_int in implementation.");
// We prefer casting to atomic_int instead of declaring pthread_once_t to be atomic_int directly.
// Because using the second method pollutes pthread.h, and causes an error when compiling libcxx.
atomic_int* once_control_ptr = reinterpret_cast<atomic_int*>(once_control);
// First check if the once is already initialized. This will be the common
// case and we want to make this as fast as possible. Note that this still
// requires a load_acquire operation here to ensure that all the
// stores performed by the initialization function are observable on
// this CPU after we exit.
int old_value = atomic_load_explicit(once_control_ptr, memory_order_acquire);
while (true) {
if (__predict_true(old_value == ONCE_INITIALIZATION_COMPLETE)) {
return 0;
}
// Try to atomically set the initialization underway flag. This requires a compare exchange
// in a loop, and we may need to exit prematurely if the initialization is complete.
if (!atomic_compare_exchange_weak_explicit(once_control_ptr, &old_value,
ONCE_INITIALIZATION_UNDERWAY,
memory_order_acquire, memory_order_acquire)) {
continue;
}
if (old_value == ONCE_INITIALIZATION_NOT_YET_STARTED) {
// We got here first, we can handle the initialization.
(*init_routine)();
// Do a store_release indicating that initialization is complete.
atomic_store_explicit(once_control_ptr, ONCE_INITIALIZATION_COMPLETE, memory_order_release);
// Wake up any waiters, if any.
__futex_wake_ex(once_control_ptr, 0, INT_MAX);
return 0;
}
// The initialization is underway, wait for its finish.
__futex_wait_ex(once_control_ptr, 0, old_value, NULL);
old_value = atomic_load_explicit(once_control_ptr, memory_order_acquire);
}
}
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