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