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 */
41int 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