cleanup: spelling fixes in comments
[urcu.git] / src / urcu-qsbr.c
1 /*
2 * urcu-qsbr.c
3 *
4 * Userspace RCU QSBR 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 URCU_NO_COMPAT_IDENTIFIERS
27 #define _LGPL_SOURCE
28 #include <stdio.h>
29 #include <pthread.h>
30 #include <signal.h>
31 #include <assert.h>
32 #include <stdlib.h>
33 #include <stdint.h>
34 #include <string.h>
35 #include <errno.h>
36 #include <poll.h>
37
38 #include <urcu/wfcqueue.h>
39 #include <urcu/map/urcu-qsbr.h>
40 #define BUILD_QSBR_LIB
41 #include <urcu/static/urcu-qsbr.h>
42 #include <urcu/pointer.h>
43 #include <urcu/tls-compat.h>
44
45 #include "urcu-die.h"
46 #include "urcu-wait.h"
47 #include "urcu-utils.h"
48
49 #define URCU_API_MAP
50 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
51 #undef _LGPL_SOURCE
52 #include <urcu/urcu-qsbr.h>
53 #define _LGPL_SOURCE
54
55 void __attribute__((destructor)) urcu_qsbr_exit(void);
56
57 /*
58 * rcu_gp_lock ensures mutual exclusion between threads calling
59 * synchronize_rcu().
60 */
61 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
62 /*
63 * rcu_registry_lock ensures mutual exclusion between threads
64 * registering and unregistering themselves to/from the registry, and
65 * with threads reading that registry from synchronize_rcu(). However,
66 * this lock is not held all the way through the completion of awaiting
67 * for the grace period. It is sporadically released between iterations
68 * on the registry.
69 * rcu_registry_lock may nest inside rcu_gp_lock.
70 */
71 static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
72 struct urcu_gp urcu_qsbr_gp = { .ctr = URCU_QSBR_GP_ONLINE };
73
74 /*
75 * Active attempts to check for reader Q.S. before calling futex().
76 */
77 #define RCU_QS_ACTIVE_ATTEMPTS 100
78
79 /*
80 * Written to only by each individual reader. Read by both the reader and the
81 * writers.
82 */
83 DEFINE_URCU_TLS(struct urcu_qsbr_reader, urcu_qsbr_reader);
84
85 static CDS_LIST_HEAD(registry);
86
87 /*
88 * Queue keeping threads awaiting to wait for a grace period. Contains
89 * struct gp_waiters_thread objects.
90 */
91 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
92
93 static void mutex_lock(pthread_mutex_t *mutex)
94 {
95 int ret;
96
97 #ifndef DISTRUST_SIGNALS_EXTREME
98 ret = pthread_mutex_lock(mutex);
99 if (ret)
100 urcu_die(ret);
101 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
102 while ((ret = pthread_mutex_trylock(mutex)) != 0) {
103 if (ret != EBUSY && ret != EINTR)
104 urcu_die(ret);
105 poll(NULL,0,10);
106 }
107 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
108 }
109
110 static void mutex_unlock(pthread_mutex_t *mutex)
111 {
112 int ret;
113
114 ret = pthread_mutex_unlock(mutex);
115 if (ret)
116 urcu_die(ret);
117 }
118
119 /*
120 * synchronize_rcu() waiting. Single thread.
121 */
122 static void wait_gp(void)
123 {
124 /* Read reader_gp before read futex */
125 cmm_smp_rmb();
126 if (uatomic_read(&urcu_qsbr_gp.futex) != -1)
127 return;
128 while (futex_noasync(&urcu_qsbr_gp.futex, FUTEX_WAIT, -1,
129 NULL, NULL, 0)) {
130 switch (errno) {
131 case EWOULDBLOCK:
132 /* Value already changed. */
133 return;
134 case EINTR:
135 /* Retry if interrupted by signal. */
136 break; /* Get out of switch. */
137 default:
138 /* Unexpected error. */
139 urcu_die(errno);
140 }
141 }
142 }
143
144 /*
145 * Always called with rcu_registry lock held. Releases this lock between
146 * iterations and grabs it again. Holds the lock when it returns.
147 */
148 static void wait_for_readers(struct cds_list_head *input_readers,
149 struct cds_list_head *cur_snap_readers,
150 struct cds_list_head *qsreaders)
151 {
152 unsigned int wait_loops = 0;
153 struct urcu_qsbr_reader *index, *tmp;
154
155 /*
156 * Wait for each thread URCU_TLS(urcu_qsbr_reader).ctr to either
157 * indicate quiescence (offline), or for them to observe the
158 * current urcu_qsbr_gp.ctr value.
159 */
160 for (;;) {
161 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
162 wait_loops++;
163 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
164 uatomic_set(&urcu_qsbr_gp.futex, -1);
165 /*
166 * Write futex before write waiting (the other side
167 * reads them in the opposite order).
168 */
169 cmm_smp_wmb();
170 cds_list_for_each_entry(index, input_readers, node) {
171 _CMM_STORE_SHARED(index->waiting, 1);
172 }
173 /* Write futex before read reader_gp */
174 cmm_smp_mb();
175 }
176 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
177 switch (urcu_qsbr_reader_state(&index->ctr)) {
178 case URCU_READER_ACTIVE_CURRENT:
179 if (cur_snap_readers) {
180 cds_list_move(&index->node,
181 cur_snap_readers);
182 break;
183 }
184 /* Fall-through */
185 case URCU_READER_INACTIVE:
186 cds_list_move(&index->node, qsreaders);
187 break;
188 case URCU_READER_ACTIVE_OLD:
189 /*
190 * Old snapshot. Leaving node in
191 * input_readers will make us busy-loop
192 * until the snapshot becomes current or
193 * the reader becomes inactive.
194 */
195 break;
196 }
197 }
198
199 if (cds_list_empty(input_readers)) {
200 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
201 /* Read reader_gp before write futex */
202 cmm_smp_mb();
203 uatomic_set(&urcu_qsbr_gp.futex, 0);
204 }
205 break;
206 } else {
207 /* Temporarily unlock the registry lock. */
208 mutex_unlock(&rcu_registry_lock);
209 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
210 wait_gp();
211 } else {
212 #ifndef HAS_INCOHERENT_CACHES
213 caa_cpu_relax();
214 #else /* #ifndef HAS_INCOHERENT_CACHES */
215 cmm_smp_mb();
216 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
217 }
218 /* Re-lock the registry lock before the next loop. */
219 mutex_lock(&rcu_registry_lock);
220 }
221 }
222 }
223
224 /*
225 * Using a two-subphases algorithm for architectures with smaller than 64-bit
226 * long-size to ensure we do not encounter an overflow bug.
227 */
228
229 #if (CAA_BITS_PER_LONG < 64)
230 void urcu_qsbr_synchronize_rcu(void)
231 {
232 CDS_LIST_HEAD(cur_snap_readers);
233 CDS_LIST_HEAD(qsreaders);
234 unsigned long was_online;
235 DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
236 struct urcu_waiters waiters;
237
238 was_online = urcu_qsbr_read_ongoing();
239
240 /* All threads should read qparity before accessing data structure
241 * where new ptr points to. In the "then" case, rcu_thread_offline
242 * includes a memory barrier.
243 *
244 * Mark the writer thread offline to make sure we don't wait for
245 * our own quiescent state. This allows using synchronize_rcu()
246 * in threads registered as readers.
247 */
248 if (was_online)
249 urcu_qsbr_thread_offline();
250 else
251 cmm_smp_mb();
252
253 /*
254 * Add ourself to gp_waiters queue of threads awaiting to wait
255 * for a grace period. Proceed to perform the grace period only
256 * if we are the first thread added into the queue.
257 */
258 if (urcu_wait_add(&gp_waiters, &wait) != 0) {
259 /* Not first in queue: will be awakened by another thread. */
260 urcu_adaptative_busy_wait(&wait);
261 goto gp_end;
262 }
263 /* We won't need to wake ourself up */
264 urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
265
266 mutex_lock(&rcu_gp_lock);
267
268 /*
269 * Move all waiters into our local queue.
270 */
271 urcu_move_waiters(&waiters, &gp_waiters);
272
273 mutex_lock(&rcu_registry_lock);
274
275 if (cds_list_empty(&registry))
276 goto out;
277
278 /*
279 * Wait for readers to observe original parity or be quiescent.
280 * wait_for_readers() can release and grab again rcu_registry_lock
281 * internally.
282 */
283 wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
284
285 /*
286 * Must finish waiting for quiescent state for original parity
287 * before committing next urcu_qsbr_gp.ctr update to memory. Failure
288 * to do so could result in the writer waiting forever while new
289 * readers are always accessing data (no progress). Enforce
290 * compiler-order of load URCU_TLS(urcu_qsbr_reader).ctr before store
291 * to urcu_qsbr_gp.ctr.
292 */
293 cmm_barrier();
294
295 /*
296 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
297 * model easier to understand. It does not have a big performance impact
298 * anyway, given this is the write-side.
299 */
300 cmm_smp_mb();
301
302 /* Switch parity: 0 -> 1, 1 -> 0 */
303 CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr ^ URCU_QSBR_GP_CTR);
304
305 /*
306 * Must commit urcu_qsbr_gp.ctr update to memory before waiting for
307 * quiescent state. Failure to do so could result in the writer
308 * waiting forever while new readers are always accessing data
309 * (no progress). Enforce compiler-order of store to urcu_qsbr_gp.ctr
310 * before load URCU_TLS(urcu_qsbr_reader).ctr.
311 */
312 cmm_barrier();
313
314 /*
315 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
316 * model easier to understand. It does not have a big performance impact
317 * anyway, given this is the write-side.
318 */
319 cmm_smp_mb();
320
321 /*
322 * Wait for readers to observe new parity or be quiescent.
323 * wait_for_readers() can release and grab again rcu_registry_lock
324 * internally.
325 */
326 wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
327
328 /*
329 * Put quiescent reader list back into registry.
330 */
331 cds_list_splice(&qsreaders, &registry);
332 out:
333 mutex_unlock(&rcu_registry_lock);
334 mutex_unlock(&rcu_gp_lock);
335 urcu_wake_all_waiters(&waiters);
336 gp_end:
337 /*
338 * Finish waiting for reader threads before letting the old ptr being
339 * freed.
340 */
341 if (was_online)
342 urcu_qsbr_thread_online();
343 else
344 cmm_smp_mb();
345 }
346 #else /* !(CAA_BITS_PER_LONG < 64) */
347 void urcu_qsbr_synchronize_rcu(void)
348 {
349 CDS_LIST_HEAD(qsreaders);
350 unsigned long was_online;
351 DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
352 struct urcu_waiters waiters;
353
354 was_online = urcu_qsbr_read_ongoing();
355
356 /*
357 * Mark the writer thread offline to make sure we don't wait for
358 * our own quiescent state. This allows using synchronize_rcu()
359 * in threads registered as readers.
360 */
361 if (was_online)
362 urcu_qsbr_thread_offline();
363 else
364 cmm_smp_mb();
365
366 /*
367 * Add ourself to gp_waiters queue of threads awaiting to wait
368 * for a grace period. Proceed to perform the grace period only
369 * if we are the first thread added into the queue.
370 */
371 if (urcu_wait_add(&gp_waiters, &wait) != 0) {
372 /* Not first in queue: will be awakened by another thread. */
373 urcu_adaptative_busy_wait(&wait);
374 goto gp_end;
375 }
376 /* We won't need to wake ourself up */
377 urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
378
379 mutex_lock(&rcu_gp_lock);
380
381 /*
382 * Move all waiters into our local queue.
383 */
384 urcu_move_waiters(&waiters, &gp_waiters);
385
386 mutex_lock(&rcu_registry_lock);
387
388 if (cds_list_empty(&registry))
389 goto out;
390
391 /* Increment current G.P. */
392 CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr + URCU_QSBR_GP_CTR);
393
394 /*
395 * Must commit urcu_qsbr_gp.ctr update to memory before waiting for
396 * quiescent state. Failure to do so could result in the writer
397 * waiting forever while new readers are always accessing data
398 * (no progress). Enforce compiler-order of store to urcu_qsbr_gp.ctr
399 * before load URCU_TLS(urcu_qsbr_reader).ctr.
400 */
401 cmm_barrier();
402
403 /*
404 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
405 * model easier to understand. It does not have a big performance impact
406 * anyway, given this is the write-side.
407 */
408 cmm_smp_mb();
409
410 /*
411 * Wait for readers to observe new count of be quiescent.
412 * wait_for_readers() can release and grab again rcu_registry_lock
413 * internally.
414 */
415 wait_for_readers(&registry, NULL, &qsreaders);
416
417 /*
418 * Put quiescent reader list back into registry.
419 */
420 cds_list_splice(&qsreaders, &registry);
421 out:
422 mutex_unlock(&rcu_registry_lock);
423 mutex_unlock(&rcu_gp_lock);
424 urcu_wake_all_waiters(&waiters);
425 gp_end:
426 if (was_online)
427 urcu_qsbr_thread_online();
428 else
429 cmm_smp_mb();
430 }
431 #endif /* !(CAA_BITS_PER_LONG < 64) */
432
433 /*
434 * library wrappers to be used by non-LGPL compatible source code.
435 */
436
437 void urcu_qsbr_read_lock(void)
438 {
439 _urcu_qsbr_read_lock();
440 }
441
442 void urcu_qsbr_read_unlock(void)
443 {
444 _urcu_qsbr_read_unlock();
445 }
446
447 int urcu_qsbr_read_ongoing(void)
448 {
449 return _urcu_qsbr_read_ongoing();
450 }
451 void rcu_read_ongoing_qsbr();
452
453 void urcu_qsbr_quiescent_state(void)
454 {
455 _urcu_qsbr_quiescent_state();
456 }
457 void rcu_quiescent_state_qsbr();
458
459 void urcu_qsbr_thread_offline(void)
460 {
461 _urcu_qsbr_thread_offline();
462 }
463 void rcu_thread_offline_qsbr();
464
465 void urcu_qsbr_thread_online(void)
466 {
467 _urcu_qsbr_thread_online();
468 }
469
470 void urcu_qsbr_register_thread(void)
471 {
472 URCU_TLS(urcu_qsbr_reader).tid = pthread_self();
473 assert(URCU_TLS(urcu_qsbr_reader).ctr == 0);
474
475 mutex_lock(&rcu_registry_lock);
476 assert(!URCU_TLS(urcu_qsbr_reader).registered);
477 URCU_TLS(urcu_qsbr_reader).registered = 1;
478 cds_list_add(&URCU_TLS(urcu_qsbr_reader).node, &registry);
479 mutex_unlock(&rcu_registry_lock);
480 _urcu_qsbr_thread_online();
481 }
482
483 void urcu_qsbr_unregister_thread(void)
484 {
485 /*
486 * We have to make the thread offline otherwise we end up dealocking
487 * with a waiting writer.
488 */
489 _urcu_qsbr_thread_offline();
490 assert(URCU_TLS(urcu_qsbr_reader).registered);
491 URCU_TLS(urcu_qsbr_reader).registered = 0;
492 mutex_lock(&rcu_registry_lock);
493 cds_list_del(&URCU_TLS(urcu_qsbr_reader).node);
494 mutex_unlock(&rcu_registry_lock);
495 }
496
497 void urcu_qsbr_exit(void)
498 {
499 /*
500 * Assertion disabled because call_rcu threads are now rcu
501 * readers, and left running at exit.
502 * assert(cds_list_empty(&registry));
503 */
504 }
505
506 DEFINE_RCU_FLAVOR(rcu_flavor);
507
508 #include "urcu-call-rcu-impl.h"
509 #include "urcu-defer-impl.h"
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