67a3eb21c8996b0751f9fc1d14ed9d71379f8bf5
[urcu.git] / urcu.c
1 /*
2 * urcu.c
3 *
4 * Userspace RCU library
5 *
6 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
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 #include <stdio.h>
28 #include <pthread.h>
29 #include <signal.h>
30 #include <assert.h>
31 #include <stdlib.h>
32 #include <string.h>
33 #include <errno.h>
34 #include <poll.h>
35
36 #include "urcu-static.h"
37 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
38 #include "urcu.h"
39
40 #ifdef RCU_MEMBARRIER
41 static int init_done;
42 int has_sys_membarrier;
43
44 void __attribute__((constructor)) rcu_init(void);
45 #endif
46
47 #ifdef RCU_MB
48 void rcu_init(void)
49 {
50 }
51 #endif
52
53 #ifdef RCU_SIGNAL
54 static int init_done;
55
56 void __attribute__((constructor)) rcu_init(void);
57 void __attribute__((destructor)) rcu_exit(void);
58 #endif
59
60 static pthread_mutex_t rcu_mutex = PTHREAD_MUTEX_INITIALIZER;
61
62 int gp_futex;
63
64 /*
65 * Global grace period counter.
66 * Contains the current RCU_GP_CTR_PHASE.
67 * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
68 * Written to only by writer with mutex taken. Read by both writer and readers.
69 */
70 long rcu_gp_ctr = RCU_GP_COUNT;
71
72 /*
73 * Written to only by each individual reader. Read by both the reader and the
74 * writers.
75 */
76 struct rcu_reader __thread rcu_reader;
77
78 #ifdef DEBUG_YIELD
79 unsigned int yield_active;
80 unsigned int __thread rand_yield;
81 #endif
82
83 static LIST_HEAD(registry);
84
85 static void internal_rcu_lock(void)
86 {
87 int ret;
88
89 #ifndef DISTRUST_SIGNALS_EXTREME
90 ret = pthread_mutex_lock(&rcu_mutex);
91 if (ret) {
92 perror("Error in pthread mutex lock");
93 exit(-1);
94 }
95 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
96 while ((ret = pthread_mutex_trylock(&rcu_mutex)) != 0) {
97 if (ret != EBUSY && ret != EINTR) {
98 printf("ret = %d, errno = %d\n", ret, errno);
99 perror("Error in pthread mutex lock");
100 exit(-1);
101 }
102 if (rcu_reader.need_mb) {
103 smp_mb();
104 rcu_reader.need_mb = 0;
105 smp_mb();
106 }
107 poll(NULL,0,10);
108 }
109 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
110 }
111
112 static void internal_rcu_unlock(void)
113 {
114 int ret;
115
116 ret = pthread_mutex_unlock(&rcu_mutex);
117 if (ret) {
118 perror("Error in pthread mutex unlock");
119 exit(-1);
120 }
121 }
122
123 /*
124 * called with rcu_mutex held.
125 */
126 static void switch_next_rcu_qparity(void)
127 {
128 STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
129 }
130
131 #ifdef RCU_MEMBARRIER
132 static void smp_mb_heavy(void)
133 {
134 if (likely(has_sys_membarrier))
135 membarrier(1);
136 else
137 smp_mb();
138 }
139 #endif
140
141 #ifdef RCU_MB
142 static void smp_mb_heavy(void)
143 {
144 smp_mb();
145 }
146 #endif
147
148 #ifdef RCU_SIGNAL
149 static void force_mb_all_readers(void)
150 {
151 struct rcu_reader *index;
152
153 /*
154 * Ask for each threads to execute a smp_mb() so we can consider the
155 * compiler barriers around rcu read lock as real memory barriers.
156 */
157 if (list_empty(&registry))
158 return;
159 /*
160 * pthread_kill has a smp_mb(). But beware, we assume it performs
161 * a cache flush on architectures with non-coherent cache. Let's play
162 * safe and don't assume anything : we use smp_mc() to make sure the
163 * cache flush is enforced.
164 */
165 list_for_each_entry(index, &registry, head) {
166 index->need_mb = 1;
167 smp_mc(); /* write need_mb before sending the signal */
168 pthread_kill(index->tid, SIGRCU);
169 }
170 /*
171 * Wait for sighandler (and thus mb()) to execute on every thread.
172 *
173 * Note that the pthread_kill() will never be executed on systems
174 * that correctly deliver signals in a timely manner. However, it
175 * is not uncommon for kernels to have bugs that can result in
176 * lost or unduly delayed signals.
177 *
178 * If you are seeing the below pthread_kill() executing much at
179 * all, we suggest testing the underlying kernel and filing the
180 * relevant bug report. For Linux kernels, we recommend getting
181 * the Linux Test Project (LTP).
182 */
183 list_for_each_entry(index, &registry, head) {
184 while (index->need_mb) {
185 pthread_kill(index->tid, SIGRCU);
186 poll(NULL, 0, 1);
187 }
188 }
189 smp_mb(); /* read ->need_mb before ending the barrier */
190 }
191
192 static void smp_mb_heavy(void)
193 {
194 force_mb_all_readers();
195 }
196 #endif /* #ifdef RCU_SIGNAL */
197
198 /*
199 * synchronize_rcu() waiting. Single thread.
200 */
201 static void wait_gp(void)
202 {
203 /* Read reader_gp before read futex */
204 smp_mb_heavy();
205 if (uatomic_read(&gp_futex) == -1)
206 futex_async(&gp_futex, FUTEX_WAIT, -1,
207 NULL, NULL, 0);
208 }
209
210 void wait_for_quiescent_state(void)
211 {
212 LIST_HEAD(qsreaders);
213 int wait_loops = 0;
214 struct rcu_reader *index, *tmp;
215
216 if (list_empty(&registry))
217 return;
218 /*
219 * Wait for each thread rcu_reader.ctr count to become 0.
220 */
221 for (;;) {
222 wait_loops++;
223 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
224 uatomic_dec(&gp_futex);
225 /* Write futex before read reader_gp */
226 smp_mb_heavy();
227 }
228
229 list_for_each_entry_safe(index, tmp, &registry, head) {
230 if (!rcu_old_gp_ongoing(&index->ctr))
231 list_move(&index->head, &qsreaders);
232 }
233
234 #ifndef HAS_INCOHERENT_CACHES
235 if (list_empty(&registry)) {
236 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
237 /* Read reader_gp before write futex */
238 smp_mb_heavy();
239 uatomic_set(&gp_futex, 0);
240 }
241 break;
242 } else {
243 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
244 wait_gp();
245 else
246 cpu_relax();
247 }
248 #else /* #ifndef HAS_INCOHERENT_CACHES */
249 /*
250 * BUSY-LOOP. Force the reader thread to commit its
251 * rcu_reader.ctr update to memory if we wait for too long.
252 */
253 if (list_empty(&registry)) {
254 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
255 /* Read reader_gp before write futex */
256 smp_mb_heavy();
257 uatomic_set(&gp_futex, 0);
258 }
259 break;
260 } else {
261 switch (wait_loops) {
262 case RCU_QS_ACTIVE_ATTEMPTS:
263 wait_gp();
264 break; /* only escape switch */
265 case KICK_READER_LOOPS:
266 smp_mb_heavy();
267 wait_loops = 0;
268 break; /* only escape switch */
269 default:
270 cpu_relax();
271 }
272 }
273 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
274 }
275 /* put back the reader list in the registry */
276 list_splice(&qsreaders, &registry);
277 }
278
279 void synchronize_rcu(void)
280 {
281 internal_rcu_lock();
282
283 /* All threads should read qparity before accessing data structure
284 * where new ptr points to. Must be done within internal_rcu_lock
285 * because it iterates on reader threads.*/
286 /* Write new ptr before changing the qparity */
287 smp_mb_heavy();
288
289 switch_next_rcu_qparity(); /* 0 -> 1 */
290
291 /*
292 * Must commit qparity update to memory before waiting for parity
293 * 0 quiescent state. Failure to do so could result in the writer
294 * waiting forever while new readers are always accessing data (no
295 * progress).
296 * Ensured by STORE_SHARED and LOAD_SHARED.
297 */
298
299 /*
300 * Adding a smp_mb() which is _not_ formally required, but makes the
301 * model easier to understand. It does not have a big performance impact
302 * anyway, given this is the write-side.
303 */
304 smp_mb();
305
306 /*
307 * Wait for previous parity to be empty of readers.
308 */
309 wait_for_quiescent_state(); /* Wait readers in parity 0 */
310
311 /*
312 * Must finish waiting for quiescent state for parity 0 before
313 * committing qparity update to memory. Failure to do so could result in
314 * the writer waiting forever while new readers are always accessing
315 * data (no progress).
316 * Ensured by STORE_SHARED and LOAD_SHARED.
317 */
318
319 /*
320 * Adding a smp_mb() which is _not_ formally required, but makes the
321 * model easier to understand. It does not have a big performance impact
322 * anyway, given this is the write-side.
323 */
324 smp_mb();
325
326 switch_next_rcu_qparity(); /* 1 -> 0 */
327
328 /*
329 * Must commit qparity update to memory before waiting for parity
330 * 1 quiescent state. Failure to do so could result in the writer
331 * waiting forever while new readers are always accessing data (no
332 * progress).
333 * Ensured by STORE_SHARED and LOAD_SHARED.
334 */
335
336 /*
337 * Adding a smp_mb() which is _not_ formally required, but makes the
338 * model easier to understand. It does not have a big performance impact
339 * anyway, given this is the write-side.
340 */
341 smp_mb();
342
343 /*
344 * Wait for previous parity to be empty of readers.
345 */
346 wait_for_quiescent_state(); /* Wait readers in parity 1 */
347
348 /* Finish waiting for reader threads before letting the old ptr being
349 * freed. Must be done within internal_rcu_lock because it iterates on
350 * reader threads. */
351 smp_mb_heavy();
352
353 internal_rcu_unlock();
354 }
355
356 /*
357 * library wrappers to be used by non-LGPL compatible source code.
358 */
359
360 void rcu_read_lock(void)
361 {
362 _rcu_read_lock();
363 }
364
365 void rcu_read_unlock(void)
366 {
367 _rcu_read_unlock();
368 }
369
370 void rcu_register_thread(void)
371 {
372 rcu_reader.tid = pthread_self();
373 assert(rcu_reader.need_mb == 0);
374 assert(rcu_reader.ctr == 0);
375
376 internal_rcu_lock();
377 rcu_init(); /* In case gcc does not support constructor attribute */
378 list_add(&rcu_reader.head, &registry);
379 internal_rcu_unlock();
380 }
381
382 void rcu_unregister_thread(void)
383 {
384 internal_rcu_lock();
385 list_del(&rcu_reader.head);
386 internal_rcu_unlock();
387 }
388
389 #ifdef RCU_MEMBARRIER
390 void rcu_init(void)
391 {
392 if (init_done)
393 return;
394 init_done = 1;
395 if (!membarrier(1))
396 has_sys_membarrier = 1;
397 }
398 #endif
399
400 #ifdef RCU_SIGNAL
401 static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
402 {
403 /*
404 * Executing this smp_mb() is the only purpose of this signal handler.
405 * It punctually promotes barrier() into smp_mb() on every thread it is
406 * executed on.
407 */
408 smp_mb();
409 rcu_reader.need_mb = 0;
410 smp_mb();
411 }
412
413 /*
414 * rcu_init constructor. Called when the library is linked, but also when
415 * reader threads are calling rcu_register_thread().
416 * Should only be called by a single thread at a given time. This is ensured by
417 * holing the internal_rcu_lock() from rcu_register_thread() or by running at
418 * library load time, which should not be executed by multiple threads nor
419 * concurrently with rcu_register_thread() anyway.
420 */
421 void rcu_init(void)
422 {
423 struct sigaction act;
424 int ret;
425
426 if (init_done)
427 return;
428 init_done = 1;
429
430 act.sa_sigaction = sigrcu_handler;
431 act.sa_flags = SA_SIGINFO | SA_RESTART;
432 sigemptyset(&act.sa_mask);
433 ret = sigaction(SIGRCU, &act, NULL);
434 if (ret) {
435 perror("Error in sigaction");
436 exit(-1);
437 }
438 }
439
440 void rcu_exit(void)
441 {
442 struct sigaction act;
443 int ret;
444
445 ret = sigaction(SIGRCU, NULL, &act);
446 if (ret) {
447 perror("Error in sigaction");
448 exit(-1);
449 }
450 assert(act.sa_sigaction == sigrcu_handler);
451 assert(list_empty(&registry));
452 }
453 #endif /* #ifdef RCU_SIGNAL */
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