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(const pthread_barrierattr_t * attr,int * pshared)45 int pthread_barrierattr_getpshared(const 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