da6839f0c25b88b4262e9a615bd197220789c185
[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 #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/wfcqueue.h"
40 #include "urcu/map/urcu.h"
41 #include "urcu/static/urcu.h"
42 #include "urcu-pointer.h"
43 #include "urcu/tls-compat.h"
44
45 #include "urcu-die.h"
46 #include "urcu-wait.h"
47
48 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
49 #undef _LGPL_SOURCE
50 #include "urcu.h"
51 #define _LGPL_SOURCE
52
53 /*
54 * If a reader is really non-cooperative and refuses to commit its
55 * rcu_active_readers count to memory (there is no barrier in the reader
56 * per-se), kick it after a few loops waiting for it.
57 */
58 #define KICK_READER_LOOPS 10000
59
60 /*
61 * Active attempts to check for reader Q.S. before calling futex().
62 */
63 #define RCU_QS_ACTIVE_ATTEMPTS 100
64
65 #ifdef RCU_MEMBARRIER
66 static int init_done;
67 int rcu_has_sys_membarrier;
68
69 void __attribute__((constructor)) rcu_init(void);
70 #endif
71
72 #ifdef RCU_MB
73 void rcu_init(void)
74 {
75 }
76 #endif
77
78 #ifdef RCU_SIGNAL
79 static int init_done;
80
81 void __attribute__((constructor)) rcu_init(void);
82 void __attribute__((destructor)) rcu_exit(void);
83 #endif
84
85 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
86 struct urcu_gp rcu_gp = { .ctr = RCU_GP_COUNT };
87
88 /*
89 * Written to only by each individual reader. Read by both the reader and the
90 * writers.
91 */
92 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
93
94 #ifdef DEBUG_YIELD
95 unsigned int rcu_yield_active;
96 DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
97 #endif
98
99 static CDS_LIST_HEAD(registry);
100
101 /*
102 * Queue keeping threads awaiting to wait for a grace period. Contains
103 * struct gp_waiters_thread objects.
104 */
105 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
106
107 static void mutex_lock(pthread_mutex_t *mutex)
108 {
109 int ret;
110
111 #ifndef DISTRUST_SIGNALS_EXTREME
112 ret = pthread_mutex_lock(mutex);
113 if (ret)
114 urcu_die(ret);
115 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
116 while ((ret = pthread_mutex_trylock(mutex)) != 0) {
117 if (ret != EBUSY && ret != EINTR)
118 urcu_die(ret);
119 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
120 cmm_smp_mb();
121 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
122 cmm_smp_mb();
123 }
124 poll(NULL,0,10);
125 }
126 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
127 }
128
129 static void mutex_unlock(pthread_mutex_t *mutex)
130 {
131 int ret;
132
133 ret = pthread_mutex_unlock(mutex);
134 if (ret)
135 urcu_die(ret);
136 }
137
138 #ifdef RCU_MEMBARRIER
139 static void smp_mb_master(int group)
140 {
141 if (caa_likely(rcu_has_sys_membarrier))
142 membarrier(MEMBARRIER_EXPEDITED);
143 else
144 cmm_smp_mb();
145 }
146 #endif
147
148 #ifdef RCU_MB
149 static void smp_mb_master(int group)
150 {
151 cmm_smp_mb();
152 }
153 #endif
154
155 #ifdef RCU_SIGNAL
156 static void force_mb_all_readers(void)
157 {
158 struct rcu_reader *index;
159
160 /*
161 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
162 * compiler barriers around rcu read lock as real memory barriers.
163 */
164 if (cds_list_empty(&registry))
165 return;
166 /*
167 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
168 * a cache flush on architectures with non-coherent cache. Let's play
169 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
170 * cache flush is enforced.
171 */
172 cds_list_for_each_entry(index, &registry, node) {
173 CMM_STORE_SHARED(index->need_mb, 1);
174 pthread_kill(index->tid, SIGRCU);
175 }
176 /*
177 * Wait for sighandler (and thus mb()) to execute on every thread.
178 *
179 * Note that the pthread_kill() will never be executed on systems
180 * that correctly deliver signals in a timely manner. However, it
181 * is not uncommon for kernels to have bugs that can result in
182 * lost or unduly delayed signals.
183 *
184 * If you are seeing the below pthread_kill() executing much at
185 * all, we suggest testing the underlying kernel and filing the
186 * relevant bug report. For Linux kernels, we recommend getting
187 * the Linux Test Project (LTP).
188 */
189 cds_list_for_each_entry(index, &registry, node) {
190 while (CMM_LOAD_SHARED(index->need_mb)) {
191 pthread_kill(index->tid, SIGRCU);
192 poll(NULL, 0, 1);
193 }
194 }
195 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
196 }
197
198 static void smp_mb_master(int group)
199 {
200 force_mb_all_readers();
201 }
202 #endif /* #ifdef RCU_SIGNAL */
203
204 /*
205 * synchronize_rcu() waiting. Single thread.
206 */
207 static void wait_gp(void)
208 {
209 /* Read reader_gp before read futex */
210 smp_mb_master(RCU_MB_GROUP);
211 if (uatomic_read(&rcu_gp.futex) == -1)
212 futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
213 NULL, NULL, 0);
214 }
215
216 static void wait_for_readers(struct cds_list_head *input_readers,
217 struct cds_list_head *cur_snap_readers,
218 struct cds_list_head *qsreaders)
219 {
220 int wait_loops = 0;
221 struct rcu_reader *index, *tmp;
222
223 /*
224 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
225 * indicate quiescence (not nested), or observe the current
226 * rcu_gp.ctr value.
227 */
228 for (;;) {
229 wait_loops++;
230 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
231 uatomic_dec(&rcu_gp.futex);
232 /* Write futex before read reader_gp */
233 smp_mb_master(RCU_MB_GROUP);
234 }
235
236 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
237 switch (rcu_reader_state(&index->ctr)) {
238 case RCU_READER_ACTIVE_CURRENT:
239 if (cur_snap_readers) {
240 cds_list_move(&index->node,
241 cur_snap_readers);
242 break;
243 }
244 /* Fall-through */
245 case RCU_READER_INACTIVE:
246 cds_list_move(&index->node, qsreaders);
247 break;
248 case RCU_READER_ACTIVE_OLD:
249 /*
250 * Old snapshot. Leaving node in
251 * input_readers will make us busy-loop
252 * until the snapshot becomes current or
253 * the reader becomes inactive.
254 */
255 break;
256 }
257 }
258
259 #ifndef HAS_INCOHERENT_CACHES
260 if (cds_list_empty(input_readers)) {
261 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
262 /* Read reader_gp before write futex */
263 smp_mb_master(RCU_MB_GROUP);
264 uatomic_set(&rcu_gp.futex, 0);
265 }
266 break;
267 } else {
268 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
269 wait_gp();
270 else
271 caa_cpu_relax();
272 }
273 #else /* #ifndef HAS_INCOHERENT_CACHES */
274 /*
275 * BUSY-LOOP. Force the reader thread to commit its
276 * URCU_TLS(rcu_reader).ctr update to memory if we wait
277 * for too long.
278 */
279 if (cds_list_empty(input_readers)) {
280 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
281 /* Read reader_gp before write futex */
282 smp_mb_master(RCU_MB_GROUP);
283 uatomic_set(&rcu_gp.futex, 0);
284 }
285 break;
286 } else {
287 switch (wait_loops) {
288 case RCU_QS_ACTIVE_ATTEMPTS:
289 wait_gp();
290 break; /* only escape switch */
291 case KICK_READER_LOOPS:
292 smp_mb_master(RCU_MB_GROUP);
293 wait_loops = 0;
294 break; /* only escape switch */
295 default:
296 caa_cpu_relax();
297 }
298 }
299 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
300 }
301 }
302
303 void synchronize_rcu(void)
304 {
305 CDS_LIST_HEAD(cur_snap_readers);
306 CDS_LIST_HEAD(qsreaders);
307 DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
308 struct urcu_waiters waiters;
309
310 /*
311 * Add ourself to gp_waiters queue of threads awaiting to wait
312 * for a grace period. Proceed to perform the grace period only
313 * if we are the first thread added into the queue.
314 * The implicit memory barrier before urcu_wait_add()
315 * orders prior memory accesses of threads put into the wait
316 * queue before their insertion into the wait queue.
317 */
318 if (urcu_wait_add(&gp_waiters, &wait) != 0) {
319 /* Not first in queue: will be awakened by another thread. */
320 urcu_adaptative_busy_wait(&wait);
321 /* Order following memory accesses after grace period. */
322 cmm_smp_mb();
323 return;
324 }
325 /* We won't need to wake ourself up */
326 urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
327
328 mutex_lock(&rcu_gp_lock);
329
330 /*
331 * Move all waiters into our local queue.
332 */
333 urcu_move_waiters(&waiters, &gp_waiters);
334
335 if (cds_list_empty(&registry))
336 goto out;
337
338 /* All threads should read qparity before accessing data structure
339 * where new ptr points to. Must be done within rcu_gp_lock because it
340 * iterates on reader threads.*/
341 /* Write new ptr before changing the qparity */
342 smp_mb_master(RCU_MB_GROUP);
343
344 /*
345 * Wait for readers to observe original parity or be quiescent.
346 */
347 wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
348
349 /*
350 * Must finish waiting for quiescent state for original parity before
351 * committing next rcu_gp.ctr update to memory. Failure to do so could
352 * result in the writer waiting forever while new readers are always
353 * accessing data (no progress). Enforce compiler-order of load
354 * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
355 */
356 cmm_barrier();
357
358 /*
359 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
360 * model easier to understand. It does not have a big performance impact
361 * anyway, given this is the write-side.
362 */
363 cmm_smp_mb();
364
365 /* Switch parity: 0 -> 1, 1 -> 0 */
366 CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR_PHASE);
367
368 /*
369 * Must commit rcu_gp.ctr update to memory before waiting for quiescent
370 * state. Failure to do so could result in the writer waiting forever
371 * while new readers are always accessing data (no progress). Enforce
372 * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
373 */
374 cmm_barrier();
375
376 /*
377 *
378 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
379 * model easier to understand. It does not have a big performance impact
380 * anyway, given this is the write-side.
381 */
382 cmm_smp_mb();
383
384 /*
385 * Wait for readers to observe new parity or be quiescent.
386 */
387 wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
388
389 /*
390 * Put quiescent reader list back into registry.
391 */
392 cds_list_splice(&qsreaders, &registry);
393
394 /* Finish waiting for reader threads before letting the old ptr being
395 * freed. Must be done within rcu_gp_lock because it iterates on reader
396 * threads. */
397 smp_mb_master(RCU_MB_GROUP);
398 out:
399 mutex_unlock(&rcu_gp_lock);
400
401 /*
402 * Wakeup waiters only after we have completed the grace period
403 * and have ensured the memory barriers at the end of the grace
404 * period have been issued.
405 */
406 urcu_wake_all_waiters(&waiters);
407 }
408
409 /*
410 * library wrappers to be used by non-LGPL compatible source code.
411 */
412
413 void rcu_read_lock(void)
414 {
415 _rcu_read_lock();
416 }
417
418 void rcu_read_unlock(void)
419 {
420 _rcu_read_unlock();
421 }
422
423 int rcu_read_ongoing(void)
424 {
425 return _rcu_read_ongoing();
426 }
427
428 void rcu_register_thread(void)
429 {
430 URCU_TLS(rcu_reader).tid = pthread_self();
431 assert(URCU_TLS(rcu_reader).need_mb == 0);
432 assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
433
434 mutex_lock(&rcu_gp_lock);
435 rcu_init(); /* In case gcc does not support constructor attribute */
436 cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
437 mutex_unlock(&rcu_gp_lock);
438 }
439
440 void rcu_unregister_thread(void)
441 {
442 mutex_lock(&rcu_gp_lock);
443 cds_list_del(&URCU_TLS(rcu_reader).node);
444 mutex_unlock(&rcu_gp_lock);
445 }
446
447 #ifdef RCU_MEMBARRIER
448 void rcu_init(void)
449 {
450 if (init_done)
451 return;
452 init_done = 1;
453 if (!membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY))
454 rcu_has_sys_membarrier = 1;
455 }
456 #endif
457
458 #ifdef RCU_SIGNAL
459 static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
460 {
461 /*
462 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
463 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
464 * executed on.
465 */
466 cmm_smp_mb();
467 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
468 cmm_smp_mb();
469 }
470
471 /*
472 * rcu_init constructor. Called when the library is linked, but also when
473 * reader threads are calling rcu_register_thread().
474 * Should only be called by a single thread at a given time. This is ensured by
475 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
476 * load time, which should not be executed by multiple threads nor concurrently
477 * with rcu_register_thread() anyway.
478 */
479 void rcu_init(void)
480 {
481 struct sigaction act;
482 int ret;
483
484 if (init_done)
485 return;
486 init_done = 1;
487
488 act.sa_sigaction = sigrcu_handler;
489 act.sa_flags = SA_SIGINFO | SA_RESTART;
490 sigemptyset(&act.sa_mask);
491 ret = sigaction(SIGRCU, &act, NULL);
492 if (ret)
493 urcu_die(errno);
494 }
495
496 void rcu_exit(void)
497 {
498 struct sigaction act;
499 int ret;
500
501 ret = sigaction(SIGRCU, NULL, &act);
502 if (ret)
503 urcu_die(errno);
504 assert(act.sa_sigaction == sigrcu_handler);
505 assert(cds_list_empty(&registry));
506 }
507
508 #endif /* #ifdef RCU_SIGNAL */
509
510 DEFINE_RCU_FLAVOR(rcu_flavor);
511
512 #include "urcu-call-rcu-impl.h"
513 #include "urcu-defer-impl.h"
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