1 /*
2 * Copyright (C) 2015 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 #include <stdint.h>
32
33 #include "private/bionic_futex.h"
34
pthread_barrierattr_init(pthread_barrierattr_t * attr)35 int pthread_barrierattr_init(pthread_barrierattr_t* attr) {
36 *attr = 0;
37 return 0;
38 }
39
pthread_barrierattr_destroy(pthread_barrierattr_t * attr)40 int pthread_barrierattr_destroy(pthread_barrierattr_t* attr) {
41 *attr = 0;
42 return 0;
43 }
44
pthread_barrierattr_getpshared(pthread_barrierattr_t * attr,int * pshared)45 int pthread_barrierattr_getpshared(pthread_barrierattr_t* attr, int* pshared) {
46 *pshared = (*attr & 1) ? PTHREAD_PROCESS_SHARED : PTHREAD_PROCESS_PRIVATE;
47 return 0;
48 }
49
pthread_barrierattr_setpshared(pthread_barrierattr_t * attr,int pshared)50 int pthread_barrierattr_setpshared(pthread_barrierattr_t* attr, int pshared) {
51 if (pshared == PTHREAD_PROCESS_SHARED) {
52 *attr |= 1;
53 } else {
54 *attr &= ~1;
55 }
56 return 0;
57 }
58
59 enum BarrierState {
60 WAIT,
61 RELEASE,
62 };
63
64 struct pthread_barrier_internal_t {
65 // One barrier can be used for unlimited number of cycles. In each cycle, [init_count]
66 // threads must call pthread_barrier_wait() before any of them successfully return from
67 // the call. It is undefined behavior if there are more than [init_count] threads call
68 // pthread_barrier_wait() in one cycle.
69 uint32_t init_count;
70 // Barrier state. It is WAIT if waiting for more threads to enter the barrier in this cycle,
71 // otherwise threads are leaving the barrier.
72 _Atomic(BarrierState) state;
73 // Number of threads having entered but not left the barrier in this cycle.
74 atomic_uint wait_count;
75 // Whether the barrier is shared across processes.
76 bool pshared;
77 uint32_t __reserved[4];
78 };
79
80 static_assert(sizeof(pthread_barrier_t) == sizeof(pthread_barrier_internal_t),
81 "pthread_barrier_t should actually be pthread_barrier_internal_t in implementation."
82 );
83
84 static_assert(alignof(pthread_barrier_t) >= 4,
85 "pthread_barrier_t should fulfill the alignment of pthread_barrier_internal_t.");
86
__get_internal_barrier(pthread_barrier_t * barrier)87 static inline pthread_barrier_internal_t* __get_internal_barrier(pthread_barrier_t* barrier) {
88 return reinterpret_cast<pthread_barrier_internal_t*>(barrier);
89 }
90
pthread_barrier_init(pthread_barrier_t * barrier_interface,const pthread_barrierattr_t * attr,unsigned count)91 int pthread_barrier_init(pthread_barrier_t* barrier_interface, const pthread_barrierattr_t* attr,
92 unsigned count) {
93 pthread_barrier_internal_t* barrier = __get_internal_barrier(barrier_interface);
94 if (count == 0) {
95 return EINVAL;
96 }
97 barrier->init_count = count;
98 atomic_init(&barrier->state, WAIT);
99 atomic_init(&barrier->wait_count, 0);
100 barrier->pshared = false;
101 if (attr != nullptr && (*attr & 1)) {
102 barrier->pshared = true;
103 }
104 return 0;
105 }
106
107 // According to POSIX standard, pthread_barrier_wait() synchronizes memory between participating
108 // threads. It means all memory operations made by participating threads before calling
109 // pthread_barrier_wait() can be seen by all participating threads after the function call.
110 // We establish this by making a happens-before relation between all threads entering the barrier
111 // with the last thread entering the barrier, and a happens-before relation between the last
112 // thread entering the barrier with all threads leaving the barrier.
pthread_barrier_wait(pthread_barrier_t * barrier_interface)113 int pthread_barrier_wait(pthread_barrier_t* barrier_interface) {
114 pthread_barrier_internal_t* barrier = __get_internal_barrier(barrier_interface);
115
116 // Wait until all threads for the previous cycle have left the barrier. This is needed
117 // as a participating thread can call pthread_barrier_wait() again before other
118 // threads have left the barrier. Use acquire operation here to synchronize with
119 // the last thread leaving the previous cycle, so we can read correct wait_count below.
120 while(atomic_load_explicit(&barrier->state, memory_order_acquire) == RELEASE) {
121 __futex_wait_ex(&barrier->state, barrier->pshared, RELEASE, false, nullptr);
122 }
123
124 uint32_t prev_wait_count = atomic_load_explicit(&barrier->wait_count, memory_order_relaxed);
125 while (true) {
126 // It happens when there are more than [init_count] threads trying to enter the barrier
127 // at one cycle. We read the POSIX standard as disallowing this, since additional arriving
128 // threads are not synchronized with respect to the barrier reset. We also don't know of
129 // any reasonable cases in which this would be intentional.
130 if (prev_wait_count >= barrier->init_count) {
131 return EINVAL;
132 }
133 // Use memory_order_acq_rel operation here to synchronize between all threads entering
134 // the barrier with the last thread entering the barrier.
135 if (atomic_compare_exchange_weak_explicit(&barrier->wait_count, &prev_wait_count,
136 prev_wait_count + 1u, memory_order_acq_rel,
137 memory_order_relaxed)) {
138 break;
139 }
140 }
141
142 int result = 0;
143 if (prev_wait_count + 1 == barrier->init_count) {
144 result = PTHREAD_BARRIER_SERIAL_THREAD;
145 if (prev_wait_count != 0) {
146 // Use release operation here to synchronize between the last thread entering the
147 // barrier with all threads leaving the barrier.
148 atomic_store_explicit(&barrier->state, RELEASE, memory_order_release);
149 __futex_wake_ex(&barrier->state, barrier->pshared, prev_wait_count);
150 }
151 } else {
152 // Use acquire operation here to synchronize between the last thread entering the
153 // barrier with all threads leaving the barrier.
154 while (atomic_load_explicit(&barrier->state, memory_order_acquire) == WAIT) {
155 __futex_wait_ex(&barrier->state, barrier->pshared, WAIT, false, nullptr);
156 }
157 }
158 // Use release operation here to make it not reordered with previous operations.
159 if (atomic_fetch_sub_explicit(&barrier->wait_count, 1, memory_order_release) == 1) {
160 // Use release operation here to synchronize with threads entering the barrier for
161 // the next cycle, or the thread calling pthread_barrier_destroy().
162 atomic_store_explicit(&barrier->state, WAIT, memory_order_release);
163 __futex_wake_ex(&barrier->state, barrier->pshared, barrier->init_count);
164 }
165 return result;
166 }
167
pthread_barrier_destroy(pthread_barrier_t * barrier_interface)168 int pthread_barrier_destroy(pthread_barrier_t* barrier_interface) {
169 pthread_barrier_internal_t* barrier = __get_internal_barrier(barrier_interface);
170 if (barrier->init_count == 0) {
171 return EINVAL;
172 }
173 // Use acquire operation here to synchronize with the last thread leaving the barrier.
174 // So we can read correct wait_count below.
175 while (atomic_load_explicit(&barrier->state, memory_order_acquire) == RELEASE) {
176 __futex_wait_ex(&barrier->state, barrier->pshared, RELEASE, false, nullptr);
177 }
178 if (atomic_load_explicit(&barrier->wait_count, memory_order_relaxed) != 0) {
179 return EBUSY;
180 }
181 barrier->init_count = 0;
182 return 0;
183 }
184