ccd9706dda6fe6a8b6bfcba2fced7893054cf3ae
[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 int rcu_has_sys_membarrier;
81
82 void __attribute__((constructor)) rcu_init(void);
83 #endif
84
85 #ifdef RCU_MB
86 void rcu_init(void)
87 {
88 }
89 #endif
90
91 #ifdef RCU_SIGNAL
92 static int init_done;
93
94 void __attribute__((constructor)) rcu_init(void);
95 void __attribute__((destructor)) rcu_exit(void);
96 #endif
97
98 /*
99 * rcu_gp_lock ensures mutual exclusion between threads calling
100 * synchronize_rcu().
101 */
102 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
103 /*
104 * rcu_registry_lock ensures mutual exclusion between threads
105 * registering and unregistering themselves to/from the registry, and
106 * with threads reading that registry from synchronize_rcu(). However,
107 * this lock is not held all the way through the completion of awaiting
108 * for the grace period. It is sporadically released between iterations
109 * on the registry.
110 * rcu_registry_lock may nest inside rcu_gp_lock.
111 */
112 static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
113 struct rcu_gp rcu_gp = { .ctr = RCU_GP_COUNT };
114
115 /*
116 * Written to only by each individual reader. Read by both the reader and the
117 * writers.
118 */
119 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
120
121 static CDS_LIST_HEAD(registry);
122
123 /*
124 * Queue keeping threads awaiting to wait for a grace period. Contains
125 * struct gp_waiters_thread objects.
126 */
127 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
128
129 static void mutex_lock(pthread_mutex_t *mutex)
130 {
131 int ret;
132
133 #ifndef DISTRUST_SIGNALS_EXTREME
134 ret = pthread_mutex_lock(mutex);
135 if (ret)
136 urcu_die(ret);
137 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
138 while ((ret = pthread_mutex_trylock(mutex)) != 0) {
139 if (ret != EBUSY && ret != EINTR)
140 urcu_die(ret);
141 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
142 cmm_smp_mb();
143 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
144 cmm_smp_mb();
145 }
146 (void) poll(NULL, 0, 10);
147 }
148 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
149 }
150
151 static void mutex_unlock(pthread_mutex_t *mutex)
152 {
153 int ret;
154
155 ret = pthread_mutex_unlock(mutex);
156 if (ret)
157 urcu_die(ret);
158 }
159
160 #ifdef RCU_MEMBARRIER
161 static void smp_mb_master(void)
162 {
163 if (caa_likely(rcu_has_sys_membarrier))
164 (void) membarrier(MEMBARRIER_CMD_SHARED, 0);
165 else
166 cmm_smp_mb();
167 }
168 #endif
169
170 #ifdef RCU_MB
171 static void smp_mb_master(void)
172 {
173 cmm_smp_mb();
174 }
175 #endif
176
177 #ifdef RCU_SIGNAL
178 static void force_mb_all_readers(void)
179 {
180 struct rcu_reader *index;
181
182 /*
183 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
184 * compiler barriers around rcu read lock as real memory barriers.
185 */
186 if (cds_list_empty(&registry))
187 return;
188 /*
189 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
190 * a cache flush on architectures with non-coherent cache. Let's play
191 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
192 * cache flush is enforced.
193 */
194 cds_list_for_each_entry(index, &registry, node) {
195 CMM_STORE_SHARED(index->need_mb, 1);
196 pthread_kill(index->tid, SIGRCU);
197 }
198 /*
199 * Wait for sighandler (and thus mb()) to execute on every thread.
200 *
201 * Note that the pthread_kill() will never be executed on systems
202 * that correctly deliver signals in a timely manner. However, it
203 * is not uncommon for kernels to have bugs that can result in
204 * lost or unduly delayed signals.
205 *
206 * If you are seeing the below pthread_kill() executing much at
207 * all, we suggest testing the underlying kernel and filing the
208 * relevant bug report. For Linux kernels, we recommend getting
209 * the Linux Test Project (LTP).
210 */
211 cds_list_for_each_entry(index, &registry, node) {
212 while (CMM_LOAD_SHARED(index->need_mb)) {
213 pthread_kill(index->tid, SIGRCU);
214 (void) poll(NULL, 0, 1);
215 }
216 }
217 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
218 }
219
220 static void smp_mb_master(void)
221 {
222 force_mb_all_readers();
223 }
224 #endif /* #ifdef RCU_SIGNAL */
225
226 /*
227 * synchronize_rcu() waiting. Single thread.
228 * Always called with rcu_registry lock held. Releases this lock and
229 * grabs it again. Holds the lock when it returns.
230 */
231 static void wait_gp(void)
232 {
233 /*
234 * Read reader_gp before read futex. smp_mb_master() needs to
235 * be called with the rcu registry lock held in RCU_SIGNAL
236 * flavor.
237 */
238 smp_mb_master();
239 /* Temporarily unlock the registry lock. */
240 mutex_unlock(&rcu_registry_lock);
241 if (uatomic_read(&rcu_gp.futex) != -1)
242 goto end;
243 while (futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
244 NULL, NULL, 0)) {
245 switch (errno) {
246 case EWOULDBLOCK:
247 /* Value already changed. */
248 goto end;
249 case EINTR:
250 /* Retry if interrupted by signal. */
251 break; /* Get out of switch. */
252 default:
253 /* Unexpected error. */
254 urcu_die(errno);
255 }
256 }
257 end:
258 /*
259 * Re-lock the registry lock before the next loop.
260 */
261 mutex_lock(&rcu_registry_lock);
262 }
263
264 /*
265 * Always called with rcu_registry lock held. Releases this lock between
266 * iterations and grabs it again. Holds the lock when it returns.
267 */
268 static void wait_for_readers(struct cds_list_head *input_readers,
269 struct cds_list_head *cur_snap_readers,
270 struct cds_list_head *qsreaders)
271 {
272 unsigned int wait_loops = 0;
273 struct rcu_reader *index, *tmp;
274 #ifdef HAS_INCOHERENT_CACHES
275 unsigned int wait_gp_loops = 0;
276 #endif /* HAS_INCOHERENT_CACHES */
277
278 /*
279 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
280 * indicate quiescence (not nested), or observe the current
281 * rcu_gp.ctr value.
282 */
283 for (;;) {
284 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
285 wait_loops++;
286 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
287 uatomic_dec(&rcu_gp.futex);
288 /* Write futex before read reader_gp */
289 smp_mb_master();
290 }
291
292 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
293 switch (rcu_reader_state(&index->ctr)) {
294 case RCU_READER_ACTIVE_CURRENT:
295 if (cur_snap_readers) {
296 cds_list_move(&index->node,
297 cur_snap_readers);
298 break;
299 }
300 /* Fall-through */
301 case RCU_READER_INACTIVE:
302 cds_list_move(&index->node, qsreaders);
303 break;
304 case RCU_READER_ACTIVE_OLD:
305 /*
306 * Old snapshot. Leaving node in
307 * input_readers will make us busy-loop
308 * until the snapshot becomes current or
309 * the reader becomes inactive.
310 */
311 break;
312 }
313 }
314
315 #ifndef HAS_INCOHERENT_CACHES
316 if (cds_list_empty(input_readers)) {
317 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
318 /* Read reader_gp before write futex */
319 smp_mb_master();
320 uatomic_set(&rcu_gp.futex, 0);
321 }
322 break;
323 } else {
324 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
325 /* wait_gp unlocks/locks registry lock. */
326 wait_gp();
327 } else {
328 /* Temporarily unlock the registry lock. */
329 mutex_unlock(&rcu_registry_lock);
330 caa_cpu_relax();
331 /*
332 * Re-lock the registry lock before the
333 * next loop.
334 */
335 mutex_lock(&rcu_registry_lock);
336 }
337 }
338 #else /* #ifndef HAS_INCOHERENT_CACHES */
339 /*
340 * BUSY-LOOP. Force the reader thread to commit its
341 * URCU_TLS(rcu_reader).ctr update to memory if we wait
342 * for too long.
343 */
344 if (cds_list_empty(input_readers)) {
345 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
346 /* Read reader_gp before write futex */
347 smp_mb_master();
348 uatomic_set(&rcu_gp.futex, 0);
349 }
350 break;
351 } else {
352 if (wait_gp_loops == KICK_READER_LOOPS) {
353 smp_mb_master();
354 wait_gp_loops = 0;
355 }
356 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
357 /* wait_gp unlocks/locks registry lock. */
358 wait_gp();
359 wait_gp_loops++;
360 } else {
361 /* Temporarily unlock the registry lock. */
362 mutex_unlock(&rcu_registry_lock);
363 caa_cpu_relax();
364 /*
365 * Re-lock the registry lock before the
366 * next loop.
367 */
368 mutex_lock(&rcu_registry_lock);
369 }
370 }
371 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
372 }
373 }
374
375 void synchronize_rcu(void)
376 {
377 CDS_LIST_HEAD(cur_snap_readers);
378 CDS_LIST_HEAD(qsreaders);
379 DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
380 struct urcu_waiters waiters;
381
382 /*
383 * Add ourself to gp_waiters queue of threads awaiting to wait
384 * for a grace period. Proceed to perform the grace period only
385 * if we are the first thread added into the queue.
386 * The implicit memory barrier before urcu_wait_add()
387 * orders prior memory accesses of threads put into the wait
388 * queue before their insertion into the wait queue.
389 */
390 if (urcu_wait_add(&gp_waiters, &wait) != 0) {
391 /* Not first in queue: will be awakened by another thread. */
392 urcu_adaptative_busy_wait(&wait);
393 /* Order following memory accesses after grace period. */
394 cmm_smp_mb();
395 return;
396 }
397 /* We won't need to wake ourself up */
398 urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
399
400 mutex_lock(&rcu_gp_lock);
401
402 /*
403 * Move all waiters into our local queue.
404 */
405 urcu_move_waiters(&waiters, &gp_waiters);
406
407 mutex_lock(&rcu_registry_lock);
408
409 if (cds_list_empty(&registry))
410 goto out;
411
412 /*
413 * All threads should read qparity before accessing data structure
414 * where new ptr points to. Must be done within rcu_registry_lock
415 * because it iterates on reader threads.
416 */
417 /* Write new ptr before changing the qparity */
418 smp_mb_master();
419
420 /*
421 * Wait for readers to observe original parity or be quiescent.
422 * wait_for_readers() can release and grab again rcu_registry_lock
423 * interally.
424 */
425 wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
426
427 /*
428 * Must finish waiting for quiescent state for original parity before
429 * committing next rcu_gp.ctr update to memory. Failure to do so could
430 * result in the writer waiting forever while new readers are always
431 * accessing data (no progress). Enforce compiler-order of load
432 * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
433 */
434 cmm_barrier();
435
436 /*
437 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
438 * model easier to understand. It does not have a big performance impact
439 * anyway, given this is the write-side.
440 */
441 cmm_smp_mb();
442
443 /* Switch parity: 0 -> 1, 1 -> 0 */
444 CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR_PHASE);
445
446 /*
447 * Must commit rcu_gp.ctr update to memory before waiting for quiescent
448 * state. Failure to do so could result in the writer waiting forever
449 * while new readers are always accessing data (no progress). Enforce
450 * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
451 */
452 cmm_barrier();
453
454 /*
455 *
456 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
457 * model easier to understand. It does not have a big performance impact
458 * anyway, given this is the write-side.
459 */
460 cmm_smp_mb();
461
462 /*
463 * Wait for readers to observe new parity or be quiescent.
464 * wait_for_readers() can release and grab again rcu_registry_lock
465 * interally.
466 */
467 wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
468
469 /*
470 * Put quiescent reader list back into registry.
471 */
472 cds_list_splice(&qsreaders, &registry);
473
474 /*
475 * Finish waiting for reader threads before letting the old ptr
476 * being freed. Must be done within rcu_registry_lock because it
477 * iterates on reader threads.
478 */
479 smp_mb_master();
480 out:
481 mutex_unlock(&rcu_registry_lock);
482 mutex_unlock(&rcu_gp_lock);
483
484 /*
485 * Wakeup waiters only after we have completed the grace period
486 * and have ensured the memory barriers at the end of the grace
487 * period have been issued.
488 */
489 urcu_wake_all_waiters(&waiters);
490 }
491
492 /*
493 * library wrappers to be used by non-LGPL compatible source code.
494 */
495
496 void rcu_read_lock(void)
497 {
498 _rcu_read_lock();
499 }
500
501 void rcu_read_unlock(void)
502 {
503 _rcu_read_unlock();
504 }
505
506 int rcu_read_ongoing(void)
507 {
508 return _rcu_read_ongoing();
509 }
510
511 void rcu_register_thread(void)
512 {
513 URCU_TLS(rcu_reader).tid = pthread_self();
514 assert(URCU_TLS(rcu_reader).need_mb == 0);
515 assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
516
517 mutex_lock(&rcu_registry_lock);
518 assert(!URCU_TLS(rcu_reader).registered);
519 URCU_TLS(rcu_reader).registered = 1;
520 rcu_init(); /* In case gcc does not support constructor attribute */
521 cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
522 mutex_unlock(&rcu_registry_lock);
523 }
524
525 void rcu_unregister_thread(void)
526 {
527 mutex_lock(&rcu_registry_lock);
528 assert(URCU_TLS(rcu_reader).registered);
529 URCU_TLS(rcu_reader).registered = 0;
530 cds_list_del(&URCU_TLS(rcu_reader).node);
531 mutex_unlock(&rcu_registry_lock);
532 }
533
534 #ifdef RCU_MEMBARRIER
535 void rcu_init(void)
536 {
537 int ret;
538
539 if (init_done)
540 return;
541 init_done = 1;
542 ret = membarrier(MEMBARRIER_CMD_QUERY, 0);
543 if (ret >= 0 && (ret & MEMBARRIER_CMD_SHARED)) {
544 rcu_has_sys_membarrier = 1;
545 }
546 }
547 #endif
548
549 #ifdef RCU_SIGNAL
550 static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
551 {
552 /*
553 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
554 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
555 * executed on.
556 */
557 cmm_smp_mb();
558 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
559 cmm_smp_mb();
560 }
561
562 /*
563 * rcu_init constructor. Called when the library is linked, but also when
564 * reader threads are calling rcu_register_thread().
565 * Should only be called by a single thread at a given time. This is ensured by
566 * holing the rcu_registry_lock from rcu_register_thread() or by running
567 * at library load time, which should not be executed by multiple
568 * threads nor concurrently with rcu_register_thread() anyway.
569 */
570 void rcu_init(void)
571 {
572 struct sigaction act;
573 int ret;
574
575 if (init_done)
576 return;
577 init_done = 1;
578
579 act.sa_sigaction = sigrcu_handler;
580 act.sa_flags = SA_SIGINFO | SA_RESTART;
581 sigemptyset(&act.sa_mask);
582 ret = sigaction(SIGRCU, &act, NULL);
583 if (ret)
584 urcu_die(errno);
585 }
586
587 void rcu_exit(void)
588 {
589 /*
590 * Don't unregister the SIGRCU signal handler anymore, because
591 * call_rcu threads could still be using it shortly before the
592 * application exits.
593 * Assertion disabled because call_rcu threads are now rcu
594 * readers, and left running at exit.
595 * assert(cds_list_empty(&registry));
596 */
597 }
598
599 #endif /* #ifdef RCU_SIGNAL */
600
601 DEFINE_RCU_FLAVOR(rcu_flavor);
602
603 #include "urcu-call-rcu-impl.h"
604 #include "urcu-defer-impl.h"
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