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 10 loops waiting for it.
58 #define KICK_READER_LOOPS 10
61 * Active attempts to check for reader Q.S. before calling futex().
63 #define RCU_QS_ACTIVE_ATTEMPTS 100
66 * RCU_MEMBARRIER is only possibly available on Linux.
68 #if defined(RCU_MEMBARRIER) && defined(__linux__)
72 /* If the headers do not support SYS_membarrier, fall back on RCU_MB */
74 # define membarrier(...) syscall(SYS_membarrier, __VA_ARGS__)
76 # define membarrier(...) -ENOSYS
79 #define MEMBARRIER_EXPEDITED (1 << 0)
80 #define MEMBARRIER_DELAYED (1 << 1)
81 #define MEMBARRIER_QUERY (1 << 16)
85 int rcu_has_sys_membarrier
;
87 void __attribute__((constructor
)) rcu_init(void);
99 void __attribute__((constructor
)) rcu_init(void);
100 void __attribute__((destructor
)) rcu_exit(void);
103 static pthread_mutex_t rcu_gp_lock
= PTHREAD_MUTEX_INITIALIZER
;
104 struct rcu_gp rcu_gp
= { .ctr
= RCU_GP_COUNT
};
107 * Written to only by each individual reader. Read by both the reader and the
110 __DEFINE_URCU_TLS_GLOBAL(struct rcu_reader
, rcu_reader
);
112 static CDS_LIST_HEAD(registry
);
115 * Queue keeping threads awaiting to wait for a grace period. Contains
116 * struct gp_waiters_thread objects.
118 static DEFINE_URCU_WAIT_QUEUE(gp_waiters
);
120 static void mutex_lock(pthread_mutex_t
*mutex
)
124 #ifndef DISTRUST_SIGNALS_EXTREME
125 ret
= pthread_mutex_lock(mutex
);
128 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
129 while ((ret
= pthread_mutex_trylock(mutex
)) != 0) {
130 if (ret
!= EBUSY
&& ret
!= EINTR
)
132 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader
).need_mb
)) {
134 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
139 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
142 static void mutex_unlock(pthread_mutex_t
*mutex
)
146 ret
= pthread_mutex_unlock(mutex
);
151 #ifdef RCU_MEMBARRIER
152 static void smp_mb_master(int group
)
154 if (caa_likely(rcu_has_sys_membarrier
))
155 (void) membarrier(MEMBARRIER_EXPEDITED
);
162 static void smp_mb_master(int group
)
169 static void force_mb_all_readers(void)
171 struct rcu_reader
*index
;
174 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
175 * compiler barriers around rcu read lock as real memory barriers.
177 if (cds_list_empty(®istry
))
180 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
181 * a cache flush on architectures with non-coherent cache. Let's play
182 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
183 * cache flush is enforced.
185 cds_list_for_each_entry(index
, ®istry
, node
) {
186 CMM_STORE_SHARED(index
->need_mb
, 1);
187 pthread_kill(index
->tid
, SIGRCU
);
190 * Wait for sighandler (and thus mb()) to execute on every thread.
192 * Note that the pthread_kill() will never be executed on systems
193 * that correctly deliver signals in a timely manner. However, it
194 * is not uncommon for kernels to have bugs that can result in
195 * lost or unduly delayed signals.
197 * If you are seeing the below pthread_kill() executing much at
198 * all, we suggest testing the underlying kernel and filing the
199 * relevant bug report. For Linux kernels, we recommend getting
200 * the Linux Test Project (LTP).
202 cds_list_for_each_entry(index
, ®istry
, node
) {
203 while (CMM_LOAD_SHARED(index
->need_mb
)) {
204 pthread_kill(index
->tid
, SIGRCU
);
208 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
211 static void smp_mb_master(int group
)
213 force_mb_all_readers();
215 #endif /* #ifdef RCU_SIGNAL */
218 * synchronize_rcu() waiting. Single thread.
220 static void wait_gp(void)
222 /* Read reader_gp before read futex */
223 smp_mb_master(RCU_MB_GROUP
);
224 if (uatomic_read(&rcu_gp
.futex
) == -1)
225 futex_async(&rcu_gp
.futex
, FUTEX_WAIT
, -1,
229 static void wait_for_readers(struct cds_list_head
*input_readers
,
230 struct cds_list_head
*cur_snap_readers
,
231 struct cds_list_head
*qsreaders
)
233 unsigned int wait_loops
= 0;
234 struct rcu_reader
*index
, *tmp
;
235 #ifdef HAS_INCOHERENT_CACHES
236 unsigned int wait_gp_loops
= 0;
237 #endif /* HAS_INCOHERENT_CACHES */
240 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
241 * indicate quiescence (not nested), or observe the current
245 if (wait_loops
< RCU_QS_ACTIVE_ATTEMPTS
)
247 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
248 uatomic_dec(&rcu_gp
.futex
);
249 /* Write futex before read reader_gp */
250 smp_mb_master(RCU_MB_GROUP
);
253 cds_list_for_each_entry_safe(index
, tmp
, input_readers
, node
) {
254 switch (rcu_reader_state(&index
->ctr
)) {
255 case RCU_READER_ACTIVE_CURRENT
:
256 if (cur_snap_readers
) {
257 cds_list_move(&index
->node
,
262 case RCU_READER_INACTIVE
:
263 cds_list_move(&index
->node
, qsreaders
);
265 case RCU_READER_ACTIVE_OLD
:
267 * Old snapshot. Leaving node in
268 * input_readers will make us busy-loop
269 * until the snapshot becomes current or
270 * the reader becomes inactive.
276 #ifndef HAS_INCOHERENT_CACHES
277 if (cds_list_empty(input_readers
)) {
278 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
279 /* Read reader_gp before write futex */
280 smp_mb_master(RCU_MB_GROUP
);
281 uatomic_set(&rcu_gp
.futex
, 0);
285 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
)
290 #else /* #ifndef HAS_INCOHERENT_CACHES */
292 * BUSY-LOOP. Force the reader thread to commit its
293 * URCU_TLS(rcu_reader).ctr update to memory if we wait
296 if (cds_list_empty(input_readers
)) {
297 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
298 /* Read reader_gp before write futex */
299 smp_mb_master(RCU_MB_GROUP
);
300 uatomic_set(&rcu_gp
.futex
, 0);
304 if (wait_gp_loops
== KICK_READER_LOOPS
) {
305 smp_mb_master(RCU_MB_GROUP
);
308 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
315 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
319 void synchronize_rcu(void)
321 CDS_LIST_HEAD(cur_snap_readers
);
322 CDS_LIST_HEAD(qsreaders
);
323 DEFINE_URCU_WAIT_NODE(wait
, URCU_WAIT_WAITING
);
324 struct urcu_waiters waiters
;
327 * Add ourself to gp_waiters queue of threads awaiting to wait
328 * for a grace period. Proceed to perform the grace period only
329 * if we are the first thread added into the queue.
330 * The implicit memory barrier before urcu_wait_add()
331 * orders prior memory accesses of threads put into the wait
332 * queue before their insertion into the wait queue.
334 if (urcu_wait_add(&gp_waiters
, &wait
) != 0) {
335 /* Not first in queue: will be awakened by another thread. */
336 urcu_adaptative_busy_wait(&wait
);
337 /* Order following memory accesses after grace period. */
341 /* We won't need to wake ourself up */
342 urcu_wait_set_state(&wait
, URCU_WAIT_RUNNING
);
344 mutex_lock(&rcu_gp_lock
);
347 * Move all waiters into our local queue.
349 urcu_move_waiters(&waiters
, &gp_waiters
);
351 if (cds_list_empty(®istry
))
354 /* All threads should read qparity before accessing data structure
355 * where new ptr points to. Must be done within rcu_gp_lock because it
356 * iterates on reader threads.*/
357 /* Write new ptr before changing the qparity */
358 smp_mb_master(RCU_MB_GROUP
);
361 * Wait for readers to observe original parity or be quiescent.
363 wait_for_readers(®istry
, &cur_snap_readers
, &qsreaders
);
366 * Must finish waiting for quiescent state for original parity before
367 * committing next rcu_gp.ctr update to memory. Failure to do so could
368 * result in the writer waiting forever while new readers are always
369 * accessing data (no progress). Enforce compiler-order of load
370 * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
375 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
376 * model easier to understand. It does not have a big performance impact
377 * anyway, given this is the write-side.
381 /* Switch parity: 0 -> 1, 1 -> 0 */
382 CMM_STORE_SHARED(rcu_gp
.ctr
, rcu_gp
.ctr
^ RCU_GP_CTR_PHASE
);
385 * Must commit rcu_gp.ctr update to memory before waiting for quiescent
386 * state. Failure to do so could result in the writer waiting forever
387 * while new readers are always accessing data (no progress). Enforce
388 * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
394 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
395 * model easier to understand. It does not have a big performance impact
396 * anyway, given this is the write-side.
401 * Wait for readers to observe new parity or be quiescent.
403 wait_for_readers(&cur_snap_readers
, NULL
, &qsreaders
);
406 * Put quiescent reader list back into registry.
408 cds_list_splice(&qsreaders
, ®istry
);
410 /* Finish waiting for reader threads before letting the old ptr being
411 * freed. Must be done within rcu_gp_lock because it iterates on reader
413 smp_mb_master(RCU_MB_GROUP
);
415 mutex_unlock(&rcu_gp_lock
);
418 * Wakeup waiters only after we have completed the grace period
419 * and have ensured the memory barriers at the end of the grace
420 * period have been issued.
422 urcu_wake_all_waiters(&waiters
);
426 * library wrappers to be used by non-LGPL compatible source code.
429 void rcu_read_lock(void)
434 void rcu_read_unlock(void)
439 int rcu_read_ongoing(void)
441 return _rcu_read_ongoing();
444 void rcu_register_thread(void)
446 URCU_TLS(rcu_reader
).tid
= pthread_self();
447 assert(URCU_TLS(rcu_reader
).need_mb
== 0);
448 assert(!(URCU_TLS(rcu_reader
).ctr
& RCU_GP_CTR_NEST_MASK
));
450 mutex_lock(&rcu_gp_lock
);
451 rcu_init(); /* In case gcc does not support constructor attribute */
452 cds_list_add(&URCU_TLS(rcu_reader
).node
, ®istry
);
453 mutex_unlock(&rcu_gp_lock
);
456 void rcu_unregister_thread(void)
458 mutex_lock(&rcu_gp_lock
);
459 cds_list_del(&URCU_TLS(rcu_reader
).node
);
460 mutex_unlock(&rcu_gp_lock
);
463 #ifdef RCU_MEMBARRIER
469 if (!membarrier(MEMBARRIER_EXPEDITED
| MEMBARRIER_QUERY
))
470 rcu_has_sys_membarrier
= 1;
475 static void sigrcu_handler(int signo
, siginfo_t
*siginfo
, void *context
)
478 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
479 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
483 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
488 * rcu_init constructor. Called when the library is linked, but also when
489 * reader threads are calling rcu_register_thread().
490 * Should only be called by a single thread at a given time. This is ensured by
491 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
492 * load time, which should not be executed by multiple threads nor concurrently
493 * with rcu_register_thread() anyway.
497 struct sigaction act
;
504 act
.sa_sigaction
= sigrcu_handler
;
505 act
.sa_flags
= SA_SIGINFO
| SA_RESTART
;
506 sigemptyset(&act
.sa_mask
);
507 ret
= sigaction(SIGRCU
, &act
, NULL
);
515 * Don't unregister the SIGRCU signal handler anymore, because
516 * call_rcu threads could still be using it shortly before the
518 * Assertion disabled because call_rcu threads are now rcu
519 * readers, and left running at exit.
520 * assert(cds_list_empty(®istry));
524 #endif /* #ifdef RCU_SIGNAL */
526 DEFINE_RCU_FLAVOR(rcu_flavor
);
528 #include "urcu-call-rcu-impl.h"
529 #include "urcu-defer-impl.h"