Cleanup: cast poll delay return value to void
[urcu.git] / urcu.c
1 /*
2 * urcu.c
3 *
4 * Userspace RCU library
5 *
6 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
7 * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
8 *
9 * This library is free software; you can redistribute it and/or
10 * modify it under the terms of the GNU Lesser General Public
11 * License as published by the Free Software Foundation; either
12 * version 2.1 of the License, or (at your option) any later version.
13 *
14 * This library is distributed in the hope that it will be useful,
15 * but WITHOUT ANY WARRANTY; without even the implied warranty of
16 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
17 * Lesser General Public License for more details.
18 *
19 * You should have received a copy of the GNU Lesser General Public
20 * License along with this library; if not, write to the Free Software
21 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
22 *
23 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
24 */
25
26 #define _BSD_SOURCE
27 #define _GNU_SOURCE
28 #define _LGPL_SOURCE
29 #define _DEFAULT_SOURCE
30 #include <stdio.h>
31 #include <pthread.h>
32 #include <signal.h>
33 #include <assert.h>
34 #include <stdlib.h>
35 #include <stdint.h>
36 #include <string.h>
37 #include <errno.h>
38 #include <poll.h>
39
40 #include "urcu/wfcqueue.h"
41 #include "urcu/map/urcu.h"
42 #include "urcu/static/urcu.h"
43 #include "urcu-pointer.h"
44 #include "urcu/tls-compat.h"
45
46 #include "urcu-die.h"
47 #include "urcu-wait.h"
48
49 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
50 #undef _LGPL_SOURCE
51 #include "urcu.h"
52 #define _LGPL_SOURCE
53
54 /*
55 * If a reader is really non-cooperative and refuses to commit its
56 * rcu_active_readers count to memory (there is no barrier in the reader
57 * per-se), kick it after 10 loops waiting for it.
58 */
59 #define KICK_READER_LOOPS 10
60
61 /*
62 * Active attempts to check for reader Q.S. before calling futex().
63 */
64 #define RCU_QS_ACTIVE_ATTEMPTS 100
65
66 /*
67 * RCU_MEMBARRIER is only possibly available on Linux.
68 */
69 #if defined(RCU_MEMBARRIER) && defined(__linux__)
70 #include <urcu/syscall-compat.h>
71 #endif
72
73 /* If the headers do not support SYS_membarrier, fall back on RCU_MB */
74 #ifdef SYS_membarrier
75 # define membarrier(...) syscall(SYS_membarrier, __VA_ARGS__)
76 #else
77 # define membarrier(...) -ENOSYS
78 #endif
79
80 #define MEMBARRIER_EXPEDITED (1 << 0)
81 #define MEMBARRIER_DELAYED (1 << 1)
82 #define MEMBARRIER_QUERY (1 << 16)
83
84 #ifdef RCU_MEMBARRIER
85 static int init_done;
86 int rcu_has_sys_membarrier;
87
88 void __attribute__((constructor)) rcu_init(void);
89 #endif
90
91 #ifdef RCU_MB
92 void rcu_init(void)
93 {
94 }
95 #endif
96
97 #ifdef RCU_SIGNAL
98 static int init_done;
99
100 void __attribute__((constructor)) rcu_init(void);
101 void __attribute__((destructor)) rcu_exit(void);
102 #endif
103
104 /*
105 * rcu_gp_lock ensures mutual exclusion between threads calling
106 * synchronize_rcu().
107 */
108 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
109 /*
110 * rcu_registry_lock ensures mutual exclusion between threads
111 * registering and unregistering themselves to/from the registry, and
112 * with threads reading that registry from synchronize_rcu(). However,
113 * this lock is not held all the way through the completion of awaiting
114 * for the grace period. It is sporadically released between iterations
115 * on the registry.
116 * rcu_registry_lock may nest inside rcu_gp_lock.
117 */
118 static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
119 struct rcu_gp rcu_gp = { .ctr = RCU_GP_COUNT };
120
121 /*
122 * Written to only by each individual reader. Read by both the reader and the
123 * writers.
124 */
125 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
126
127 static CDS_LIST_HEAD(registry);
128
129 /*
130 * Queue keeping threads awaiting to wait for a grace period. Contains
131 * struct gp_waiters_thread objects.
132 */
133 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
134
135 static void mutex_lock(pthread_mutex_t *mutex)
136 {
137 int ret;
138
139 #ifndef DISTRUST_SIGNALS_EXTREME
140 ret = pthread_mutex_lock(mutex);
141 if (ret)
142 urcu_die(ret);
143 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
144 while ((ret = pthread_mutex_trylock(mutex)) != 0) {
145 if (ret != EBUSY && ret != EINTR)
146 urcu_die(ret);
147 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
148 cmm_smp_mb();
149 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
150 cmm_smp_mb();
151 }
152 (void) poll(NULL, 0, 10);
153 }
154 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
155 }
156
157 static void mutex_unlock(pthread_mutex_t *mutex)
158 {
159 int ret;
160
161 ret = pthread_mutex_unlock(mutex);
162 if (ret)
163 urcu_die(ret);
164 }
165
166 #ifdef RCU_MEMBARRIER
167 static void smp_mb_master(int group)
168 {
169 if (caa_likely(rcu_has_sys_membarrier))
170 (void) membarrier(MEMBARRIER_EXPEDITED);
171 else
172 cmm_smp_mb();
173 }
174 #endif
175
176 #ifdef RCU_MB
177 static void smp_mb_master(int group)
178 {
179 cmm_smp_mb();
180 }
181 #endif
182
183 #ifdef RCU_SIGNAL
184 static void force_mb_all_readers(void)
185 {
186 struct rcu_reader *index;
187
188 /*
189 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
190 * compiler barriers around rcu read lock as real memory barriers.
191 */
192 if (cds_list_empty(&registry))
193 return;
194 /*
195 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
196 * a cache flush on architectures with non-coherent cache. Let's play
197 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
198 * cache flush is enforced.
199 */
200 cds_list_for_each_entry(index, &registry, node) {
201 CMM_STORE_SHARED(index->need_mb, 1);
202 pthread_kill(index->tid, SIGRCU);
203 }
204 /*
205 * Wait for sighandler (and thus mb()) to execute on every thread.
206 *
207 * Note that the pthread_kill() will never be executed on systems
208 * that correctly deliver signals in a timely manner. However, it
209 * is not uncommon for kernels to have bugs that can result in
210 * lost or unduly delayed signals.
211 *
212 * If you are seeing the below pthread_kill() executing much at
213 * all, we suggest testing the underlying kernel and filing the
214 * relevant bug report. For Linux kernels, we recommend getting
215 * the Linux Test Project (LTP).
216 */
217 cds_list_for_each_entry(index, &registry, node) {
218 while (CMM_LOAD_SHARED(index->need_mb)) {
219 pthread_kill(index->tid, SIGRCU);
220 (void) poll(NULL, 0, 1);
221 }
222 }
223 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
224 }
225
226 static void smp_mb_master(int group)
227 {
228 force_mb_all_readers();
229 }
230 #endif /* #ifdef RCU_SIGNAL */
231
232 /*
233 * synchronize_rcu() waiting. Single thread.
234 */
235 static void wait_gp(void)
236 {
237 /* Read reader_gp before read futex */
238 smp_mb_master(RCU_MB_GROUP);
239 if (uatomic_read(&rcu_gp.futex) == -1)
240 futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
241 NULL, NULL, 0);
242 }
243
244 /*
245 * Always called with rcu_registry lock held. Releases this lock between
246 * iterations and grabs it again. Holds the lock when it returns.
247 */
248 static void wait_for_readers(struct cds_list_head *input_readers,
249 struct cds_list_head *cur_snap_readers,
250 struct cds_list_head *qsreaders)
251 {
252 unsigned int wait_loops = 0;
253 struct rcu_reader *index, *tmp;
254 #ifdef HAS_INCOHERENT_CACHES
255 unsigned int wait_gp_loops = 0;
256 #endif /* HAS_INCOHERENT_CACHES */
257
258 /*
259 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
260 * indicate quiescence (not nested), or observe the current
261 * rcu_gp.ctr value.
262 */
263 for (;;) {
264 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
265 wait_loops++;
266 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
267 uatomic_dec(&rcu_gp.futex);
268 /* Write futex before read reader_gp */
269 smp_mb_master(RCU_MB_GROUP);
270 }
271
272 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
273 switch (rcu_reader_state(&index->ctr)) {
274 case RCU_READER_ACTIVE_CURRENT:
275 if (cur_snap_readers) {
276 cds_list_move(&index->node,
277 cur_snap_readers);
278 break;
279 }
280 /* Fall-through */
281 case RCU_READER_INACTIVE:
282 cds_list_move(&index->node, qsreaders);
283 break;
284 case RCU_READER_ACTIVE_OLD:
285 /*
286 * Old snapshot. Leaving node in
287 * input_readers will make us busy-loop
288 * until the snapshot becomes current or
289 * the reader becomes inactive.
290 */
291 break;
292 }
293 }
294
295 #ifndef HAS_INCOHERENT_CACHES
296 if (cds_list_empty(input_readers)) {
297 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
298 /* Read reader_gp before write futex */
299 smp_mb_master(RCU_MB_GROUP);
300 uatomic_set(&rcu_gp.futex, 0);
301 }
302 break;
303 } else {
304 /* Temporarily unlock the registry lock. */
305 mutex_unlock(&rcu_registry_lock);
306 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS)
307 wait_gp();
308 else
309 caa_cpu_relax();
310 /* Re-lock the registry lock before the next loop. */
311 mutex_lock(&rcu_registry_lock);
312 }
313 #else /* #ifndef HAS_INCOHERENT_CACHES */
314 /*
315 * BUSY-LOOP. Force the reader thread to commit its
316 * URCU_TLS(rcu_reader).ctr update to memory if we wait
317 * for too long.
318 */
319 if (cds_list_empty(input_readers)) {
320 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
321 /* Read reader_gp before write futex */
322 smp_mb_master(RCU_MB_GROUP);
323 uatomic_set(&rcu_gp.futex, 0);
324 }
325 break;
326 } else {
327 if (wait_gp_loops == KICK_READER_LOOPS) {
328 smp_mb_master(RCU_MB_GROUP);
329 wait_gp_loops = 0;
330 }
331 /* Temporarily unlock the registry lock. */
332 mutex_unlock(&rcu_registry_lock);
333 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
334 wait_gp();
335 wait_gp_loops++;
336 } else {
337 caa_cpu_relax();
338 }
339 /* Re-lock the registry lock before the next loop. */
340 mutex_lock(&rcu_registry_lock);
341 }
342 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
343 }
344 }
345
346 void synchronize_rcu(void)
347 {
348 CDS_LIST_HEAD(cur_snap_readers);
349 CDS_LIST_HEAD(qsreaders);
350 DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
351 struct urcu_waiters waiters;
352
353 /*
354 * Add ourself to gp_waiters queue of threads awaiting to wait
355 * for a grace period. Proceed to perform the grace period only
356 * if we are the first thread added into the queue.
357 * The implicit memory barrier before urcu_wait_add()
358 * orders prior memory accesses of threads put into the wait
359 * queue before their insertion into the wait queue.
360 */
361 if (urcu_wait_add(&gp_waiters, &wait) != 0) {
362 /* Not first in queue: will be awakened by another thread. */
363 urcu_adaptative_busy_wait(&wait);
364 /* Order following memory accesses after grace period. */
365 cmm_smp_mb();
366 return;
367 }
368 /* We won't need to wake ourself up */
369 urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
370
371 mutex_lock(&rcu_gp_lock);
372
373 /*
374 * Move all waiters into our local queue.
375 */
376 urcu_move_waiters(&waiters, &gp_waiters);
377
378 mutex_lock(&rcu_registry_lock);
379
380 if (cds_list_empty(&registry))
381 goto out;
382
383 /*
384 * All threads should read qparity before accessing data structure
385 * where new ptr points to. Must be done within rcu_registry_lock
386 * because it iterates on reader threads.
387 */
388 /* Write new ptr before changing the qparity */
389 smp_mb_master(RCU_MB_GROUP);
390
391 /*
392 * Wait for readers to observe original parity or be quiescent.
393 * wait_for_readers() can release and grab again rcu_registry_lock
394 * interally.
395 */
396 wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
397
398 /*
399 * Must finish waiting for quiescent state for original parity before
400 * committing next rcu_gp.ctr update to memory. Failure to do so could
401 * result in the writer waiting forever while new readers are always
402 * accessing data (no progress). Enforce compiler-order of load
403 * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
404 */
405 cmm_barrier();
406
407 /*
408 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
409 * model easier to understand. It does not have a big performance impact
410 * anyway, given this is the write-side.
411 */
412 cmm_smp_mb();
413
414 /* Switch parity: 0 -> 1, 1 -> 0 */
415 CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR_PHASE);
416
417 /*
418 * Must commit rcu_gp.ctr update to memory before waiting for quiescent
419 * state. Failure to do so could result in the writer waiting forever
420 * while new readers are always accessing data (no progress). Enforce
421 * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
422 */
423 cmm_barrier();
424
425 /*
426 *
427 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
428 * model easier to understand. It does not have a big performance impact
429 * anyway, given this is the write-side.
430 */
431 cmm_smp_mb();
432
433 /*
434 * Wait for readers to observe new parity or be quiescent.
435 * wait_for_readers() can release and grab again rcu_registry_lock
436 * interally.
437 */
438 wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
439
440 /*
441 * Put quiescent reader list back into registry.
442 */
443 cds_list_splice(&qsreaders, &registry);
444
445 /*
446 * Finish waiting for reader threads before letting the old ptr
447 * being freed. Must be done within rcu_registry_lock because it
448 * iterates on reader threads.
449 */
450 smp_mb_master(RCU_MB_GROUP);
451 out:
452 mutex_unlock(&rcu_registry_lock);
453 mutex_unlock(&rcu_gp_lock);
454
455 /*
456 * Wakeup waiters only after we have completed the grace period
457 * and have ensured the memory barriers at the end of the grace
458 * period have been issued.
459 */
460 urcu_wake_all_waiters(&waiters);
461 }
462
463 /*
464 * library wrappers to be used by non-LGPL compatible source code.
465 */
466
467 void rcu_read_lock(void)
468 {
469 _rcu_read_lock();
470 }
471
472 void rcu_read_unlock(void)
473 {
474 _rcu_read_unlock();
475 }
476
477 int rcu_read_ongoing(void)
478 {
479 return _rcu_read_ongoing();
480 }
481
482 void rcu_register_thread(void)
483 {
484 URCU_TLS(rcu_reader).tid = pthread_self();
485 assert(URCU_TLS(rcu_reader).need_mb == 0);
486 assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
487
488 mutex_lock(&rcu_registry_lock);
489 rcu_init(); /* In case gcc does not support constructor attribute */
490 cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
491 mutex_unlock(&rcu_registry_lock);
492 }
493
494 void rcu_unregister_thread(void)
495 {
496 mutex_lock(&rcu_registry_lock);
497 cds_list_del(&URCU_TLS(rcu_reader).node);
498 mutex_unlock(&rcu_registry_lock);
499 }
500
501 #ifdef RCU_MEMBARRIER
502 void rcu_init(void)
503 {
504 if (init_done)
505 return;
506 init_done = 1;
507 if (!membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY))
508 rcu_has_sys_membarrier = 1;
509 }
510 #endif
511
512 #ifdef RCU_SIGNAL
513 static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
514 {
515 /*
516 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
517 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
518 * executed on.
519 */
520 cmm_smp_mb();
521 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
522 cmm_smp_mb();
523 }
524
525 /*
526 * rcu_init constructor. Called when the library is linked, but also when
527 * reader threads are calling rcu_register_thread().
528 * Should only be called by a single thread at a given time. This is ensured by
529 * holing the rcu_registry_lock from rcu_register_thread() or by running
530 * at library load time, which should not be executed by multiple
531 * threads nor concurrently with rcu_register_thread() anyway.
532 */
533 void rcu_init(void)
534 {
535 struct sigaction act;
536 int ret;
537
538 if (init_done)
539 return;
540 init_done = 1;
541
542 act.sa_sigaction = sigrcu_handler;
543 act.sa_flags = SA_SIGINFO | SA_RESTART;
544 sigemptyset(&act.sa_mask);
545 ret = sigaction(SIGRCU, &act, NULL);
546 if (ret)
547 urcu_die(errno);
548 }
549
550 void rcu_exit(void)
551 {
552 /*
553 * Don't unregister the SIGRCU signal handler anymore, because
554 * call_rcu threads could still be using it shortly before the
555 * application exits.
556 * Assertion disabled because call_rcu threads are now rcu
557 * readers, and left running at exit.
558 * assert(cds_list_empty(&registry));
559 */
560 }
561
562 #endif /* #ifdef RCU_SIGNAL */
563
564 DEFINE_RCU_FLAVOR(rcu_flavor);
565
566 #include "urcu-call-rcu-impl.h"
567 #include "urcu-defer-impl.h"
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