Allow forcing the use of sys membarrier
[urcu.git] / src / 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 _LGPL_SOURCE
28 #define _DEFAULT_SOURCE
29 #include <stdio.h>
30 #include <pthread.h>
31 #include <signal.h>
32 #include <assert.h>
33 #include <stdlib.h>
34 #include <stdint.h>
35 #include <string.h>
36 #include <errno.h>
37 #include <poll.h>
38
39 #include "urcu/arch.h"
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 /* If the headers do not support membarrier system call, fall back on RCU_MB */
67 #ifdef __NR_membarrier
68 # define membarrier(...) syscall(__NR_membarrier, __VA_ARGS__)
69 #else
70 # define membarrier(...) -ENOSYS
71 #endif
72
73 enum membarrier_cmd {
74 MEMBARRIER_CMD_QUERY = 0,
75 MEMBARRIER_CMD_SHARED = (1 << 0),
76 };
77
78 #ifdef RCU_MEMBARRIER
79 static int init_done;
80 #ifndef CONFIG_RCU_FORCE_SYS_MEMBARRIER
81 int rcu_has_sys_membarrier_memb;
82 #endif
83
84 void __attribute__((constructor)) rcu_init(void);
85 #endif
86
87 #ifdef RCU_MB
88 void rcu_init(void)
89 {
90 }
91 #endif
92
93 #ifdef RCU_SIGNAL
94 static int init_done;
95
96 void __attribute__((constructor)) rcu_init(void);
97 void __attribute__((destructor)) rcu_exit(void);
98 #endif
99
100 /*
101 * rcu_gp_lock ensures mutual exclusion between threads calling
102 * synchronize_rcu().
103 */
104 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
105 /*
106 * rcu_registry_lock ensures mutual exclusion between threads
107 * registering and unregistering themselves to/from the registry, and
108 * with threads reading that registry from synchronize_rcu(). However,
109 * this lock is not held all the way through the completion of awaiting
110 * for the grace period. It is sporadically released between iterations
111 * on the registry.
112 * rcu_registry_lock may nest inside rcu_gp_lock.
113 */
114 static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
115 struct rcu_gp rcu_gp = { .ctr = RCU_GP_COUNT };
116
117 /*
118 * Written to only by each individual reader. Read by both the reader and the
119 * writers.
120 */
121 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
122
123 static CDS_LIST_HEAD(registry);
124
125 /*
126 * Queue keeping threads awaiting to wait for a grace period. Contains
127 * struct gp_waiters_thread objects.
128 */
129 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
130
131 static void mutex_lock(pthread_mutex_t *mutex)
132 {
133 int ret;
134
135 #ifndef DISTRUST_SIGNALS_EXTREME
136 ret = pthread_mutex_lock(mutex);
137 if (ret)
138 urcu_die(ret);
139 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
140 while ((ret = pthread_mutex_trylock(mutex)) != 0) {
141 if (ret != EBUSY && ret != EINTR)
142 urcu_die(ret);
143 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
144 cmm_smp_mb();
145 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
146 cmm_smp_mb();
147 }
148 (void) poll(NULL, 0, 10);
149 }
150 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
151 }
152
153 static void mutex_unlock(pthread_mutex_t *mutex)
154 {
155 int ret;
156
157 ret = pthread_mutex_unlock(mutex);
158 if (ret)
159 urcu_die(ret);
160 }
161
162 #ifdef RCU_MEMBARRIER
163 static void smp_mb_master(void)
164 {
165 if (caa_likely(rcu_has_sys_membarrier_memb))
166 (void) membarrier(MEMBARRIER_CMD_SHARED, 0);
167 else
168 cmm_smp_mb();
169 }
170 #endif
171
172 #ifdef RCU_MB
173 static void smp_mb_master(void)
174 {
175 cmm_smp_mb();
176 }
177 #endif
178
179 #ifdef RCU_SIGNAL
180 static void force_mb_all_readers(void)
181 {
182 struct rcu_reader *index;
183
184 /*
185 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
186 * compiler barriers around rcu read lock as real memory barriers.
187 */
188 if (cds_list_empty(&registry))
189 return;
190 /*
191 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
192 * a cache flush on architectures with non-coherent cache. Let's play
193 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
194 * cache flush is enforced.
195 */
196 cds_list_for_each_entry(index, &registry, node) {
197 CMM_STORE_SHARED(index->need_mb, 1);
198 pthread_kill(index->tid, SIGRCU);
199 }
200 /*
201 * Wait for sighandler (and thus mb()) to execute on every thread.
202 *
203 * Note that the pthread_kill() will never be executed on systems
204 * that correctly deliver signals in a timely manner. However, it
205 * is not uncommon for kernels to have bugs that can result in
206 * lost or unduly delayed signals.
207 *
208 * If you are seeing the below pthread_kill() executing much at
209 * all, we suggest testing the underlying kernel and filing the
210 * relevant bug report. For Linux kernels, we recommend getting
211 * the Linux Test Project (LTP).
212 */
213 cds_list_for_each_entry(index, &registry, node) {
214 while (CMM_LOAD_SHARED(index->need_mb)) {
215 pthread_kill(index->tid, SIGRCU);
216 (void) poll(NULL, 0, 1);
217 }
218 }
219 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
220 }
221
222 static void smp_mb_master(void)
223 {
224 force_mb_all_readers();
225 }
226 #endif /* #ifdef RCU_SIGNAL */
227
228 /*
229 * synchronize_rcu() waiting. Single thread.
230 * Always called with rcu_registry lock held. Releases this lock and
231 * grabs it again. Holds the lock when it returns.
232 */
233 static void wait_gp(void)
234 {
235 /*
236 * Read reader_gp before read futex. smp_mb_master() needs to
237 * be called with the rcu registry lock held in RCU_SIGNAL
238 * flavor.
239 */
240 smp_mb_master();
241 /* Temporarily unlock the registry lock. */
242 mutex_unlock(&rcu_registry_lock);
243 if (uatomic_read(&rcu_gp.futex) != -1)
244 goto end;
245 while (futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
246 NULL, NULL, 0)) {
247 switch (errno) {
248 case EWOULDBLOCK:
249 /* Value already changed. */
250 goto end;
251 case EINTR:
252 /* Retry if interrupted by signal. */
253 break; /* Get out of switch. */
254 default:
255 /* Unexpected error. */
256 urcu_die(errno);
257 }
258 }
259 end:
260 /*
261 * Re-lock the registry lock before the next loop.
262 */
263 mutex_lock(&rcu_registry_lock);
264 }
265
266 /*
267 * Always called with rcu_registry lock held. Releases this lock between
268 * iterations and grabs it again. Holds the lock when it returns.
269 */
270 static void wait_for_readers(struct cds_list_head *input_readers,
271 struct cds_list_head *cur_snap_readers,
272 struct cds_list_head *qsreaders)
273 {
274 unsigned int wait_loops = 0;
275 struct rcu_reader *index, *tmp;
276 #ifdef HAS_INCOHERENT_CACHES
277 unsigned int wait_gp_loops = 0;
278 #endif /* HAS_INCOHERENT_CACHES */
279
280 /*
281 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
282 * indicate quiescence (not nested), or observe the current
283 * rcu_gp.ctr value.
284 */
285 for (;;) {
286 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
287 wait_loops++;
288 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
289 uatomic_dec(&rcu_gp.futex);
290 /* Write futex before read reader_gp */
291 smp_mb_master();
292 }
293
294 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
295 switch (rcu_reader_state(&index->ctr)) {
296 case RCU_READER_ACTIVE_CURRENT:
297 if (cur_snap_readers) {
298 cds_list_move(&index->node,
299 cur_snap_readers);
300 break;
301 }
302 /* Fall-through */
303 case RCU_READER_INACTIVE:
304 cds_list_move(&index->node, qsreaders);
305 break;
306 case RCU_READER_ACTIVE_OLD:
307 /*
308 * Old snapshot. Leaving node in
309 * input_readers will make us busy-loop
310 * until the snapshot becomes current or
311 * the reader becomes inactive.
312 */
313 break;
314 }
315 }
316
317 #ifndef HAS_INCOHERENT_CACHES
318 if (cds_list_empty(input_readers)) {
319 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
320 /* Read reader_gp before write futex */
321 smp_mb_master();
322 uatomic_set(&rcu_gp.futex, 0);
323 }
324 break;
325 } else {
326 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
327 /* wait_gp unlocks/locks registry lock. */
328 wait_gp();
329 } else {
330 /* Temporarily unlock the registry lock. */
331 mutex_unlock(&rcu_registry_lock);
332 caa_cpu_relax();
333 /*
334 * Re-lock the registry lock before the
335 * next loop.
336 */
337 mutex_lock(&rcu_registry_lock);
338 }
339 }
340 #else /* #ifndef HAS_INCOHERENT_CACHES */
341 /*
342 * BUSY-LOOP. Force the reader thread to commit its
343 * URCU_TLS(rcu_reader).ctr update to memory if we wait
344 * for too long.
345 */
346 if (cds_list_empty(input_readers)) {
347 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
348 /* Read reader_gp before write futex */
349 smp_mb_master();
350 uatomic_set(&rcu_gp.futex, 0);
351 }
352 break;
353 } else {
354 if (wait_gp_loops == KICK_READER_LOOPS) {
355 smp_mb_master();
356 wait_gp_loops = 0;
357 }
358 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
359 /* wait_gp unlocks/locks registry lock. */
360 wait_gp();
361 wait_gp_loops++;
362 } else {
363 /* Temporarily unlock the registry lock. */
364 mutex_unlock(&rcu_registry_lock);
365 caa_cpu_relax();
366 /*
367 * Re-lock the registry lock before the
368 * next loop.
369 */
370 mutex_lock(&rcu_registry_lock);
371 }
372 }
373 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
374 }
375 }
376
377 void synchronize_rcu(void)
378 {
379 CDS_LIST_HEAD(cur_snap_readers);
380 CDS_LIST_HEAD(qsreaders);
381 DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
382 struct urcu_waiters waiters;
383
384 /*
385 * Add ourself to gp_waiters queue of threads awaiting to wait
386 * for a grace period. Proceed to perform the grace period only
387 * if we are the first thread added into the queue.
388 * The implicit memory barrier before urcu_wait_add()
389 * orders prior memory accesses of threads put into the wait
390 * queue before their insertion into the wait queue.
391 */
392 if (urcu_wait_add(&gp_waiters, &wait) != 0) {
393 /* Not first in queue: will be awakened by another thread. */
394 urcu_adaptative_busy_wait(&wait);
395 /* Order following memory accesses after grace period. */
396 cmm_smp_mb();
397 return;
398 }
399 /* We won't need to wake ourself up */
400 urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
401
402 mutex_lock(&rcu_gp_lock);
403
404 /*
405 * Move all waiters into our local queue.
406 */
407 urcu_move_waiters(&waiters, &gp_waiters);
408
409 mutex_lock(&rcu_registry_lock);
410
411 if (cds_list_empty(&registry))
412 goto out;
413
414 /*
415 * All threads should read qparity before accessing data structure
416 * where new ptr points to. Must be done within rcu_registry_lock
417 * because it iterates on reader threads.
418 */
419 /* Write new ptr before changing the qparity */
420 smp_mb_master();
421
422 /*
423 * Wait for readers to observe original parity or be quiescent.
424 * wait_for_readers() can release and grab again rcu_registry_lock
425 * interally.
426 */
427 wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
428
429 /*
430 * Must finish waiting for quiescent state for original parity before
431 * committing next rcu_gp.ctr update to memory. Failure to do so could
432 * result in the writer waiting forever while new readers are always
433 * accessing data (no progress). Enforce compiler-order of load
434 * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
435 */
436 cmm_barrier();
437
438 /*
439 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
440 * model easier to understand. It does not have a big performance impact
441 * anyway, given this is the write-side.
442 */
443 cmm_smp_mb();
444
445 /* Switch parity: 0 -> 1, 1 -> 0 */
446 CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR_PHASE);
447
448 /*
449 * Must commit rcu_gp.ctr update to memory before waiting for quiescent
450 * state. Failure to do so could result in the writer waiting forever
451 * while new readers are always accessing data (no progress). Enforce
452 * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
453 */
454 cmm_barrier();
455
456 /*
457 *
458 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
459 * model easier to understand. It does not have a big performance impact
460 * anyway, given this is the write-side.
461 */
462 cmm_smp_mb();
463
464 /*
465 * Wait for readers to observe new parity or be quiescent.
466 * wait_for_readers() can release and grab again rcu_registry_lock
467 * interally.
468 */
469 wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
470
471 /*
472 * Put quiescent reader list back into registry.
473 */
474 cds_list_splice(&qsreaders, &registry);
475
476 /*
477 * Finish waiting for reader threads before letting the old ptr
478 * being freed. Must be done within rcu_registry_lock because it
479 * iterates on reader threads.
480 */
481 smp_mb_master();
482 out:
483 mutex_unlock(&rcu_registry_lock);
484 mutex_unlock(&rcu_gp_lock);
485
486 /*
487 * Wakeup waiters only after we have completed the grace period
488 * and have ensured the memory barriers at the end of the grace
489 * period have been issued.
490 */
491 urcu_wake_all_waiters(&waiters);
492 }
493
494 /*
495 * library wrappers to be used by non-LGPL compatible source code.
496 */
497
498 void rcu_read_lock(void)
499 {
500 _rcu_read_lock();
501 }
502
503 void rcu_read_unlock(void)
504 {
505 _rcu_read_unlock();
506 }
507
508 int rcu_read_ongoing(void)
509 {
510 return _rcu_read_ongoing();
511 }
512
513 void rcu_register_thread(void)
514 {
515 URCU_TLS(rcu_reader).tid = pthread_self();
516 assert(URCU_TLS(rcu_reader).need_mb == 0);
517 assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
518
519 mutex_lock(&rcu_registry_lock);
520 assert(!URCU_TLS(rcu_reader).registered);
521 URCU_TLS(rcu_reader).registered = 1;
522 rcu_init(); /* In case gcc does not support constructor attribute */
523 cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
524 mutex_unlock(&rcu_registry_lock);
525 }
526
527 void rcu_unregister_thread(void)
528 {
529 mutex_lock(&rcu_registry_lock);
530 assert(URCU_TLS(rcu_reader).registered);
531 URCU_TLS(rcu_reader).registered = 0;
532 cds_list_del(&URCU_TLS(rcu_reader).node);
533 mutex_unlock(&rcu_registry_lock);
534 }
535
536 #ifdef RCU_MEMBARRIER
537
538 #ifdef CONFIG_RCU_FORCE_SYS_MEMBARRIER
539 static
540 void rcu_sys_membarrier_status(int available)
541 {
542 if (!available)
543 abort();
544 }
545 #else
546 static
547 void rcu_sys_membarrier_status(int available)
548 {
549 if (available)
550 rcu_has_sys_membarrier_memb = 1;
551 }
552 #endif
553
554 void rcu_init(void)
555 {
556 int ret;
557
558 if (init_done)
559 return;
560 init_done = 1;
561 ret = membarrier(MEMBARRIER_CMD_QUERY, 0);
562 rcu_sys_membarrier_status(ret >= 0 && (ret & MEMBARRIER_CMD_SHARED));
563 }
564 #endif
565
566 #ifdef RCU_SIGNAL
567 static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
568 {
569 /*
570 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
571 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
572 * executed on.
573 */
574 cmm_smp_mb();
575 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
576 cmm_smp_mb();
577 }
578
579 /*
580 * rcu_init constructor. Called when the library is linked, but also when
581 * reader threads are calling rcu_register_thread().
582 * Should only be called by a single thread at a given time. This is ensured by
583 * holing the rcu_registry_lock from rcu_register_thread() or by running
584 * at library load time, which should not be executed by multiple
585 * threads nor concurrently with rcu_register_thread() anyway.
586 */
587 void rcu_init(void)
588 {
589 struct sigaction act;
590 int ret;
591
592 if (init_done)
593 return;
594 init_done = 1;
595
596 act.sa_sigaction = sigrcu_handler;
597 act.sa_flags = SA_SIGINFO | SA_RESTART;
598 sigemptyset(&act.sa_mask);
599 ret = sigaction(SIGRCU, &act, NULL);
600 if (ret)
601 urcu_die(errno);
602 }
603
604 void rcu_exit(void)
605 {
606 /*
607 * Don't unregister the SIGRCU signal handler anymore, because
608 * call_rcu threads could still be using it shortly before the
609 * application exits.
610 * Assertion disabled because call_rcu threads are now rcu
611 * readers, and left running at exit.
612 * assert(cds_list_empty(&registry));
613 */
614 }
615
616 #endif /* #ifdef RCU_SIGNAL */
617
618 DEFINE_RCU_FLAVOR(rcu_flavor);
619
620 #include "urcu-call-rcu-impl.h"
621 #include "urcu-defer-impl.h"
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