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[urcu.git] / 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 #include <stdio.h>
27 #include <pthread.h>
28 #include <signal.h>
29 #include <assert.h>
30 #include <stdlib.h>
31 #include <string.h>
32 #include <errno.h>
33 #include <poll.h>
34
35 #define BUILD_QSBR_LIB
36 #include "urcu-qsbr-static.h"
37 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
38 #include "urcu-qsbr.h"
39
40 void __attribute__((destructor)) rcu_exit(void);
41
42 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
43
44 int gp_futex;
45
46 /*
47 * Global grace period counter.
48 */
49 unsigned long rcu_gp_ctr = RCU_GP_ONLINE;
50
51 /*
52 * Written to only by each individual reader. Read by both the reader and the
53 * writers.
54 */
55 struct rcu_reader __thread rcu_reader;
56
57 #ifdef DEBUG_YIELD
58 unsigned int yield_active;
59 unsigned int __thread rand_yield;
60 #endif
61
62 static LIST_HEAD(registry);
63
64 static void mutex_lock(pthread_mutex_t *mutex)
65 {
66 int ret;
67
68 #ifndef DISTRUST_SIGNALS_EXTREME
69 ret = pthread_mutex_lock(mutex);
70 if (ret) {
71 perror("Error in pthread mutex lock");
72 exit(-1);
73 }
74 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
75 while ((ret = pthread_mutex_trylock(mutex)) != 0) {
76 if (ret != EBUSY && ret != EINTR) {
77 printf("ret = %d, errno = %d\n", ret, errno);
78 perror("Error in pthread mutex lock");
79 exit(-1);
80 }
81 poll(NULL,0,10);
82 }
83 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
84 }
85
86 static void mutex_unlock(pthread_mutex_t *mutex)
87 {
88 int ret;
89
90 ret = pthread_mutex_unlock(mutex);
91 if (ret) {
92 perror("Error in pthread mutex unlock");
93 exit(-1);
94 }
95 }
96
97 /*
98 * synchronize_rcu() waiting. Single thread.
99 */
100 static void wait_gp(void)
101 {
102 /* Read reader_gp before read futex */
103 smp_rmb();
104 if (uatomic_read(&gp_futex) == -1)
105 futex_noasync(&gp_futex, FUTEX_WAIT, -1,
106 NULL, NULL, 0);
107 }
108
109 static void update_counter_and_wait(void)
110 {
111 LIST_HEAD(qsreaders);
112 int wait_loops = 0;
113 struct rcu_reader *index, *tmp;
114
115 #if (BITS_PER_LONG < 64)
116 /* Switch parity: 0 -> 1, 1 -> 0 */
117 STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
118 #else /* !(BITS_PER_LONG < 64) */
119 /* Increment current G.P. */
120 STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
121 #endif /* !(BITS_PER_LONG < 64) */
122
123 /*
124 * Must commit rcu_gp_ctr update to memory before waiting for quiescent
125 * state. Failure to do so could result in the writer waiting forever
126 * while new readers are always accessing data (no progress). Enforce
127 * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
128 */
129 barrier();
130
131 /*
132 * Adding a smp_mb() which is _not_ formally required, but makes the
133 * model easier to understand. It does not have a big performance impact
134 * anyway, given this is the write-side.
135 */
136 smp_mb();
137
138 /*
139 * Wait for each thread rcu_reader_qs_gp count to become 0.
140 */
141 for (;;) {
142 wait_loops++;
143 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
144 uatomic_dec(&gp_futex);
145 /* Write futex before read reader_gp */
146 smp_mb();
147 }
148
149 list_for_each_entry_safe(index, tmp, &registry, node) {
150 if (!rcu_gp_ongoing(&index->ctr))
151 list_move(&index->node, &qsreaders);
152 }
153
154 if (list_empty(&registry)) {
155 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
156 /* Read reader_gp before write futex */
157 smp_mb();
158 uatomic_set(&gp_futex, 0);
159 }
160 break;
161 } else {
162 if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
163 wait_gp();
164 } else {
165 #ifndef HAS_INCOHERENT_CACHES
166 cpu_relax();
167 #else /* #ifndef HAS_INCOHERENT_CACHES */
168 smp_mb();
169 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
170 }
171 }
172 }
173 /* put back the reader list in the registry */
174 list_splice(&qsreaders, &registry);
175 }
176
177 /*
178 * Using a two-subphases algorithm for architectures with smaller than 64-bit
179 * long-size to ensure we do not encounter an overflow bug.
180 */
181
182 #if (BITS_PER_LONG < 64)
183 void synchronize_rcu(void)
184 {
185 unsigned long was_online;
186
187 was_online = rcu_reader.ctr;
188
189 /* All threads should read qparity before accessing data structure
190 * where new ptr points to.
191 */
192 /* Write new ptr before changing the qparity */
193 smp_mb();
194
195 /*
196 * Mark the writer thread offline to make sure we don't wait for
197 * our own quiescent state. This allows using synchronize_rcu() in
198 * threads registered as readers.
199 */
200 if (was_online)
201 STORE_SHARED(rcu_reader.ctr, 0);
202
203 mutex_lock(&rcu_gp_lock);
204
205 if (list_empty(&registry))
206 goto out;
207
208 /*
209 * Wait for previous parity to be empty of readers.
210 */
211 update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */
212
213 /*
214 * Must finish waiting for quiescent state for parity 0 before
215 * committing next rcu_gp_ctr update to memory. Failure to do so could
216 * result in the writer waiting forever while new readers are always
217 * accessing data (no progress). Enforce compiler-order of load
218 * rcu_reader ctr before store to rcu_gp_ctr.
219 */
220 barrier();
221
222 /*
223 * Adding a smp_mb() which is _not_ formally required, but makes the
224 * model easier to understand. It does not have a big performance impact
225 * anyway, given this is the write-side.
226 */
227 smp_mb();
228
229 /*
230 * Wait for previous parity to be empty of readers.
231 */
232 update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */
233 out:
234 mutex_unlock(&rcu_gp_lock);
235
236 /*
237 * Finish waiting for reader threads before letting the old ptr being
238 * freed.
239 */
240 if (was_online)
241 _STORE_SHARED(rcu_reader.ctr, LOAD_SHARED(rcu_gp_ctr));
242 smp_mb();
243 }
244 #else /* !(BITS_PER_LONG < 64) */
245 void synchronize_rcu(void)
246 {
247 unsigned long was_online;
248
249 was_online = rcu_reader.ctr;
250
251 /*
252 * Mark the writer thread offline to make sure we don't wait for
253 * our own quiescent state. This allows using synchronize_rcu() in
254 * threads registered as readers.
255 */
256 smp_mb();
257 if (was_online)
258 STORE_SHARED(rcu_reader.ctr, 0);
259
260 mutex_lock(&rcu_gp_lock);
261 if (list_empty(&registry))
262 goto out;
263 update_counter_and_wait();
264 out:
265 mutex_unlock(&rcu_gp_lock);
266
267 if (was_online)
268 _STORE_SHARED(rcu_reader.ctr, LOAD_SHARED(rcu_gp_ctr));
269 smp_mb();
270 }
271 #endif /* !(BITS_PER_LONG < 64) */
272
273 /*
274 * library wrappers to be used by non-LGPL compatible source code.
275 */
276
277 void rcu_read_lock(void)
278 {
279 _rcu_read_lock();
280 }
281
282 void rcu_read_unlock(void)
283 {
284 _rcu_read_unlock();
285 }
286
287 void rcu_quiescent_state(void)
288 {
289 _rcu_quiescent_state();
290 }
291
292 void rcu_thread_offline(void)
293 {
294 _rcu_thread_offline();
295 }
296
297 void rcu_thread_online(void)
298 {
299 _rcu_thread_online();
300 }
301
302 void rcu_register_thread(void)
303 {
304 rcu_reader.tid = pthread_self();
305 assert(rcu_reader.ctr == 0);
306
307 mutex_lock(&rcu_gp_lock);
308 list_add(&rcu_reader.node, &registry);
309 mutex_unlock(&rcu_gp_lock);
310 _rcu_thread_online();
311 }
312
313 void rcu_unregister_thread(void)
314 {
315 /*
316 * We have to make the thread offline otherwise we end up dealocking
317 * with a waiting writer.
318 */
319 _rcu_thread_offline();
320 mutex_lock(&rcu_gp_lock);
321 list_del(&rcu_reader.node);
322 mutex_unlock(&rcu_gp_lock);
323 }
324
325 void rcu_exit(void)
326 {
327 assert(list_empty(&registry));
328 }
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