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  */
pthread_once(pthread_once_t * once_control,void (* init_routine)(void))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