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/wfcqueue.h"
40 #include "urcu/map/urcu.h"
41 #include "urcu/static/urcu.h"
42 #include "urcu-pointer.h"
43 #include "urcu/tls-compat.h"
46 #include "urcu-wait.h"
48 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
54 * If a reader is really non-cooperative and refuses to commit its
55 * rcu_active_readers count to memory (there is no barrier in the reader
56 * per-se), kick it after a few loops waiting for it.
58 #define KICK_READER_LOOPS 10000
61 * Active attempts to check for reader Q.S. before calling futex().
63 #define RCU_QS_ACTIVE_ATTEMPTS 100
67 int rcu_has_sys_membarrier
;
69 void __attribute__((constructor
)) rcu_init(void);
81 void __attribute__((constructor
)) rcu_init(void);
82 void __attribute__((destructor
)) rcu_exit(void);
85 static pthread_mutex_t rcu_gp_lock
= PTHREAD_MUTEX_INITIALIZER
;
86 struct urcu_gp rcu_gp
= { .ctr
= RCU_GP_COUNT
};
89 * Written to only by each individual reader. Read by both the reader and the
92 DEFINE_URCU_TLS(struct rcu_reader
, rcu_reader
);
95 unsigned int rcu_yield_active
;
96 DEFINE_URCU_TLS(unsigned int, rcu_rand_yield
);
99 static CDS_LIST_HEAD(registry
);
102 * Queue keeping threads awaiting to wait for a grace period. Contains
103 * struct gp_waiters_thread objects.
105 static DEFINE_URCU_WAIT_QUEUE(gp_waiters
);
107 static void mutex_lock(pthread_mutex_t
*mutex
)
111 #ifndef DISTRUST_SIGNALS_EXTREME
112 ret
= pthread_mutex_lock(mutex
);
115 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
116 while ((ret
= pthread_mutex_trylock(mutex
)) != 0) {
117 if (ret
!= EBUSY
&& ret
!= EINTR
)
119 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader
).need_mb
)) {
121 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
126 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
129 static void mutex_unlock(pthread_mutex_t
*mutex
)
133 ret
= pthread_mutex_unlock(mutex
);
138 #ifdef RCU_MEMBARRIER
139 static void smp_mb_master(int group
)
141 if (caa_likely(rcu_has_sys_membarrier
))
142 membarrier(MEMBARRIER_EXPEDITED
);
149 static void smp_mb_master(int group
)
156 static void force_mb_all_readers(void)
158 struct rcu_reader
*index
;
161 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
162 * compiler barriers around rcu read lock as real memory barriers.
164 if (cds_list_empty(®istry
))
167 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
168 * a cache flush on architectures with non-coherent cache. Let's play
169 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
170 * cache flush is enforced.
172 cds_list_for_each_entry(index
, ®istry
, node
) {
173 CMM_STORE_SHARED(index
->need_mb
, 1);
174 pthread_kill(index
->tid
, SIGRCU
);
177 * Wait for sighandler (and thus mb()) to execute on every thread.
179 * Note that the pthread_kill() will never be executed on systems
180 * that correctly deliver signals in a timely manner. However, it
181 * is not uncommon for kernels to have bugs that can result in
182 * lost or unduly delayed signals.
184 * If you are seeing the below pthread_kill() executing much at
185 * all, we suggest testing the underlying kernel and filing the
186 * relevant bug report. For Linux kernels, we recommend getting
187 * the Linux Test Project (LTP).
189 cds_list_for_each_entry(index
, ®istry
, node
) {
190 while (CMM_LOAD_SHARED(index
->need_mb
)) {
191 pthread_kill(index
->tid
, SIGRCU
);
195 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
198 static void smp_mb_master(int group
)
200 force_mb_all_readers();
202 #endif /* #ifdef RCU_SIGNAL */
205 * synchronize_rcu() waiting. Single thread.
207 static void wait_gp(void)
209 /* Read reader_gp before read futex */
210 smp_mb_master(RCU_MB_GROUP
);
211 if (uatomic_read(&rcu_gp
.futex
) == -1)
212 futex_async(&rcu_gp
.futex
, FUTEX_WAIT
, -1,
216 static void wait_for_readers(struct cds_list_head
*input_readers
,
217 struct cds_list_head
*cur_snap_readers
,
218 struct cds_list_head
*qsreaders
)
221 struct rcu_reader
*index
, *tmp
;
224 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
225 * indicate quiescence (not nested), or observe the current
230 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
231 uatomic_dec(&rcu_gp
.futex
);
232 /* Write futex before read reader_gp */
233 smp_mb_master(RCU_MB_GROUP
);
236 cds_list_for_each_entry_safe(index
, tmp
, input_readers
, node
) {
237 switch (rcu_reader_state(&index
->ctr
)) {
238 case RCU_READER_ACTIVE_CURRENT
:
239 if (cur_snap_readers
) {
240 cds_list_move(&index
->node
,
245 case RCU_READER_INACTIVE
:
246 cds_list_move(&index
->node
, qsreaders
);
248 case RCU_READER_ACTIVE_OLD
:
250 * Old snapshot. Leaving node in
251 * input_readers will make us busy-loop
252 * until the snapshot becomes current or
253 * the reader becomes inactive.
259 #ifndef HAS_INCOHERENT_CACHES
260 if (cds_list_empty(input_readers
)) {
261 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
262 /* Read reader_gp before write futex */
263 smp_mb_master(RCU_MB_GROUP
);
264 uatomic_set(&rcu_gp
.futex
, 0);
268 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
)
273 #else /* #ifndef HAS_INCOHERENT_CACHES */
275 * BUSY-LOOP. Force the reader thread to commit its
276 * URCU_TLS(rcu_reader).ctr update to memory if we wait
279 if (cds_list_empty(input_readers
)) {
280 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
281 /* Read reader_gp before write futex */
282 smp_mb_master(RCU_MB_GROUP
);
283 uatomic_set(&rcu_gp
.futex
, 0);
287 switch (wait_loops
) {
288 case RCU_QS_ACTIVE_ATTEMPTS
:
290 break; /* only escape switch */
291 case KICK_READER_LOOPS
:
292 smp_mb_master(RCU_MB_GROUP
);
294 break; /* only escape switch */
299 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
303 void synchronize_rcu(void)
305 CDS_LIST_HEAD(cur_snap_readers
);
306 CDS_LIST_HEAD(qsreaders
);
307 DEFINE_URCU_WAIT_NODE(wait
, URCU_WAIT_WAITING
);
308 struct urcu_waiters waiters
;
311 * Add ourself to gp_waiters queue of threads awaiting to wait
312 * for a grace period. Proceed to perform the grace period only
313 * if we are the first thread added into the queue.
314 * The implicit memory barrier before urcu_wait_add()
315 * orders prior memory accesses of threads put into the wait
316 * queue before their insertion into the wait queue.
318 if (urcu_wait_add(&gp_waiters
, &wait
) != 0) {
319 /* Not first in queue: will be awakened by another thread. */
320 urcu_adaptative_busy_wait(&wait
);
321 /* Order following memory accesses after grace period. */
325 /* We won't need to wake ourself up */
326 urcu_wait_set_state(&wait
, URCU_WAIT_RUNNING
);
328 mutex_lock(&rcu_gp_lock
);
331 * Move all waiters into our local queue.
333 urcu_move_waiters(&waiters
, &gp_waiters
);
335 if (cds_list_empty(®istry
))
338 /* All threads should read qparity before accessing data structure
339 * where new ptr points to. Must be done within rcu_gp_lock because it
340 * iterates on reader threads.*/
341 /* Write new ptr before changing the qparity */
342 smp_mb_master(RCU_MB_GROUP
);
345 * Wait for readers to observe original parity or be quiescent.
347 wait_for_readers(®istry
, &cur_snap_readers
, &qsreaders
);
350 * Must finish waiting for quiescent state for original parity before
351 * committing next rcu_gp.ctr update to memory. Failure to do so could
352 * result in the writer waiting forever while new readers are always
353 * accessing data (no progress). Enforce compiler-order of load
354 * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
359 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
360 * model easier to understand. It does not have a big performance impact
361 * anyway, given this is the write-side.
365 /* Switch parity: 0 -> 1, 1 -> 0 */
366 CMM_STORE_SHARED(rcu_gp
.ctr
, rcu_gp
.ctr
^ RCU_GP_CTR_PHASE
);
369 * Must commit rcu_gp.ctr update to memory before waiting for quiescent
370 * state. Failure to do so could result in the writer waiting forever
371 * while new readers are always accessing data (no progress). Enforce
372 * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
378 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
379 * model easier to understand. It does not have a big performance impact
380 * anyway, given this is the write-side.
385 * Wait for readers to observe new parity or be quiescent.
387 wait_for_readers(&cur_snap_readers
, NULL
, &qsreaders
);
390 * Put quiescent reader list back into registry.
392 cds_list_splice(&qsreaders
, ®istry
);
394 /* Finish waiting for reader threads before letting the old ptr being
395 * freed. Must be done within rcu_gp_lock because it iterates on reader
397 smp_mb_master(RCU_MB_GROUP
);
399 mutex_unlock(&rcu_gp_lock
);
402 * Wakeup waiters only after we have completed the grace period
403 * and have ensured the memory barriers at the end of the grace
404 * period have been issued.
406 urcu_wake_all_waiters(&waiters
);
410 * library wrappers to be used by non-LGPL compatible source code.
413 void rcu_read_lock(void)
418 void rcu_read_unlock(void)
423 int rcu_read_ongoing(void)
425 return _rcu_read_ongoing();
428 void rcu_register_thread(void)
430 URCU_TLS(rcu_reader
).tid
= pthread_self();
431 assert(URCU_TLS(rcu_reader
).need_mb
== 0);
432 assert(!(URCU_TLS(rcu_reader
).ctr
& RCU_GP_CTR_NEST_MASK
));
434 mutex_lock(&rcu_gp_lock
);
435 rcu_init(); /* In case gcc does not support constructor attribute */
436 cds_list_add(&URCU_TLS(rcu_reader
).node
, ®istry
);
437 mutex_unlock(&rcu_gp_lock
);
440 void rcu_unregister_thread(void)
442 mutex_lock(&rcu_gp_lock
);
443 cds_list_del(&URCU_TLS(rcu_reader
).node
);
444 mutex_unlock(&rcu_gp_lock
);
447 #ifdef RCU_MEMBARRIER
453 if (!membarrier(MEMBARRIER_EXPEDITED
| MEMBARRIER_QUERY
))
454 rcu_has_sys_membarrier
= 1;
459 static void sigrcu_handler(int signo
, siginfo_t
*siginfo
, void *context
)
462 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
463 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
467 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
472 * rcu_init constructor. Called when the library is linked, but also when
473 * reader threads are calling rcu_register_thread().
474 * Should only be called by a single thread at a given time. This is ensured by
475 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
476 * load time, which should not be executed by multiple threads nor concurrently
477 * with rcu_register_thread() anyway.
481 struct sigaction act
;
488 act
.sa_sigaction
= sigrcu_handler
;
489 act
.sa_flags
= SA_SIGINFO
| SA_RESTART
;
490 sigemptyset(&act
.sa_mask
);
491 ret
= sigaction(SIGRCU
, &act
, NULL
);
498 struct sigaction act
;
501 ret
= sigaction(SIGRCU
, NULL
, &act
);
504 assert(act
.sa_sigaction
== sigrcu_handler
);
505 assert(cds_list_empty(®istry
));
508 #endif /* #ifdef RCU_SIGNAL */
510 DEFINE_RCU_FLAVOR(rcu_flavor
);
512 #include "urcu-call-rcu-impl.h"
513 #include "urcu-defer-impl.h"