pthread_cond.cpp revision c9a659c57b256001fd63f9825bde69e660c2655b 
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
31#include <errno.h>
32#include <limits.h>
33#include <stdatomic.h>
34#include <sys/mman.h>
35#include <time.h>
36#include <unistd.h>
37
38#include "pthread_internal.h"
39
40#include "private/bionic_futex.h"
41#include "private/bionic_time_conversions.h"
42#include "private/bionic_tls.h"
43
44// XXX *technically* there is a race condition that could allow
45// XXX a signal to be missed.  If thread A is preempted in _wait()
46// XXX after unlocking the mutex and before waiting, and if other
47// XXX threads call signal or broadcast UINT_MAX/2 times (exactly),
48// XXX before thread A is scheduled again and calls futex_wait(),
49// XXX then the signal will be lost.
50
51// We use one bit in pthread_condattr_t (long) values as the 'shared' flag
52// and one bit for the clock type (CLOCK_REALTIME is ((clockid_t) 1), and
53// CLOCK_MONOTONIC is ((clockid_t) 0).). The rest of the bits are a counter.
54//
55// The 'value' field pthread_cond_t has the same layout.
56
57#define COND_SHARED_MASK 0x0001
58#define COND_CLOCK_MASK 0x0002
59#define COND_COUNTER_STEP 0x0004
60#define COND_FLAGS_MASK (COND_SHARED_MASK | COND_CLOCK_MASK)
61#define COND_COUNTER_MASK (~COND_FLAGS_MASK)
62
63#define COND_IS_SHARED(c) (((c) & COND_SHARED_MASK) != 0)
64#define COND_GET_CLOCK(c) (((c) & COND_CLOCK_MASK) >> 1)
65#define COND_SET_CLOCK(attr, c) ((attr) | (c << 1))
66
67int pthread_condattr_init(pthread_condattr_t* attr) {
68  *attr = 0;
69  *attr |= PTHREAD_PROCESS_PRIVATE;
70  *attr |= (CLOCK_REALTIME << 1);
71  return 0;
72}
73
74int pthread_condattr_getpshared(const pthread_condattr_t* attr, int* pshared) {
75  *pshared = static_cast<int>(COND_IS_SHARED(*attr));
76  return 0;
77}
78
79int pthread_condattr_setpshared(pthread_condattr_t* attr, int pshared) {
80  if (pshared != PTHREAD_PROCESS_SHARED && pshared != PTHREAD_PROCESS_PRIVATE) {
81    return EINVAL;
82  }
83
84  *attr |= pshared;
85  return 0;
86}
87
88int pthread_condattr_getclock(const pthread_condattr_t* attr, clockid_t* clock) {
89  *clock = COND_GET_CLOCK(*attr);
90  return 0;
91}
92
93int pthread_condattr_setclock(pthread_condattr_t* attr, clockid_t clock) {
94  if (clock != CLOCK_MONOTONIC && clock != CLOCK_REALTIME) {
95    return EINVAL;
96  }
97
98  *attr = COND_SET_CLOCK(*attr, clock);
99  return 0;
100}
101
102int pthread_condattr_destroy(pthread_condattr_t* attr) {
103  *attr = 0xdeada11d;
104  return 0;
105}
106
107struct pthread_cond_internal_t {
108  atomic_uint state;
109
110  bool process_shared() {
111    return COND_IS_SHARED(atomic_load_explicit(&state, memory_order_relaxed));
112  }
113
114  bool use_realtime_clock() {
115    return COND_GET_CLOCK(atomic_load_explicit(&state, memory_order_relaxed)) == CLOCK_REALTIME;
116  }
117
118#if defined(__LP64__)
119  char __reserved[44];
120#endif
121};
122
123static_assert(sizeof(pthread_cond_t) == sizeof(pthread_cond_internal_t),
124              "pthread_cond_t should actually be pthread_cond_internal_t in implementation.");
125
126// For binary compatibility with old version of pthread_cond_t, we can't use more strict alignment
127// than 4-byte alignment.
128static_assert(alignof(pthread_cond_t) == 4,
129              "pthread_cond_t should fulfill the alignment requirement of pthread_cond_internal_t.");
130
131static pthread_cond_internal_t* __get_internal_cond(pthread_cond_t* cond_interface) {
132  return reinterpret_cast<pthread_cond_internal_t*>(cond_interface);
133}
134
135int pthread_cond_init(pthread_cond_t* cond_interface, const pthread_condattr_t* attr) {
136  pthread_cond_internal_t* cond = __get_internal_cond(cond_interface);
137
138  unsigned int init_state = 0;
139  if (attr != NULL) {
140    init_state = (*attr & COND_FLAGS_MASK);
141  }
142  atomic_init(&cond->state, init_state);
143
144  return 0;
145}
146
147int pthread_cond_destroy(pthread_cond_t* cond_interface) {
148  pthread_cond_internal_t* cond = __get_internal_cond(cond_interface);
149  atomic_store_explicit(&cond->state, 0xdeadc04d, memory_order_relaxed);
150  return 0;
151}
152
153// This function is used by pthread_cond_broadcast and
154// pthread_cond_signal to atomically decrement the counter
155// then wake up thread_count threads.
156static int __pthread_cond_pulse(pthread_cond_internal_t* cond, int thread_count) {
157  // We don't use a release/seq_cst fence here. Because pthread_cond_wait/signal can't be
158  // used as a method for memory synchronization by itself. It should always be used with
159  // pthread mutexes. Note that Spurious wakeups from pthread_cond_wait/timedwait may occur,
160  // so when using condition variables there is always a boolean predicate involving shared
161  // variables associated with each condition wait that is true if the thread should proceed.
162  // If the predicate is seen true before a condition wait, pthread_cond_wait/timedwait will
163  // not be called. That's why pthread_wait/signal pair can't be used as a method for memory
164  // synchronization. And it doesn't help even if we use any fence here.
165
166  // The increase of value should leave flags alone, even if the value can overflows.
167  atomic_fetch_add_explicit(&cond->state, COND_COUNTER_STEP, memory_order_relaxed);
168
169  __futex_wake_ex(&cond->state, cond->process_shared(), thread_count);
170  return 0;
171}
172
173static int __pthread_cond_timedwait(pthread_cond_internal_t* cond, pthread_mutex_t* mutex,
174                                    bool use_realtime_clock, const timespec* abs_timeout_or_null) {
175  int result = check_timespec(abs_timeout_or_null);
176  if (result != 0) {
177    return result;
178  }
179
180  unsigned int old_state = atomic_load_explicit(&cond->state, memory_order_relaxed);
181  pthread_mutex_unlock(mutex);
182  int status = __futex_wait_ex(&cond->state, cond->process_shared(), old_state,
183                               use_realtime_clock, abs_timeout_or_null);
184  pthread_mutex_lock(mutex);
185
186  if (status == -ETIMEDOUT) {
187    return ETIMEDOUT;
188  }
189  return 0;
190}
191
192int pthread_cond_broadcast(pthread_cond_t* cond_interface) {
193  return __pthread_cond_pulse(__get_internal_cond(cond_interface), INT_MAX);
194}
195
196int pthread_cond_signal(pthread_cond_t* cond_interface) {
197  return __pthread_cond_pulse(__get_internal_cond(cond_interface), 1);
198}
199
200int pthread_cond_wait(pthread_cond_t* cond_interface, pthread_mutex_t* mutex) {
201  pthread_cond_internal_t* cond = __get_internal_cond(cond_interface);
202  return __pthread_cond_timedwait(cond, mutex, false, nullptr);
203}
204
205int pthread_cond_timedwait(pthread_cond_t *cond_interface, pthread_mutex_t * mutex,
206                           const timespec *abstime) {
207
208  pthread_cond_internal_t* cond = __get_internal_cond(cond_interface);
209  return __pthread_cond_timedwait(cond, mutex, cond->use_realtime_clock(), abstime);
210}
211
212#if !defined(__LP64__)
213// TODO: this exists only for backward binary compatibility on 32 bit platforms.
214extern "C" int pthread_cond_timedwait_monotonic(pthread_cond_t* cond_interface,
215                                                pthread_mutex_t* mutex,
216                                                const timespec* abs_timeout) {
217
218  return __pthread_cond_timedwait(__get_internal_cond(cond_interface), mutex, false, abs_timeout);
219}
220
221extern "C" int pthread_cond_timedwait_monotonic_np(pthread_cond_t* cond_interface,
222                                                   pthread_mutex_t* mutex,
223                                                   const timespec* abs_timeout) {
224  return pthread_cond_timedwait_monotonic(cond_interface, mutex, abs_timeout);
225}
226
227extern "C" int pthread_cond_timedwait_relative_np(pthread_cond_t* cond_interface,
228                                                  pthread_mutex_t* mutex,
229                                                  const timespec* rel_timeout) {
230  timespec ts;
231  timespec* abs_timeout = nullptr;
232  if (rel_timeout != nullptr) {
233    absolute_timespec_from_timespec(ts, *rel_timeout, CLOCK_REALTIME);
234    abs_timeout = &ts;
235  }
236  return __pthread_cond_timedwait(__get_internal_cond(cond_interface), mutex, true, abs_timeout);
237}
238
239extern "C" int pthread_cond_timeout_np(pthread_cond_t* cond_interface,
240                                       pthread_mutex_t* mutex, unsigned ms) {
241  timespec ts;
242  timespec_from_ms(ts, ms);
243  return pthread_cond_timedwait_relative_np(cond_interface, mutex, &ts);
244}
245#endif // !defined(__LP64__)
246