Fix: move wait loop increment before first conditional block
[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 10 loops waiting for it.
57 */
58 #define KICK_READER_LOOPS 10
59
60 /*
61 * Active attempts to check for reader Q.S. before calling futex().
62 */
63 #define RCU_QS_ACTIVE_ATTEMPTS 100
64
65 /*
66 * RCU_MEMBARRIER is only possibly available on Linux.
67 */
68 #if defined(RCU_MEMBARRIER) && defined(__linux__)
69 #include <urcu/syscall-compat.h>
70 #endif
71
72 /* If the headers do not support SYS_membarrier, fall back on RCU_MB */
73 #ifdef SYS_membarrier
74 # define membarrier(...) syscall(SYS_membarrier, __VA_ARGS__)
75 #else
76 # define membarrier(...) -ENOSYS
77 #endif
78
79 #define MEMBARRIER_EXPEDITED (1 << 0)
80 #define MEMBARRIER_DELAYED (1 << 1)
81 #define MEMBARRIER_QUERY (1 << 16)
82
83 #ifdef RCU_MEMBARRIER
84 static int init_done;
85 int rcu_has_sys_membarrier;
86
87 void __attribute__((constructor)) rcu_init(void);
88 #endif
89
90 #ifdef RCU_MB
91 void rcu_init(void)
92 {
93 }
94 #endif
95
96 #ifdef RCU_SIGNAL
97 static int init_done;
98
99 void __attribute__((constructor)) rcu_init(void);
100 void __attribute__((destructor)) rcu_exit(void);
101 #endif
102
103 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
104 struct rcu_gp rcu_gp = { .ctr = RCU_GP_COUNT };
105
106 /*
107 * Written to only by each individual reader. Read by both the reader and the
108 * writers.
109 */
110 DEFINE_URCU_TLS(struct rcu_reader, rcu_reader);
111
112 static CDS_LIST_HEAD(registry);
113
114 /*
115 * Queue keeping threads awaiting to wait for a grace period. Contains
116 * struct gp_waiters_thread objects.
117 */
118 static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
119
120 static void mutex_lock(pthread_mutex_t *mutex)
121 {
122 int ret;
123
124 #ifndef DISTRUST_SIGNALS_EXTREME
125 ret = pthread_mutex_lock(mutex);
126 if (ret)
127 urcu_die(ret);
128 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
129 while ((ret = pthread_mutex_trylock(mutex)) != 0) {
130 if (ret != EBUSY && ret != EINTR)
131 urcu_die(ret);
132 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader).need_mb)) {
133 cmm_smp_mb();
134 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
135 cmm_smp_mb();
136 }
137 poll(NULL,0,10);
138 }
139 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
140 }
141
142 static void mutex_unlock(pthread_mutex_t *mutex)
143 {
144 int ret;
145
146 ret = pthread_mutex_unlock(mutex);
147 if (ret)
148 urcu_die(ret);
149 }
150
151 #ifdef RCU_MEMBARRIER
152 static void smp_mb_master(int group)
153 {
154 if (caa_likely(rcu_has_sys_membarrier))
155 (void) membarrier(MEMBARRIER_EXPEDITED);
156 else
157 cmm_smp_mb();
158 }
159 #endif
160
161 #ifdef RCU_MB
162 static void smp_mb_master(int group)
163 {
164 cmm_smp_mb();
165 }
166 #endif
167
168 #ifdef RCU_SIGNAL
169 static void force_mb_all_readers(void)
170 {
171 struct rcu_reader *index;
172
173 /*
174 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
175 * compiler barriers around rcu read lock as real memory barriers.
176 */
177 if (cds_list_empty(&registry))
178 return;
179 /*
180 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
181 * a cache flush on architectures with non-coherent cache. Let's play
182 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
183 * cache flush is enforced.
184 */
185 cds_list_for_each_entry(index, &registry, node) {
186 CMM_STORE_SHARED(index->need_mb, 1);
187 pthread_kill(index->tid, SIGRCU);
188 }
189 /*
190 * Wait for sighandler (and thus mb()) to execute on every thread.
191 *
192 * Note that the pthread_kill() will never be executed on systems
193 * that correctly deliver signals in a timely manner. However, it
194 * is not uncommon for kernels to have bugs that can result in
195 * lost or unduly delayed signals.
196 *
197 * If you are seeing the below pthread_kill() executing much at
198 * all, we suggest testing the underlying kernel and filing the
199 * relevant bug report. For Linux kernels, we recommend getting
200 * the Linux Test Project (LTP).
201 */
202 cds_list_for_each_entry(index, &registry, node) {
203 while (CMM_LOAD_SHARED(index->need_mb)) {
204 pthread_kill(index->tid, SIGRCU);
205 poll(NULL, 0, 1);
206 }
207 }
208 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
209 }
210
211 static void smp_mb_master(int group)
212 {
213 force_mb_all_readers();
214 }
215 #endif /* #ifdef RCU_SIGNAL */
216
217 /*
218 * synchronize_rcu() waiting. Single thread.
219 */
220 static void wait_gp(void)
221 {
222 /* Read reader_gp before read futex */
223 smp_mb_master(RCU_MB_GROUP);
224 if (uatomic_read(&rcu_gp.futex) == -1)
225 futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
226 NULL, NULL, 0);
227 }
228
229 static void wait_for_readers(struct cds_list_head *input_readers,
230 struct cds_list_head *cur_snap_readers,
231 struct cds_list_head *qsreaders)
232 {
233 unsigned int wait_loops = 0;
234 struct rcu_reader *index, *tmp;
235 #ifdef HAS_INCOHERENT_CACHES
236 unsigned int wait_gp_loops = 0;
237 #endif /* HAS_INCOHERENT_CACHES */
238
239 /*
240 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
241 * indicate quiescence (not nested), or observe the current
242 * rcu_gp.ctr value.
243 */
244 for (;;) {
245 if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
246 wait_loops++;
247 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
248 uatomic_dec(&rcu_gp.futex);
249 /* Write futex before read reader_gp */
250 smp_mb_master(RCU_MB_GROUP);
251 }
252
253 cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
254 switch (rcu_reader_state(&index->ctr)) {
255 case RCU_READER_ACTIVE_CURRENT:
256 if (cur_snap_readers) {
257 cds_list_move(&index->node,
258 cur_snap_readers);
259 break;
260 }
261 /* Fall-through */
262 case RCU_READER_INACTIVE:
263 cds_list_move(&index->node, qsreaders);
264 break;
265 case RCU_READER_ACTIVE_OLD:
266 /*
267 * Old snapshot. Leaving node in
268 * input_readers will make us busy-loop
269 * until the snapshot becomes current or
270 * the reader becomes inactive.
271 */
272 break;
273 }
274 }
275
276 #ifndef HAS_INCOHERENT_CACHES
277 if (cds_list_empty(input_readers)) {
278 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
279 /* Read reader_gp before write futex */
280 smp_mb_master(RCU_MB_GROUP);
281 uatomic_set(&rcu_gp.futex, 0);
282 }
283 break;
284 } else {
285 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS)
286 wait_gp();
287 else
288 caa_cpu_relax();
289 }
290 #else /* #ifndef HAS_INCOHERENT_CACHES */
291 /*
292 * BUSY-LOOP. Force the reader thread to commit its
293 * URCU_TLS(rcu_reader).ctr update to memory if we wait
294 * for too long.
295 */
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 if (wait_gp_loops == KICK_READER_LOOPS) {
305 smp_mb_master(RCU_MB_GROUP);
306 wait_gp_loops = 0;
307 }
308 if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
309 wait_gp();
310 wait_gp_loops++;
311 } else {
312 caa_cpu_relax();
313 }
314 }
315 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
316 }
317 }
318
319 void synchronize_rcu(void)
320 {
321 CDS_LIST_HEAD(cur_snap_readers);
322 CDS_LIST_HEAD(qsreaders);
323 DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
324 struct urcu_waiters waiters;
325
326 /*
327 * Add ourself to gp_waiters queue of threads awaiting to wait
328 * for a grace period. Proceed to perform the grace period only
329 * if we are the first thread added into the queue.
330 * The implicit memory barrier before urcu_wait_add()
331 * orders prior memory accesses of threads put into the wait
332 * queue before their insertion into the wait queue.
333 */
334 if (urcu_wait_add(&gp_waiters, &wait) != 0) {
335 /* Not first in queue: will be awakened by another thread. */
336 urcu_adaptative_busy_wait(&wait);
337 /* Order following memory accesses after grace period. */
338 cmm_smp_mb();
339 return;
340 }
341 /* We won't need to wake ourself up */
342 urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
343
344 mutex_lock(&rcu_gp_lock);
345
346 /*
347 * Move all waiters into our local queue.
348 */
349 urcu_move_waiters(&waiters, &gp_waiters);
350
351 if (cds_list_empty(&registry))
352 goto out;
353
354 /* All threads should read qparity before accessing data structure
355 * where new ptr points to. Must be done within rcu_gp_lock because it
356 * iterates on reader threads.*/
357 /* Write new ptr before changing the qparity */
358 smp_mb_master(RCU_MB_GROUP);
359
360 /*
361 * Wait for readers to observe original parity or be quiescent.
362 */
363 wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
364
365 /*
366 * Must finish waiting for quiescent state for original parity before
367 * committing next rcu_gp.ctr update to memory. Failure to do so could
368 * result in the writer waiting forever while new readers are always
369 * accessing data (no progress). Enforce compiler-order of load
370 * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
371 */
372 cmm_barrier();
373
374 /*
375 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
376 * model easier to understand. It does not have a big performance impact
377 * anyway, given this is the write-side.
378 */
379 cmm_smp_mb();
380
381 /* Switch parity: 0 -> 1, 1 -> 0 */
382 CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR_PHASE);
383
384 /*
385 * Must commit rcu_gp.ctr update to memory before waiting for quiescent
386 * state. Failure to do so could result in the writer waiting forever
387 * while new readers are always accessing data (no progress). Enforce
388 * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
389 */
390 cmm_barrier();
391
392 /*
393 *
394 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
395 * model easier to understand. It does not have a big performance impact
396 * anyway, given this is the write-side.
397 */
398 cmm_smp_mb();
399
400 /*
401 * Wait for readers to observe new parity or be quiescent.
402 */
403 wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
404
405 /*
406 * Put quiescent reader list back into registry.
407 */
408 cds_list_splice(&qsreaders, &registry);
409
410 /* Finish waiting for reader threads before letting the old ptr being
411 * freed. Must be done within rcu_gp_lock because it iterates on reader
412 * threads. */
413 smp_mb_master(RCU_MB_GROUP);
414 out:
415 mutex_unlock(&rcu_gp_lock);
416
417 /*
418 * Wakeup waiters only after we have completed the grace period
419 * and have ensured the memory barriers at the end of the grace
420 * period have been issued.
421 */
422 urcu_wake_all_waiters(&waiters);
423 }
424
425 /*
426 * library wrappers to be used by non-LGPL compatible source code.
427 */
428
429 void rcu_read_lock(void)
430 {
431 _rcu_read_lock();
432 }
433
434 void rcu_read_unlock(void)
435 {
436 _rcu_read_unlock();
437 }
438
439 int rcu_read_ongoing(void)
440 {
441 return _rcu_read_ongoing();
442 }
443
444 void rcu_register_thread(void)
445 {
446 URCU_TLS(rcu_reader).tid = pthread_self();
447 assert(URCU_TLS(rcu_reader).need_mb == 0);
448 assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
449
450 mutex_lock(&rcu_gp_lock);
451 rcu_init(); /* In case gcc does not support constructor attribute */
452 cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
453 mutex_unlock(&rcu_gp_lock);
454 }
455
456 void rcu_unregister_thread(void)
457 {
458 mutex_lock(&rcu_gp_lock);
459 cds_list_del(&URCU_TLS(rcu_reader).node);
460 mutex_unlock(&rcu_gp_lock);
461 }
462
463 #ifdef RCU_MEMBARRIER
464 void rcu_init(void)
465 {
466 if (init_done)
467 return;
468 init_done = 1;
469 if (!membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY))
470 rcu_has_sys_membarrier = 1;
471 }
472 #endif
473
474 #ifdef RCU_SIGNAL
475 static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
476 {
477 /*
478 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
479 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
480 * executed on.
481 */
482 cmm_smp_mb();
483 _CMM_STORE_SHARED(URCU_TLS(rcu_reader).need_mb, 0);
484 cmm_smp_mb();
485 }
486
487 /*
488 * rcu_init constructor. Called when the library is linked, but also when
489 * reader threads are calling rcu_register_thread().
490 * Should only be called by a single thread at a given time. This is ensured by
491 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
492 * load time, which should not be executed by multiple threads nor concurrently
493 * with rcu_register_thread() anyway.
494 */
495 void rcu_init(void)
496 {
497 struct sigaction act;
498 int ret;
499
500 if (init_done)
501 return;
502 init_done = 1;
503
504 act.sa_sigaction = sigrcu_handler;
505 act.sa_flags = SA_SIGINFO | SA_RESTART;
506 sigemptyset(&act.sa_mask);
507 ret = sigaction(SIGRCU, &act, NULL);
508 if (ret)
509 urcu_die(errno);
510 }
511
512 void rcu_exit(void)
513 {
514 /*
515 * Don't unregister the SIGRCU signal handler anymore, because
516 * call_rcu threads could still be using it shortly before the
517 * application exits.
518 * Assertion disabled because call_rcu threads are now rcu
519 * readers, and left running at exit.
520 * assert(cds_list_empty(&registry));
521 */
522 }
523
524 #endif /* #ifdef RCU_SIGNAL */
525
526 DEFINE_RCU_FLAVOR(rcu_flavor);
527
528 #include "urcu-call-rcu-impl.h"
529 #include "urcu-defer-impl.h"
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