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"
47 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
53 * If a reader is really non-cooperative and refuses to commit its
54 * rcu_active_readers count to memory (there is no barrier in the reader
55 * per-se), kick it after 10 loops waiting for it.
57 #define KICK_READER_LOOPS 10
60 * Active attempts to check for reader Q.S. before calling futex().
62 #define RCU_QS_ACTIVE_ATTEMPTS 100
66 int has_sys_membarrier
;
68 void __attribute__((constructor
)) rcu_init(void);
80 void __attribute__((constructor
)) rcu_init(void);
81 void __attribute__((destructor
)) rcu_exit(void);
85 * rcu_gp_lock ensures mutual exclusion between threads calling
88 static pthread_mutex_t rcu_gp_lock
= PTHREAD_MUTEX_INITIALIZER
;
90 * rcu_registry_lock ensures mutual exclusion between threads
91 * registering and unregistering themselves to/from the registry, and
92 * with threads reading that registry from synchronize_rcu(). However,
93 * this lock is not held all the way through the completion of awaiting
94 * for the grace period. It is sporadically released between iterations
96 * rcu_registry_lock may nest inside rcu_gp_lock.
98 static pthread_mutex_t rcu_registry_lock
= PTHREAD_MUTEX_INITIALIZER
;
103 * Global grace period counter.
104 * Contains the current RCU_GP_CTR_PHASE.
105 * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
106 * Written to only by writer with mutex taken. Read by both writer and readers.
108 unsigned long rcu_gp_ctr
= RCU_GP_COUNT
;
110 * Written to only by each individual reader. Read by both the reader and the
113 __DEFINE_URCU_TLS_GLOBAL(struct rcu_reader
, rcu_reader
);
116 unsigned int yield_active
;
117 __DEFINE_URCU_TLS_GLOBAL(unsigned int, rand_yield
);
120 static CDS_LIST_HEAD(registry
);
122 static void mutex_lock(pthread_mutex_t
*mutex
)
126 #ifndef DISTRUST_SIGNALS_EXTREME
127 ret
= pthread_mutex_lock(mutex
);
130 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
131 while ((ret
= pthread_mutex_trylock(mutex
)) != 0) {
132 if (ret
!= EBUSY
&& ret
!= EINTR
)
134 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader
).need_mb
)) {
136 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
141 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
144 static void mutex_unlock(pthread_mutex_t
*mutex
)
148 ret
= pthread_mutex_unlock(mutex
);
153 #ifdef RCU_MEMBARRIER
154 static void smp_mb_master(int group
)
156 if (caa_likely(has_sys_membarrier
))
157 membarrier(MEMBARRIER_EXPEDITED
);
164 static void smp_mb_master(int group
)
171 static void force_mb_all_readers(void)
173 struct rcu_reader
*index
;
176 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
177 * compiler barriers around rcu read lock as real memory barriers.
179 if (cds_list_empty(®istry
))
182 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
183 * a cache flush on architectures with non-coherent cache. Let's play
184 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
185 * cache flush is enforced.
187 cds_list_for_each_entry(index
, ®istry
, node
) {
188 CMM_STORE_SHARED(index
->need_mb
, 1);
189 pthread_kill(index
->tid
, SIGRCU
);
192 * Wait for sighandler (and thus mb()) to execute on every thread.
194 * Note that the pthread_kill() will never be executed on systems
195 * that correctly deliver signals in a timely manner. However, it
196 * is not uncommon for kernels to have bugs that can result in
197 * lost or unduly delayed signals.
199 * If you are seeing the below pthread_kill() executing much at
200 * all, we suggest testing the underlying kernel and filing the
201 * relevant bug report. For Linux kernels, we recommend getting
202 * the Linux Test Project (LTP).
204 cds_list_for_each_entry(index
, ®istry
, node
) {
205 while (CMM_LOAD_SHARED(index
->need_mb
)) {
206 pthread_kill(index
->tid
, SIGRCU
);
210 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
213 static void smp_mb_master(int group
)
215 force_mb_all_readers();
217 #endif /* #ifdef RCU_SIGNAL */
220 * synchronize_rcu() waiting. Single thread.
222 static void wait_gp(void)
224 /* Read reader_gp before read futex */
225 smp_mb_master(RCU_MB_GROUP
);
226 if (uatomic_read(&gp_futex
) == -1)
227 futex_async(&gp_futex
, FUTEX_WAIT
, -1,
232 * Always called with rcu_registry lock held. Releases this lock between
233 * iterations and grabs it again. Holds the lock when it returns.
235 void update_counter_and_wait(void)
237 CDS_LIST_HEAD(qsreaders
);
238 unsigned int wait_loops
= 0;
239 struct rcu_reader
*index
, *tmp
;
240 #ifdef HAS_INCOHERENT_CACHES
241 unsigned int wait_gp_loops
= 0;
242 #endif /* HAS_INCOHERENT_CACHES */
244 /* Switch parity: 0 -> 1, 1 -> 0 */
245 CMM_STORE_SHARED(rcu_gp_ctr
, rcu_gp_ctr
^ RCU_GP_CTR_PHASE
);
248 * Must commit rcu_gp_ctr update to memory before waiting for quiescent
249 * state. Failure to do so could result in the writer waiting forever
250 * while new readers are always accessing data (no progress). Enforce
251 * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
257 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
258 * model easier to understand. It does not have a big performance impact
259 * anyway, given this is the write-side.
264 * Wait for each thread URCU_TLS(rcu_reader).ctr count to become 0.
267 if (wait_loops
< RCU_QS_ACTIVE_ATTEMPTS
)
269 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
270 uatomic_dec(&gp_futex
);
271 /* Write futex before read reader_gp */
272 smp_mb_master(RCU_MB_GROUP
);
275 cds_list_for_each_entry_safe(index
, tmp
, ®istry
, node
) {
276 if (!rcu_gp_ongoing(&index
->ctr
))
277 cds_list_move(&index
->node
, &qsreaders
);
280 #ifndef HAS_INCOHERENT_CACHES
281 if (cds_list_empty(®istry
)) {
282 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
283 /* Read reader_gp before write futex */
284 smp_mb_master(RCU_MB_GROUP
);
285 uatomic_set(&gp_futex
, 0);
289 /* Temporarily unlock the registry lock. */
290 mutex_unlock(&rcu_registry_lock
);
291 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
)
295 /* Re-lock the registry lock before the next loop. */
296 mutex_lock(&rcu_registry_lock
);
298 #else /* #ifndef HAS_INCOHERENT_CACHES */
300 * BUSY-LOOP. Force the reader thread to commit its
301 * URCU_TLS(rcu_reader).ctr update to memory if we wait
304 if (cds_list_empty(®istry
)) {
305 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
306 /* Read reader_gp before write futex */
307 smp_mb_master(RCU_MB_GROUP
);
308 uatomic_set(&gp_futex
, 0);
312 if (wait_gp_loops
== KICK_READER_LOOPS
) {
313 smp_mb_master(RCU_MB_GROUP
);
316 /* Temporarily unlock the registry lock. */
317 mutex_unlock(&rcu_registry_lock
);
318 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
324 /* Re-lock the registry lock before the next loop. */
325 mutex_lock(&rcu_registry_lock
);
327 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
329 /* put back the reader list in the registry */
330 cds_list_splice(&qsreaders
, ®istry
);
333 void synchronize_rcu(void)
335 mutex_lock(&rcu_gp_lock
);
336 mutex_lock(&rcu_registry_lock
);
338 if (cds_list_empty(®istry
))
342 * All threads should read qparity before accessing data structure
343 * where new ptr points to. Must be done within rcu_registry_lock
344 * because it iterates on reader threads.
346 /* Write new ptr before changing the qparity */
347 smp_mb_master(RCU_MB_GROUP
);
350 * Wait for previous parity to be empty of readers.
351 * update_counter_and_wait() can release and grab again
352 * rcu_registry_lock interally.
354 update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */
357 * Must finish waiting for quiescent state for parity 0 before
358 * committing next rcu_gp_ctr update to memory. Failure to do so could
359 * result in the writer waiting forever while new readers are always
360 * accessing data (no progress). Enforce compiler-order of load
361 * URCU_TLS(rcu_reader).ctr before store to rcu_gp_ctr.
366 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
367 * model easier to understand. It does not have a big performance impact
368 * anyway, given this is the write-side.
373 * Wait for previous parity to be empty of readers.
374 * update_counter_and_wait() can release and grab again
375 * rcu_registry_lock interally.
377 update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */
380 * Finish waiting for reader threads before letting the old ptr
381 * being freed. Must be done within rcu_registry_lock because it
382 * iterates on reader threads.
384 smp_mb_master(RCU_MB_GROUP
);
386 mutex_unlock(&rcu_registry_lock
);
387 mutex_unlock(&rcu_gp_lock
);
391 * library wrappers to be used by non-LGPL compatible source code.
394 void rcu_read_lock(void)
399 void rcu_read_unlock(void)
404 void rcu_register_thread(void)
406 URCU_TLS(rcu_reader
).tid
= pthread_self();
407 assert(URCU_TLS(rcu_reader
).need_mb
== 0);
408 assert(!(URCU_TLS(rcu_reader
).ctr
& RCU_GP_CTR_NEST_MASK
));
410 mutex_lock(&rcu_registry_lock
);
411 rcu_init(); /* In case gcc does not support constructor attribute */
412 cds_list_add(&URCU_TLS(rcu_reader
).node
, ®istry
);
413 mutex_unlock(&rcu_registry_lock
);
416 void rcu_unregister_thread(void)
418 mutex_lock(&rcu_registry_lock
);
419 cds_list_del(&URCU_TLS(rcu_reader
).node
);
420 mutex_unlock(&rcu_registry_lock
);
423 #ifdef RCU_MEMBARRIER
429 if (!membarrier(MEMBARRIER_EXPEDITED
| MEMBARRIER_QUERY
))
430 has_sys_membarrier
= 1;
435 static void sigrcu_handler(int signo
, siginfo_t
*siginfo
, void *context
)
438 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
439 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
443 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
448 * rcu_init constructor. Called when the library is linked, but also when
449 * reader threads are calling rcu_register_thread().
450 * Should only be called by a single thread at a given time. This is ensured by
451 * holing the rcu_registry_lock from rcu_register_thread() or by running
452 * at library load time, which should not be executed by multiple
453 * threads nor concurrently with rcu_register_thread() anyway.
457 struct sigaction act
;
464 act
.sa_sigaction
= sigrcu_handler
;
465 act
.sa_flags
= SA_SIGINFO
| SA_RESTART
;
466 sigemptyset(&act
.sa_mask
);
467 ret
= sigaction(SIGRCU
, &act
, NULL
);
474 struct sigaction act
;
477 ret
= sigaction(SIGRCU
, NULL
, &act
);
480 assert(act
.sa_sigaction
== sigrcu_handler
);
481 assert(cds_list_empty(®istry
));
484 #endif /* #ifdef RCU_SIGNAL */
486 DEFINE_RCU_FLAVOR(rcu_flavor
);
488 #include "urcu-call-rcu-impl.h"
489 #include "urcu-defer-impl.h"