| /* |
| * 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, false, nullptr); |
| old_value = atomic_load_explicit(once_control_ptr, memory_order_acquire); |
| } |
| } |