4 * Userspace RCU library
6 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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.
39 #include "urcu/wfqueue.h"
40 #include "urcu/map/urcu.h"
41 #include "urcu/static/urcu.h"
42 #include "urcu-pointer.h"
43 #include "urcu/tls-compat.h"
45 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
51 * If a reader is really non-cooperative and refuses to commit its
52 * rcu_active_readers count to memory (there is no barrier in the reader
53 * per-se), kick it after a few loops waiting for it.
55 #define KICK_READER_LOOPS 10000
58 * Active attempts to check for reader Q.S. before calling futex().
60 #define RCU_QS_ACTIVE_ATTEMPTS 100
64 int has_sys_membarrier
;
66 void __attribute__((constructor
)) rcu_init(void);
78 void __attribute__((constructor
)) rcu_init(void);
79 void __attribute__((destructor
)) rcu_exit(void);
82 static pthread_mutex_t rcu_gp_lock
= PTHREAD_MUTEX_INITIALIZER
;
87 * Global grace period counter.
88 * Contains the current RCU_GP_CTR_PHASE.
89 * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
90 * Written to only by writer with mutex taken. Read by both writer and readers.
92 unsigned long rcu_gp_ctr
= RCU_GP_COUNT
;
95 * Written to only by each individual reader. Read by both the reader and the
98 DEFINE_URCU_TLS(struct rcu_reader
, rcu_reader
);
101 unsigned int yield_active
;
102 DEFINE_URCU_TLS(unsigned int, rand_yield
);
105 static CDS_LIST_HEAD(registry
);
107 static void mutex_lock(pthread_mutex_t
*mutex
)
111 #ifndef DISTRUST_SIGNALS_EXTREME
112 ret
= pthread_mutex_lock(mutex
);
114 perror("Error in pthread mutex lock");
117 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
118 while ((ret
= pthread_mutex_trylock(mutex
)) != 0) {
119 if (ret
!= EBUSY
&& ret
!= EINTR
) {
120 printf("ret = %d, errno = %d\n", ret
, errno
);
121 perror("Error in pthread mutex lock");
124 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader
).need_mb
)) {
126 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
131 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
134 static void mutex_unlock(pthread_mutex_t
*mutex
)
138 ret
= pthread_mutex_unlock(mutex
);
140 perror("Error in pthread mutex unlock");
145 #ifdef RCU_MEMBARRIER
146 static void smp_mb_master(int group
)
148 if (caa_likely(has_sys_membarrier
))
149 membarrier(MEMBARRIER_EXPEDITED
);
156 static void smp_mb_master(int group
)
163 static void force_mb_all_readers(void)
165 struct rcu_reader
*index
;
168 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
169 * compiler barriers around rcu read lock as real memory barriers.
171 if (cds_list_empty(®istry
))
174 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
175 * a cache flush on architectures with non-coherent cache. Let's play
176 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
177 * cache flush is enforced.
179 cds_list_for_each_entry(index
, ®istry
, node
) {
180 CMM_STORE_SHARED(index
->need_mb
, 1);
181 pthread_kill(index
->tid
, SIGRCU
);
184 * Wait for sighandler (and thus mb()) to execute on every thread.
186 * Note that the pthread_kill() will never be executed on systems
187 * that correctly deliver signals in a timely manner. However, it
188 * is not uncommon for kernels to have bugs that can result in
189 * lost or unduly delayed signals.
191 * If you are seeing the below pthread_kill() executing much at
192 * all, we suggest testing the underlying kernel and filing the
193 * relevant bug report. For Linux kernels, we recommend getting
194 * the Linux Test Project (LTP).
196 cds_list_for_each_entry(index
, ®istry
, node
) {
197 while (CMM_LOAD_SHARED(index
->need_mb
)) {
198 pthread_kill(index
->tid
, SIGRCU
);
202 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
205 static void smp_mb_master(int group
)
207 force_mb_all_readers();
209 #endif /* #ifdef RCU_SIGNAL */
212 * synchronize_rcu() waiting. Single thread.
214 static void wait_gp(void)
216 /* Read reader_gp before read futex */
217 smp_mb_master(RCU_MB_GROUP
);
218 if (uatomic_read(&gp_futex
) == -1)
219 futex_async(&gp_futex
, FUTEX_WAIT
, -1,
223 void update_counter_and_wait(void)
225 CDS_LIST_HEAD(qsreaders
);
227 struct rcu_reader
*index
, *tmp
;
229 /* Switch parity: 0 -> 1, 1 -> 0 */
230 CMM_STORE_SHARED(rcu_gp_ctr
, rcu_gp_ctr
^ RCU_GP_CTR_PHASE
);
233 * Must commit rcu_gp_ctr update to memory before waiting for quiescent
234 * state. Failure to do so could result in the writer waiting forever
235 * while new readers are always accessing data (no progress). Enforce
236 * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
242 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
243 * model easier to understand. It does not have a big performance impact
244 * anyway, given this is the write-side.
249 * Wait for each thread URCU_TLS(rcu_reader).ctr count to become 0.
253 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
254 uatomic_dec(&gp_futex
);
255 /* Write futex before read reader_gp */
256 smp_mb_master(RCU_MB_GROUP
);
259 cds_list_for_each_entry_safe(index
, tmp
, ®istry
, node
) {
260 if (!rcu_gp_ongoing(&index
->ctr
))
261 cds_list_move(&index
->node
, &qsreaders
);
264 #ifndef HAS_INCOHERENT_CACHES
265 if (cds_list_empty(®istry
)) {
266 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
267 /* Read reader_gp before write futex */
268 smp_mb_master(RCU_MB_GROUP
);
269 uatomic_set(&gp_futex
, 0);
273 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
)
278 #else /* #ifndef HAS_INCOHERENT_CACHES */
280 * BUSY-LOOP. Force the reader thread to commit its
281 * URCU_TLS(rcu_reader).ctr update to memory if we wait
284 if (cds_list_empty(®istry
)) {
285 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
286 /* Read reader_gp before write futex */
287 smp_mb_master(RCU_MB_GROUP
);
288 uatomic_set(&gp_futex
, 0);
292 switch (wait_loops
) {
293 case RCU_QS_ACTIVE_ATTEMPTS
:
295 break; /* only escape switch */
296 case KICK_READER_LOOPS
:
297 smp_mb_master(RCU_MB_GROUP
);
299 break; /* only escape switch */
304 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
306 /* put back the reader list in the registry */
307 cds_list_splice(&qsreaders
, ®istry
);
310 void synchronize_rcu(void)
312 mutex_lock(&rcu_gp_lock
);
314 if (cds_list_empty(®istry
))
317 /* All threads should read qparity before accessing data structure
318 * where new ptr points to. Must be done within rcu_gp_lock because it
319 * iterates on reader threads.*/
320 /* Write new ptr before changing the qparity */
321 smp_mb_master(RCU_MB_GROUP
);
324 * Wait for previous parity to be empty of readers.
326 update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */
329 * Must finish waiting for quiescent state for parity 0 before
330 * committing next rcu_gp_ctr update to memory. Failure to do so could
331 * result in the writer waiting forever while new readers are always
332 * accessing data (no progress). Enforce compiler-order of load
333 * URCU_TLS(rcu_reader).ctr before store to rcu_gp_ctr.
338 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
339 * model easier to understand. It does not have a big performance impact
340 * anyway, given this is the write-side.
345 * Wait for previous parity to be empty of readers.
347 update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */
349 /* Finish waiting for reader threads before letting the old ptr being
350 * freed. Must be done within rcu_gp_lock because it iterates on reader
352 smp_mb_master(RCU_MB_GROUP
);
354 mutex_unlock(&rcu_gp_lock
);
358 * library wrappers to be used by non-LGPL compatible source code.
361 void rcu_read_lock(void)
366 void rcu_read_unlock(void)
371 void rcu_register_thread(void)
373 URCU_TLS(rcu_reader
).tid
= pthread_self();
374 assert(URCU_TLS(rcu_reader
).need_mb
== 0);
375 assert(!(URCU_TLS(rcu_reader
).ctr
& RCU_GP_CTR_NEST_MASK
));
377 mutex_lock(&rcu_gp_lock
);
378 rcu_init(); /* In case gcc does not support constructor attribute */
379 cds_list_add(&URCU_TLS(rcu_reader
).node
, ®istry
);
380 mutex_unlock(&rcu_gp_lock
);
383 void rcu_unregister_thread(void)
385 mutex_lock(&rcu_gp_lock
);
386 cds_list_del(&URCU_TLS(rcu_reader
).node
);
387 mutex_unlock(&rcu_gp_lock
);
390 #ifdef RCU_MEMBARRIER
396 if (!membarrier(MEMBARRIER_EXPEDITED
| MEMBARRIER_QUERY
))
397 has_sys_membarrier
= 1;
402 static void sigrcu_handler(int signo
, siginfo_t
*siginfo
, void *context
)
405 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
406 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
410 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
415 * rcu_init constructor. Called when the library is linked, but also when
416 * reader threads are calling rcu_register_thread().
417 * Should only be called by a single thread at a given time. This is ensured by
418 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
419 * load time, which should not be executed by multiple threads nor concurrently
420 * with rcu_register_thread() anyway.
424 struct sigaction act
;
431 act
.sa_sigaction
= sigrcu_handler
;
432 act
.sa_flags
= SA_SIGINFO
| SA_RESTART
;
433 sigemptyset(&act
.sa_mask
);
434 ret
= sigaction(SIGRCU
, &act
, NULL
);
436 perror("Error in sigaction");
443 struct sigaction act
;
446 ret
= sigaction(SIGRCU
, NULL
, &act
);
448 perror("Error in sigaction");
451 assert(act
.sa_sigaction
== sigrcu_handler
);
452 assert(cds_list_empty(®istry
));
455 #endif /* #ifdef RCU_SIGNAL */
457 DEFINE_RCU_FLAVOR(rcu_flavor
);
459 #include "urcu-call-rcu-impl.h"
460 #include "urcu-defer-impl.h"