1/* sem_waitcommon -- wait on a semaphore, shared code.
2 Copyright (C) 2003-2019 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4 Contributed by Paul Mackerras <paulus@au.ibm.com>, 2003.
5
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2.1 of the License, or (at your option) any later version.
10
11 The GNU C Library is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
15
16 You should have received a copy of the GNU Lesser General Public
17 License along with the GNU C Library; if not, see
18 <http://www.gnu.org/licenses/>. */
19
20#include <kernel-features.h>
21#include <errno.h>
22#include <sysdep.h>
23#include <futex-internal.h>
24#include <internaltypes.h>
25#include <semaphore.h>
26#include <sys/time.h>
27
28#include <pthreadP.h>
29#include <shlib-compat.h>
30#include <atomic.h>
31
32
33/* The semaphore provides two main operations: sem_post adds a token to the
34 semaphore; sem_wait grabs a token from the semaphore, potentially waiting
35 until there is a token available. A sem_wait needs to synchronize with
36 the sem_post that provided the token, so that whatever lead to the sem_post
37 happens before the code after sem_wait.
38
39 Conceptually, available tokens can simply be counted; let's call that the
40 value of the semaphore. However, we also want to know whether there might
41 be a sem_wait that is blocked on the value because it was zero (using a
42 futex with the value being the futex variable); if there is no blocked
43 sem_wait, sem_post does not need to execute a futex_wake call. Therefore,
44 we also need to count the number of potentially blocked sem_wait calls
45 (which we call nwaiters).
46
47 What makes this tricky is that POSIX requires that a semaphore can be
48 destroyed as soon as the last remaining sem_wait has returned, and no
49 other sem_wait or sem_post calls are executing concurrently. However, the
50 sem_post call whose token was consumed by the last sem_wait is considered
51 to have finished once it provided the token to the sem_wait.
52 Thus, sem_post must not access the semaphore struct anymore after it has
53 made a token available; IOW, it needs to be able to atomically provide
54 a token and check whether any blocked sem_wait calls might exist.
55
56 This is straightforward to do if the architecture provides 64b atomics
57 because we can just put both the value and nwaiters into one variable that
58 we access atomically: This is the data field, the value is in the
59 least-significant 32 bits, and nwaiters in the other bits. When sem_post
60 makes a value available, it can atomically check nwaiters.
61
62 If we have only 32b atomics available, we cannot put both nwaiters and
63 value into one 32b value because then we might have too few bits for both
64 of those counters. Therefore, we need to use two distinct fields.
65
66 To allow sem_post to atomically make a token available and check for
67 blocked sem_wait calls, we use one bit in value to indicate whether
68 nwaiters is nonzero. That allows sem_post to use basically the same
69 algorithm as with 64b atomics, but requires sem_wait to update the bit; it
70 can't do this atomically with another access to nwaiters, but it can compute
71 a conservative value for the bit because it's benign if the bit is set
72 even if nwaiters is zero (all we get is an unnecessary futex wake call by
73 sem_post).
74 Specifically, sem_wait will unset the bit speculatively if it believes that
75 there is no other concurrently executing sem_wait. If it misspeculated,
76 it will have to clean up by waking any other sem_wait call (i.e., what
77 sem_post would do otherwise). This does not conflict with the destruction
78 requirement because the semaphore must not be destructed while any sem_wait
79 is still executing. */
80
81#if !__HAVE_64B_ATOMICS
82static void
83__sem_wait_32_finish (struct new_sem *sem);
84#endif
85
86static void
87__sem_wait_cleanup (void *arg)
88{
89 struct new_sem *sem = (struct new_sem *) arg;
90
91#if __HAVE_64B_ATOMICS
92 /* Stop being registered as a waiter. See below for MO. */
93 atomic_fetch_add_relaxed (&sem->data, -((uint64_t) 1 << SEM_NWAITERS_SHIFT));
94#else
95 __sem_wait_32_finish (sem);
96#endif
97}
98
99/* Wait until at least one token is available, possibly with a timeout.
100 This is in a separate function in order to make sure gcc
101 puts the call site into an exception region, and thus the
102 cleanups get properly run. TODO still necessary? Other futex_wait
103 users don't seem to need it. */
104static int
105__attribute__ ((noinline))
106do_futex_wait (struct new_sem *sem, const struct timespec *abstime)
107{
108 int err;
109
110#if __HAVE_64B_ATOMICS
111 err = futex_abstimed_wait_cancelable (
112 (unsigned int *) &sem->data + SEM_VALUE_OFFSET, 0, abstime,
113 sem->private);
114#else
115 err = futex_abstimed_wait_cancelable (&sem->value, SEM_NWAITERS_MASK,
116 abstime, sem->private);
117#endif
118
119 return err;
120}
121
122/* Fast path: Try to grab a token without blocking. */
123static int
124__new_sem_wait_fast (struct new_sem *sem, int definitive_result)
125{
126 /* We need acquire MO if we actually grab a token, so that this
127 synchronizes with all token providers (i.e., the RMW operation we read
128 from or all those before it in modification order; also see sem_post).
129 We do not need to guarantee any ordering if we observed that there is
130 no token (POSIX leaves it unspecified whether functions that fail
131 synchronize memory); thus, relaxed MO is sufficient for the initial load
132 and the failure path of the CAS. If the weak CAS fails and we need a
133 definitive result, retry. */
134#if __HAVE_64B_ATOMICS
135 uint64_t d = atomic_load_relaxed (&sem->data);
136 do
137 {
138 if ((d & SEM_VALUE_MASK) == 0)
139 break;
140 if (atomic_compare_exchange_weak_acquire (&sem->data, &d, d - 1))
141 return 0;
142 }
143 while (definitive_result);
144 return -1;
145#else
146 unsigned int v = atomic_load_relaxed (&sem->value);
147 do
148 {
149 if ((v >> SEM_VALUE_SHIFT) == 0)
150 break;
151 if (atomic_compare_exchange_weak_acquire (&sem->value,
152 &v, v - (1 << SEM_VALUE_SHIFT)))
153 return 0;
154 }
155 while (definitive_result);
156 return -1;
157#endif
158}
159
160/* Slow path that blocks. */
161static int
162__attribute__ ((noinline))
163__new_sem_wait_slow (struct new_sem *sem, const struct timespec *abstime)
164{
165 int err = 0;
166
167#if __HAVE_64B_ATOMICS
168 /* Add a waiter. Relaxed MO is sufficient because we can rely on the
169 ordering provided by the RMW operations we use. */
170 uint64_t d = atomic_fetch_add_relaxed (&sem->data,
171 (uint64_t) 1 << SEM_NWAITERS_SHIFT);
172
173 pthread_cleanup_push (__sem_wait_cleanup, sem);
174
175 /* Wait for a token to be available. Retry until we can grab one. */
176 for (;;)
177 {
178 /* If there is no token available, sleep until there is. */
179 if ((d & SEM_VALUE_MASK) == 0)
180 {
181 err = do_futex_wait (sem, abstime);
182 /* A futex return value of 0 or EAGAIN is due to a real or spurious
183 wake-up, or due to a change in the number of tokens. We retry in
184 these cases.
185 If we timed out, forward this to the caller.
186 EINTR is returned if we are interrupted by a signal; we
187 forward this to the caller. (See futex_wait and related
188 documentation. Before Linux 2.6.22, EINTR was also returned on
189 spurious wake-ups; we only support more recent Linux versions,
190 so do not need to consider this here.) */
191 if (err == ETIMEDOUT || err == EINTR)
192 {
193 __set_errno (err);
194 err = -1;
195 /* Stop being registered as a waiter. */
196 atomic_fetch_add_relaxed (&sem->data,
197 -((uint64_t) 1 << SEM_NWAITERS_SHIFT));
198 break;
199 }
200 /* Relaxed MO is sufficient; see below. */
201 d = atomic_load_relaxed (&sem->data);
202 }
203 else
204 {
205 /* Try to grab both a token and stop being a waiter. We need
206 acquire MO so this synchronizes with all token providers (i.e.,
207 the RMW operation we read from or all those before it in
208 modification order; also see sem_post). On the failure path,
209 relaxed MO is sufficient because we only eventually need the
210 up-to-date value; the futex_wait or the CAS perform the real
211 work. */
212 if (atomic_compare_exchange_weak_acquire (&sem->data,
213 &d, d - 1 - ((uint64_t) 1 << SEM_NWAITERS_SHIFT)))
214 {
215 err = 0;
216 break;
217 }
218 }
219 }
220
221 pthread_cleanup_pop (0);
222#else
223 /* The main difference to the 64b-atomics implementation is that we need to
224 access value and nwaiters in separate steps, and that the nwaiters bit
225 in the value can temporarily not be set even if nwaiters is nonzero.
226 We work around incorrectly unsetting the nwaiters bit by letting sem_wait
227 set the bit again and waking the number of waiters that could grab a
228 token. There are two additional properties we need to ensure:
229 (1) We make sure that whenever unsetting the bit, we see the increment of
230 nwaiters by the other thread that set the bit. IOW, we will notice if
231 we make a mistake.
232 (2) When setting the nwaiters bit, we make sure that we see the unsetting
233 of the bit by another waiter that happened before us. This avoids having
234 to blindly set the bit whenever we need to block on it. We set/unset
235 the bit while having incremented nwaiters (i.e., are a registered
236 waiter), and the problematic case only happens when one waiter indeed
237 followed another (i.e., nwaiters was never larger than 1); thus, this
238 works similarly as with a critical section using nwaiters (see the MOs
239 and related comments below).
240
241 An alternative approach would be to unset the bit after decrementing
242 nwaiters; however, that would result in needing Dekker-like
243 synchronization and thus full memory barriers. We also would not be able
244 to prevent misspeculation, so this alternative scheme does not seem
245 beneficial. */
246 unsigned int v;
247
248 /* Add a waiter. We need acquire MO so this synchronizes with the release
249 MO we use when decrementing nwaiters below; it ensures that if another
250 waiter unset the bit before us, we see that and set it again. Also see
251 property (2) above. */
252 atomic_fetch_add_acquire (&sem->nwaiters, 1);
253
254 pthread_cleanup_push (__sem_wait_cleanup, sem);
255
256 /* Wait for a token to be available. Retry until we can grab one. */
257 /* We do not need any ordering wrt. to this load's reads-from, so relaxed
258 MO is sufficient. The acquire MO above ensures that in the problematic
259 case, we do see the unsetting of the bit by another waiter. */
260 v = atomic_load_relaxed (&sem->value);
261 do
262 {
263 do
264 {
265 /* We are about to block, so make sure that the nwaiters bit is
266 set. We need release MO on the CAS to ensure that when another
267 waiter unsets the nwaiters bit, it will also observe that we
268 incremented nwaiters in the meantime (also see the unsetting of
269 the bit below). Relaxed MO on CAS failure is sufficient (see
270 above). */
271 do
272 {
273 if ((v & SEM_NWAITERS_MASK) != 0)
274 break;
275 }
276 while (!atomic_compare_exchange_weak_release (&sem->value,
277 &v, v | SEM_NWAITERS_MASK));
278 /* If there is no token, wait. */
279 if ((v >> SEM_VALUE_SHIFT) == 0)
280 {
281 /* See __HAVE_64B_ATOMICS variant. */
282 err = do_futex_wait(sem, abstime);
283 if (err == ETIMEDOUT || err == EINTR)
284 {
285 __set_errno (err);
286 err = -1;
287 goto error;
288 }
289 err = 0;
290 /* We blocked, so there might be a token now. Relaxed MO is
291 sufficient (see above). */
292 v = atomic_load_relaxed (&sem->value);
293 }
294 }
295 /* If there is no token, we must not try to grab one. */
296 while ((v >> SEM_VALUE_SHIFT) == 0);
297 }
298 /* Try to grab a token. We need acquire MO so this synchronizes with
299 all token providers (i.e., the RMW operation we read from or all those
300 before it in modification order; also see sem_post). */
301 while (!atomic_compare_exchange_weak_acquire (&sem->value,
302 &v, v - (1 << SEM_VALUE_SHIFT)));
303
304error:
305 pthread_cleanup_pop (0);
306
307 __sem_wait_32_finish (sem);
308#endif
309
310 return err;
311}
312
313/* Stop being a registered waiter (non-64b-atomics code only). */
314#if !__HAVE_64B_ATOMICS
315static void
316__sem_wait_32_finish (struct new_sem *sem)
317{
318 /* The nwaiters bit is still set, try to unset it now if this seems
319 necessary. We do this before decrementing nwaiters so that the unsetting
320 is visible to other waiters entering after us. Relaxed MO is sufficient
321 because we are just speculating here; a stronger MO would not prevent
322 misspeculation. */
323 unsigned int wguess = atomic_load_relaxed (&sem->nwaiters);
324 if (wguess == 1)
325 /* We might be the last waiter, so unset. This needs acquire MO so that
326 it syncronizes with the release MO when setting the bit above; if we
327 overwrite someone else that set the bit, we'll read in the following
328 decrement of nwaiters at least from that release sequence, so we'll
329 see if the other waiter is still active or if another writer entered
330 in the meantime (i.e., using the check below). */
331 atomic_fetch_and_acquire (&sem->value, ~SEM_NWAITERS_MASK);
332
333 /* Now stop being a waiter, and see whether our guess was correct.
334 This needs release MO so that it synchronizes with the acquire MO when
335 a waiter increments nwaiters; this makes sure that newer writers see that
336 we reset the waiters_present bit. */
337 unsigned int wfinal = atomic_fetch_add_release (&sem->nwaiters, -1);
338 if (wfinal > 1 && wguess == 1)
339 {
340 /* We guessed wrong, and so need to clean up after the mistake and
341 unblock any waiters that could have not been woken. There is no
342 additional ordering that we need to set up, so relaxed MO is
343 sufficient. */
344 unsigned int v = atomic_fetch_or_relaxed (&sem->value,
345 SEM_NWAITERS_MASK);
346 /* If there are available tokens, then wake as many waiters. If there
347 aren't any, then there is no need to wake anyone because there is
348 none to grab for another waiter. If tokens become available
349 subsequently, then the respective sem_post calls will do the wake-up
350 due to us having set the nwaiters bit again. */
351 v >>= SEM_VALUE_SHIFT;
352 if (v > 0)
353 futex_wake (&sem->value, v, sem->private);
354 }
355}
356#endif
357