| /* |
| * 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; |
| } |