4 * Userspace RCU library
6 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
7 * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
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.
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.
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
23 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
36 #include "urcu-static.h"
37 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
42 int has_sys_membarrier
;
44 void __attribute__((constructor
)) rcu_init(void);
56 void __attribute__((constructor
)) rcu_init(void);
57 void __attribute__((destructor
)) rcu_exit(void);
60 static pthread_mutex_t rcu_gp_lock
= PTHREAD_MUTEX_INITIALIZER
;
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.
70 unsigned long rcu_gp_ctr
= RCU_GP_COUNT
;
73 * Written to only by each individual reader. Read by both the reader and the
76 struct rcu_reader __thread rcu_reader
;
79 unsigned int yield_active
;
80 unsigned int __thread rand_yield
;
83 static LIST_HEAD(registry
);
85 static void mutex_lock(pthread_mutex_t
*mutex
)
89 #ifndef DISTRUST_SIGNALS_EXTREME
90 ret
= pthread_mutex_lock(mutex
);
92 perror("Error in pthread mutex lock");
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");
102 if (LOAD_SHARED(rcu_reader
.need_mb
)) {
104 _STORE_SHARED(rcu_reader
.need_mb
, 0);
109 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
112 static void mutex_unlock(pthread_mutex_t
*mutex
)
116 ret
= pthread_mutex_unlock(mutex
);
118 perror("Error in pthread mutex unlock");
123 #ifdef RCU_MEMBARRIER
124 static void smp_mb_master(int group
)
126 if (likely(has_sys_membarrier
))
127 membarrier(MEMBARRIER_EXPEDITED
);
134 static void smp_mb_master(int group
)
141 static void force_mb_all_readers(void)
143 struct rcu_reader
*index
;
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.
149 if (list_empty(®istry
))
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.
157 list_for_each_entry(index
, ®istry
, head
) {
158 STORE_SHARED(index
->need_mb
, 1);
159 pthread_kill(index
->tid
, SIGRCU
);
162 * Wait for sighandler (and thus mb()) to execute on every thread.
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.
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).
174 list_for_each_entry(index
, ®istry
, head
) {
175 while (LOAD_SHARED(index
->need_mb
)) {
176 pthread_kill(index
->tid
, SIGRCU
);
180 smp_mb(); /* read ->need_mb before ending the barrier */
183 static void smp_mb_master(int group
)
185 force_mb_all_readers();
187 #endif /* #ifdef RCU_SIGNAL */
190 * synchronize_rcu() waiting. Single thread.
192 static void wait_gp(void)
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,
201 void update_counter_and_wait(void)
203 LIST_HEAD(qsreaders
);
205 struct rcu_reader
*index
, *tmp
;
207 /* Switch parity: 0 -> 1, 1 -> 0 */
208 STORE_SHARED(rcu_gp_ctr
, rcu_gp_ctr
^ RCU_GP_CTR_PHASE
);
211 * Must commit qparity update to memory before waiting for other parity
212 * quiescent state. Failure to do so could result in the writer waiting
213 * forever while new readers are always accessing data (no progress).
214 * Ensured by STORE_SHARED and LOAD_SHARED.
218 * Adding a smp_mb() which is _not_ formally required, but makes the
219 * model easier to understand. It does not have a big performance impact
220 * anyway, given this is the write-side.
225 * Wait for each thread rcu_reader.ctr count to become 0.
229 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
230 uatomic_dec(&gp_futex
);
231 /* Write futex before read reader_gp */
232 smp_mb_master(RCU_MB_GROUP
);
235 list_for_each_entry_safe(index
, tmp
, ®istry
, head
) {
236 if (!rcu_gp_ongoing(&index
->ctr
))
237 list_move(&index
->head
, &qsreaders
);
240 #ifndef HAS_INCOHERENT_CACHES
241 if (list_empty(®istry
)) {
242 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
243 /* Read reader_gp before write futex */
244 smp_mb_master(RCU_MB_GROUP
);
245 uatomic_set(&gp_futex
, 0);
249 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
)
254 #else /* #ifndef HAS_INCOHERENT_CACHES */
256 * BUSY-LOOP. Force the reader thread to commit its
257 * rcu_reader.ctr update to memory if we wait for too long.
259 if (list_empty(®istry
)) {
260 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
261 /* Read reader_gp before write futex */
262 smp_mb_master(RCU_MB_GROUP
);
263 uatomic_set(&gp_futex
, 0);
267 switch (wait_loops
) {
268 case RCU_QS_ACTIVE_ATTEMPTS
:
270 break; /* only escape switch */
271 case KICK_READER_LOOPS
:
272 smp_mb_master(RCU_MB_GROUP
);
274 break; /* only escape switch */
279 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
281 /* put back the reader list in the registry */
282 list_splice(&qsreaders
, ®istry
);
285 void synchronize_rcu(void)
287 mutex_lock(&rcu_gp_lock
);
289 if (list_empty(®istry
))
292 /* All threads should read qparity before accessing data structure
293 * where new ptr points to. Must be done within rcu_gp_lock because it
294 * iterates on reader threads.*/
295 /* Write new ptr before changing the qparity */
296 smp_mb_master(RCU_MB_GROUP
);
299 * Wait for previous parity to be empty of readers.
301 update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */
304 * Must finish waiting for quiescent state for parity 0 before
305 * committing qparity update to memory. Failure to do so could result in
306 * the writer waiting forever while new readers are always accessing
307 * data (no progress).
308 * Ensured by STORE_SHARED and LOAD_SHARED.
312 * Adding a smp_mb() which is _not_ formally required, but makes the
313 * model easier to understand. It does not have a big performance impact
314 * anyway, given this is the write-side.
319 * Wait for previous parity to be empty of readers.
321 update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */
323 /* Finish waiting for reader threads before letting the old ptr being
324 * freed. Must be done within rcu_gp_lock because it iterates on reader
326 smp_mb_master(RCU_MB_GROUP
);
328 mutex_unlock(&rcu_gp_lock
);
332 * library wrappers to be used by non-LGPL compatible source code.
335 void rcu_read_lock(void)
340 void rcu_read_unlock(void)
345 void rcu_register_thread(void)
347 rcu_reader
.tid
= pthread_self();
348 assert(rcu_reader
.need_mb
== 0);
349 assert(rcu_reader
.ctr
== 0);
351 mutex_lock(&rcu_gp_lock
);
352 rcu_init(); /* In case gcc does not support constructor attribute */
353 list_add(&rcu_reader
.head
, ®istry
);
354 mutex_unlock(&rcu_gp_lock
);
357 void rcu_unregister_thread(void)
359 mutex_lock(&rcu_gp_lock
);
360 list_del(&rcu_reader
.head
);
361 mutex_unlock(&rcu_gp_lock
);
364 #ifdef RCU_MEMBARRIER
370 if (!membarrier(MEMBARRIER_EXPEDITED
| MEMBARRIER_QUERY
))
371 has_sys_membarrier
= 1;
376 static void sigrcu_handler(int signo
, siginfo_t
*siginfo
, void *context
)
379 * Executing this smp_mb() is the only purpose of this signal handler.
380 * It punctually promotes barrier() into smp_mb() on every thread it is
384 _STORE_SHARED(rcu_reader
.need_mb
, 0);
389 * rcu_init constructor. Called when the library is linked, but also when
390 * reader threads are calling rcu_register_thread().
391 * Should only be called by a single thread at a given time. This is ensured by
392 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
393 * load time, which should not be executed by multiple threads nor concurrently
394 * with rcu_register_thread() anyway.
398 struct sigaction act
;
405 act
.sa_sigaction
= sigrcu_handler
;
406 act
.sa_flags
= SA_SIGINFO
| SA_RESTART
;
407 sigemptyset(&act
.sa_mask
);
408 ret
= sigaction(SIGRCU
, &act
, NULL
);
410 perror("Error in sigaction");
417 struct sigaction act
;
420 ret
= sigaction(SIGRCU
, NULL
, &act
);
422 perror("Error in sigaction");
425 assert(act
.sa_sigaction
== sigrcu_handler
);
426 assert(list_empty(®istry
));
428 #endif /* #ifdef RCU_SIGNAL */