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
35int pthread_barrierattr_init(pthread_barrierattr_t* attr) {
36  *attr = 0;
37  return 0;
38}
39
40int pthread_barrierattr_destroy(pthread_barrierattr_t* attr) {
41  *attr = 0;
42  return 0;
43}
44
45int pthread_barrierattr_getpshared(pthread_barrierattr_t* attr, int* pshared) {
46  *pshared = (*attr & 1) ? PTHREAD_PROCESS_SHARED : PTHREAD_PROCESS_PRIVATE;
47  return 0;
48}
49
50int 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
59enum BarrierState {
60  WAIT,
61  RELEASE,
62};
63
64struct 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
80static_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
84static_assert(alignof(pthread_barrier_t) >= 4,
85              "pthread_barrier_t should fulfill the alignment of pthread_barrier_internal_t.");
86
87static inline pthread_barrier_internal_t* __get_internal_barrier(pthread_barrier_t* barrier) {
88  return reinterpret_cast<pthread_barrier_internal_t*>(barrier);
89}
90
91int 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.
113int 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
168int 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