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[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 #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_gp_lock = 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 unsigned 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 mutex_lock(pthread_mutex_t *mutex)
86 {
87 int ret;
88
89 #ifndef DISTRUST_SIGNALS_EXTREME
90 ret = pthread_mutex_lock(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(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 (LOAD_SHARED(rcu_reader.need_mb)) {
103 smp_mb();
104 _STORE_SHARED(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 mutex_unlock(pthread_mutex_t *mutex)
113 {
114 int ret;
115
116 ret = pthread_mutex_unlock(mutex);
117 if (ret) {
118 perror("Error in pthread mutex unlock");
119 exit(-1);
120 }
121 }
122
123 #ifdef RCU_MEMBARRIER
124 static void smp_mb_master(int group)
125 {
126 if (likely(has_sys_membarrier))
127 membarrier(MEMBARRIER_EXPEDITED);
128 else
129 smp_mb();
130 }
131 #endif
132
133 #ifdef RCU_MB
134 static void smp_mb_master(int group)
135 {
136 smp_mb();
137 }
138 #endif
139
140 #ifdef RCU_SIGNAL
141 static void force_mb_all_readers(void)
142 {
143 struct rcu_reader *index;
144
145 /*
146 * Ask for each threads to execute a smp_mb() so we can consider the
147 * compiler barriers around rcu read lock as real memory barriers.
148 */
149 if (list_empty(&registry))
150 return;
151 /*
152 * pthread_kill has a smp_mb(). But beware, we assume it performs
153 * a cache flush on architectures with non-coherent cache. Let's play
154 * safe and don't assume anything : we use smp_mc() to make sure the
155 * cache flush is enforced.
156 */
157 list_for_each_entry(index, &registry, node) {
158 STORE_SHARED(index->need_mb, 1);
159 pthread_kill(index->tid, SIGRCU);
160 }
161 /*
162 * Wait for sighandler (and thus mb()) to execute on every thread.
163 *
164 * Note that the pthread_kill() will never be executed on systems
165 * that correctly deliver signals in a timely manner. However, it
166 * is not uncommon for kernels to have bugs that can result in
167 * lost or unduly delayed signals.
168 *
169 * If you are seeing the below pthread_kill() executing much at
170 * all, we suggest testing the underlying kernel and filing the
171 * relevant bug report. For Linux kernels, we recommend getting
172 * the Linux Test Project (LTP).
173 */
174 list_for_each_entry(index, &registry, node) {
175 while (LOAD_SHARED(index->need_mb)) {
176 pthread_kill(index->tid, SIGRCU);
177 poll(NULL, 0, 1);
178 }
179 }
180 smp_mb(); /* read ->need_mb before ending the barrier */
181 }
182
183 static void smp_mb_master(int group)
184 {
185 force_mb_all_readers();
186 }
187 #endif /* #ifdef RCU_SIGNAL */
188
189 /*
190 * synchronize_rcu() waiting. Single thread.
191 */
192 static void wait_gp(void)
193 {
194 /* Read reader_gp before read futex */
195 smp_mb_master(RCU_MB_GROUP);
196 if (uatomic_read(&gp_futex) == -1)
197 futex_async(&gp_futex, FUTEX_WAIT, -1,
198 NULL, NULL, 0);
199 }
200
201 void update_counter_and_wait(void)
202 {
203 LIST_HEAD(qsreaders);
204 int wait_loops = 0;
205 struct rcu_reader *index, *tmp;
206
207 /* Switch parity: 0 -> 1, 1 -> 0 */
208 STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
209
210 /*
211 * Must commit rcu_gp_ctr update to memory before waiting for quiescent
212 * state. Failure to do so could result in the writer waiting forever
213 * while new readers are always accessing data (no progress). Enforce
214 * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
215 */
216 barrier();
217
218 /*
219 *
220 * Adding a smp_mb() which is _not_ formally required, but makes the
221 * model easier to understand. It does not have a big performance impact
222 * anyway, given this is the write-side.
223 */
224 smp_mb();
225
226 /*
227 * Wait for each thread rcu_reader.ctr count to become 0.
228 */
229 for (;;) {
230 wait_loops++;
231 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
232 uatomic_dec(&gp_futex);
233 /* Write futex before read reader_gp */
234 smp_mb_master(RCU_MB_GROUP);
235 }
236
237 list_for_each_entry_safe(index, tmp, &registry, node) {
238 if (!rcu_gp_ongoing(&index->ctr))
239 list_move(&index->node, &qsreaders);
240 }
241
242 #ifndef HAS_INCOHERENT_CACHES
243 if (list_empty(&registry)) {
244 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
245 /* Read reader_gp before write futex */
246 smp_mb_master(RCU_MB_GROUP);
247 uatomic_set(&gp_futex, 0);
248 }
249 break;
250 } else {
251 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
252 wait_gp();
253 else
254 cpu_relax();
255 }
256 #else /* #ifndef HAS_INCOHERENT_CACHES */
257 /*
258 * BUSY-LOOP. Force the reader thread to commit its
259 * rcu_reader.ctr update to memory if we wait for too long.
260 */
261 if (list_empty(&registry)) {
262 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
263 /* Read reader_gp before write futex */
264 smp_mb_master(RCU_MB_GROUP);
265 uatomic_set(&gp_futex, 0);
266 }
267 break;
268 } else {
269 switch (wait_loops) {
270 case RCU_QS_ACTIVE_ATTEMPTS:
271 wait_gp();
272 break; /* only escape switch */
273 case KICK_READER_LOOPS:
274 smp_mb_master(RCU_MB_GROUP);
275 wait_loops = 0;
276 break; /* only escape switch */
277 default:
278 cpu_relax();
279 }
280 }
281 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
282 }
283 /* put back the reader list in the registry */
284 list_splice(&qsreaders, &registry);
285 }
286
287 void synchronize_rcu(void)
288 {
289 mutex_lock(&rcu_gp_lock);
290
291 if (list_empty(&registry))
292 goto out;
293
294 /* All threads should read qparity before accessing data structure
295 * where new ptr points to. Must be done within rcu_gp_lock because it
296 * iterates on reader threads.*/
297 /* Write new ptr before changing the qparity */
298 smp_mb_master(RCU_MB_GROUP);
299
300 /*
301 * Wait for previous parity to be empty of readers.
302 */
303 update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */
304
305 /*
306 * Must finish waiting for quiescent state for parity 0 before
307 * committing next rcu_gp_ctr update to memory. Failure to do so could
308 * result in the writer waiting forever while new readers are always
309 * accessing data (no progress). Enforce compiler-order of load
310 * rcu_reader ctr before store to rcu_gp_ctr.
311 */
312 barrier();
313
314 /*
315 * Adding a 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 smp_mb();
320
321 /*
322 * Wait for previous parity to be empty of readers.
323 */
324 update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */
325
326 /* Finish waiting for reader threads before letting the old ptr being
327 * freed. Must be done within rcu_gp_lock because it iterates on reader
328 * threads. */
329 smp_mb_master(RCU_MB_GROUP);
330 out:
331 mutex_unlock(&rcu_gp_lock);
332 }
333
334 /*
335 * library wrappers to be used by non-LGPL compatible source code.
336 */
337
338 void rcu_read_lock(void)
339 {
340 _rcu_read_lock();
341 }
342
343 void rcu_read_unlock(void)
344 {
345 _rcu_read_unlock();
346 }
347
348 void rcu_register_thread(void)
349 {
350 rcu_reader.tid = pthread_self();
351 assert(rcu_reader.need_mb == 0);
352 assert(!(rcu_reader.ctr & RCU_GP_CTR_NEST_MASK));
353
354 mutex_lock(&rcu_gp_lock);
355 rcu_init(); /* In case gcc does not support constructor attribute */
356 list_add(&rcu_reader.node, &registry);
357 mutex_unlock(&rcu_gp_lock);
358 }
359
360 void rcu_unregister_thread(void)
361 {
362 mutex_lock(&rcu_gp_lock);
363 list_del(&rcu_reader.node);
364 mutex_unlock(&rcu_gp_lock);
365 }
366
367 #ifdef RCU_MEMBARRIER
368 void rcu_init(void)
369 {
370 if (init_done)
371 return;
372 init_done = 1;
373 if (!membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY))
374 has_sys_membarrier = 1;
375 }
376 #endif
377
378 #ifdef RCU_SIGNAL
379 static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
380 {
381 /*
382 * Executing this smp_mb() is the only purpose of this signal handler.
383 * It punctually promotes barrier() into smp_mb() on every thread it is
384 * executed on.
385 */
386 smp_mb();
387 _STORE_SHARED(rcu_reader.need_mb, 0);
388 smp_mb();
389 }
390
391 /*
392 * rcu_init constructor. Called when the library is linked, but also when
393 * reader threads are calling rcu_register_thread().
394 * Should only be called by a single thread at a given time. This is ensured by
395 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
396 * load time, which should not be executed by multiple threads nor concurrently
397 * with rcu_register_thread() anyway.
398 */
399 void rcu_init(void)
400 {
401 struct sigaction act;
402 int ret;
403
404 if (init_done)
405 return;
406 init_done = 1;
407
408 act.sa_sigaction = sigrcu_handler;
409 act.sa_flags = SA_SIGINFO | SA_RESTART;
410 sigemptyset(&act.sa_mask);
411 ret = sigaction(SIGRCU, &act, NULL);
412 if (ret) {
413 perror("Error in sigaction");
414 exit(-1);
415 }
416 }
417
418 void rcu_exit(void)
419 {
420 struct sigaction act;
421 int ret;
422
423 ret = sigaction(SIGRCU, NULL, &act);
424 if (ret) {
425 perror("Error in sigaction");
426 exit(-1);
427 }
428 assert(act.sa_sigaction == sigrcu_handler);
429 assert(list_empty(&registry));
430 }
431 #endif /* #ifdef RCU_SIGNAL */
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