1 /* 2 * Copyright (C) 2008 The Android Open Source Project 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * * Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * * Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in 12 * the documentation and/or other materials provided with the 13 * distribution. 14 * 15 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 16 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 17 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 18 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 19 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, 20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, 21 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS 22 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED 23 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, 24 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT 25 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26 * SUCH DAMAGE. 27 */ 28 29 #include <pthread.h> 30 #include <stdatomic.h> 31 32 #include "private/bionic_futex.h" 33 34 #define ONCE_INITIALIZATION_NOT_YET_STARTED 0 35 #define ONCE_INITIALIZATION_UNDERWAY 1 36 #define ONCE_INITIALIZATION_COMPLETE 2 37 38 /* NOTE: this implementation doesn't support a init function that throws a C++ exception 39 * or calls fork() 40 */ 41 int pthread_once(pthread_once_t* once_control, void (*init_routine)(void)) { 42 static_assert(sizeof(atomic_int) == sizeof(pthread_once_t), 43 "pthread_once_t should actually be atomic_int in implementation."); 44 45 // We prefer casting to atomic_int instead of declaring pthread_once_t to be atomic_int directly. 46 // Because using the second method pollutes pthread.h, and causes an error when compiling libcxx. 47 atomic_int* once_control_ptr = reinterpret_cast<atomic_int*>(once_control); 48 49 // First check if the once is already initialized. This will be the common 50 // case and we want to make this as fast as possible. Note that this still 51 // requires a load_acquire operation here to ensure that all the 52 // stores performed by the initialization function are observable on 53 // this CPU after we exit. 54 int old_value = atomic_load_explicit(once_control_ptr, memory_order_acquire); 55 56 while (true) { 57 if (__predict_true(old_value == ONCE_INITIALIZATION_COMPLETE)) { 58 return 0; 59 } 60 61 // Try to atomically set the initialization underway flag. This requires a compare exchange 62 // in a loop, and we may need to exit prematurely if the initialization is complete. 63 if (!atomic_compare_exchange_weak_explicit(once_control_ptr, &old_value, 64 ONCE_INITIALIZATION_UNDERWAY, 65 memory_order_acquire, memory_order_acquire)) { 66 continue; 67 } 68 69 if (old_value == ONCE_INITIALIZATION_NOT_YET_STARTED) { 70 // We got here first, we can handle the initialization. 71 (*init_routine)(); 72 73 // Do a store_release indicating that initialization is complete. 74 atomic_store_explicit(once_control_ptr, ONCE_INITIALIZATION_COMPLETE, memory_order_release); 75 76 // Wake up any waiters, if any. 77 __futex_wake_ex(once_control_ptr, 0, INT_MAX); 78 return 0; 79 } 80 81 // The initialization is underway, wait for its finish. 82 __futex_wait_ex(once_control_ptr, 0, old_value, false, nullptr); 83 old_value = atomic_load_explicit(once_control_ptr, memory_order_acquire); 84 } 85 } 86
