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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 "private/bionic_atomic_inline.h"
#include "private/bionic_futex.h"
#define ONCE_INITIALIZING (1 << 0)
#define ONCE_COMPLETED (1 << 1)
/* 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)) {
volatile pthread_once_t* once_control_ptr = once_control;
// PTHREAD_ONCE_INIT is 0, we use the following bit flags
// bit 0 set -> initialization is under way
// bit 1 set -> initialization is complete
// 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.
if (__predict_true((*once_control_ptr & ONCE_COMPLETED) != 0)) {
ANDROID_MEMBAR_FULL();
return 0;
}
while (true) {
// Try to atomically set the INITIALIZING flag.
// This requires a cmpxchg loop, and we may need
// to exit prematurely if we detect that
// COMPLETED is now set.
int32_t old_value, new_value;
do {
old_value = *once_control_ptr;
if ((old_value & ONCE_COMPLETED) != 0) {
break;
}
new_value = old_value | ONCE_INITIALIZING;
} while (__bionic_cmpxchg(old_value, new_value, once_control_ptr) != 0);
if ((old_value & ONCE_COMPLETED) != 0) {
// We detected that COMPLETED was set while in our loop.
ANDROID_MEMBAR_FULL();
return 0;
}
if ((old_value & ONCE_INITIALIZING) == 0) {
// We got there first, we can jump out of the loop to handle the initialization.
break;
}
// Another thread is running the initialization and hasn't completed
// yet, so wait for it, then try again.
__futex_wait_ex(once_control_ptr, 0, old_value, NULL);
}
// Call the initialization function.
(*init_routine)();
// Do a store_release indicating that initialization is complete.
ANDROID_MEMBAR_FULL();
*once_control_ptr = ONCE_COMPLETED;
// Wake up any waiters, if any.
__futex_wake_ex(once_control_ptr, 0, INT_MAX);
return 0;
}
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