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.
28 #define _DEFAULT_SOURCE
39 #include "urcu/arch.h"
40 #include "urcu/wfcqueue.h"
41 #include "urcu/map/urcu.h"
42 #include "urcu/static/urcu.h"
43 #include "urcu-pointer.h"
44 #include "urcu/tls-compat.h"
47 #include "urcu-wait.h"
49 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
55 * If a reader is really non-cooperative and refuses to commit its
56 * rcu_active_readers count to memory (there is no barrier in the reader
57 * per-se), kick it after 10 loops waiting for it.
59 #define KICK_READER_LOOPS 10
62 * Active attempts to check for reader Q.S. before calling futex().
64 #define RCU_QS_ACTIVE_ATTEMPTS 100
66 /* If the headers do not support membarrier system call, fall back on RCU_MB */
67 #ifdef __NR_membarrier
68 # define membarrier(...) syscall(__NR_membarrier, __VA_ARGS__)
70 # define membarrier(...) -ENOSYS
74 MEMBARRIER_CMD_QUERY
= 0,
75 MEMBARRIER_CMD_SHARED
= (1 << 0),
80 int rcu_has_sys_membarrier
;
82 void __attribute__((constructor
)) rcu_init(void);
94 void __attribute__((constructor
)) rcu_init(void);
95 void __attribute__((destructor
)) rcu_exit(void);
99 * rcu_gp_lock ensures mutual exclusion between threads calling
102 static pthread_mutex_t rcu_gp_lock
= PTHREAD_MUTEX_INITIALIZER
;
104 * rcu_registry_lock ensures mutual exclusion between threads
105 * registering and unregistering themselves to/from the registry, and
106 * with threads reading that registry from synchronize_rcu(). However,
107 * this lock is not held all the way through the completion of awaiting
108 * for the grace period. It is sporadically released between iterations
110 * rcu_registry_lock may nest inside rcu_gp_lock.
112 static pthread_mutex_t rcu_registry_lock
= PTHREAD_MUTEX_INITIALIZER
;
113 struct rcu_gp rcu_gp
= { .ctr
= RCU_GP_COUNT
};
116 * Written to only by each individual reader. Read by both the reader and the
119 DEFINE_URCU_TLS(struct rcu_reader
, rcu_reader
);
121 static CDS_LIST_HEAD(registry
);
124 * Queue keeping threads awaiting to wait for a grace period. Contains
125 * struct gp_waiters_thread objects.
127 static DEFINE_URCU_WAIT_QUEUE(gp_waiters
);
129 static void mutex_lock(pthread_mutex_t
*mutex
)
133 #ifndef DISTRUST_SIGNALS_EXTREME
134 ret
= pthread_mutex_lock(mutex
);
137 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
138 while ((ret
= pthread_mutex_trylock(mutex
)) != 0) {
139 if (ret
!= EBUSY
&& ret
!= EINTR
)
141 if (CMM_LOAD_SHARED(URCU_TLS(rcu_reader
).need_mb
)) {
143 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
146 (void) poll(NULL
, 0, 10);
148 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
151 static void mutex_unlock(pthread_mutex_t
*mutex
)
155 ret
= pthread_mutex_unlock(mutex
);
160 #ifdef RCU_MEMBARRIER
161 static void smp_mb_master(void)
163 if (caa_likely(rcu_has_sys_membarrier
))
164 (void) membarrier(MEMBARRIER_CMD_SHARED
, 0);
171 static void smp_mb_master(void)
178 static void force_mb_all_readers(void)
180 struct rcu_reader
*index
;
183 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
184 * compiler barriers around rcu read lock as real memory barriers.
186 if (cds_list_empty(®istry
))
189 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
190 * a cache flush on architectures with non-coherent cache. Let's play
191 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
192 * cache flush is enforced.
194 cds_list_for_each_entry(index
, ®istry
, node
) {
195 CMM_STORE_SHARED(index
->need_mb
, 1);
196 pthread_kill(index
->tid
, SIGRCU
);
199 * Wait for sighandler (and thus mb()) to execute on every thread.
201 * Note that the pthread_kill() will never be executed on systems
202 * that correctly deliver signals in a timely manner. However, it
203 * is not uncommon for kernels to have bugs that can result in
204 * lost or unduly delayed signals.
206 * If you are seeing the below pthread_kill() executing much at
207 * all, we suggest testing the underlying kernel and filing the
208 * relevant bug report. For Linux kernels, we recommend getting
209 * the Linux Test Project (LTP).
211 cds_list_for_each_entry(index
, ®istry
, node
) {
212 while (CMM_LOAD_SHARED(index
->need_mb
)) {
213 pthread_kill(index
->tid
, SIGRCU
);
214 (void) poll(NULL
, 0, 1);
217 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
220 static void smp_mb_master(void)
222 force_mb_all_readers();
224 #endif /* #ifdef RCU_SIGNAL */
227 * synchronize_rcu() waiting. Single thread.
228 * Always called with rcu_registry lock held. Releases this lock and
229 * grabs it again. Holds the lock when it returns.
231 static void wait_gp(void)
234 * Read reader_gp before read futex. smp_mb_master() needs to
235 * be called with the rcu registry lock held in RCU_SIGNAL
239 /* Temporarily unlock the registry lock. */
240 mutex_unlock(&rcu_registry_lock
);
241 if (uatomic_read(&rcu_gp
.futex
) != -1)
243 while (futex_async(&rcu_gp
.futex
, FUTEX_WAIT
, -1,
247 /* Value already changed. */
250 /* Retry if interrupted by signal. */
251 break; /* Get out of switch. */
253 /* Unexpected error. */
259 * Re-lock the registry lock before the next loop.
261 mutex_lock(&rcu_registry_lock
);
265 * Always called with rcu_registry lock held. Releases this lock between
266 * iterations and grabs it again. Holds the lock when it returns.
268 static void wait_for_readers(struct cds_list_head
*input_readers
,
269 struct cds_list_head
*cur_snap_readers
,
270 struct cds_list_head
*qsreaders
)
272 unsigned int wait_loops
= 0;
273 struct rcu_reader
*index
, *tmp
;
274 #ifdef HAS_INCOHERENT_CACHES
275 unsigned int wait_gp_loops
= 0;
276 #endif /* HAS_INCOHERENT_CACHES */
279 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
280 * indicate quiescence (not nested), or observe the current
284 if (wait_loops
< RCU_QS_ACTIVE_ATTEMPTS
)
286 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
287 uatomic_dec(&rcu_gp
.futex
);
288 /* Write futex before read reader_gp */
292 cds_list_for_each_entry_safe(index
, tmp
, input_readers
, node
) {
293 switch (rcu_reader_state(&index
->ctr
)) {
294 case RCU_READER_ACTIVE_CURRENT
:
295 if (cur_snap_readers
) {
296 cds_list_move(&index
->node
,
301 case RCU_READER_INACTIVE
:
302 cds_list_move(&index
->node
, qsreaders
);
304 case RCU_READER_ACTIVE_OLD
:
306 * Old snapshot. Leaving node in
307 * input_readers will make us busy-loop
308 * until the snapshot becomes current or
309 * the reader becomes inactive.
315 #ifndef HAS_INCOHERENT_CACHES
316 if (cds_list_empty(input_readers
)) {
317 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
318 /* Read reader_gp before write futex */
320 uatomic_set(&rcu_gp
.futex
, 0);
324 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
325 /* wait_gp unlocks/locks registry lock. */
328 /* Temporarily unlock the registry lock. */
329 mutex_unlock(&rcu_registry_lock
);
332 * Re-lock the registry lock before the
335 mutex_lock(&rcu_registry_lock
);
338 #else /* #ifndef HAS_INCOHERENT_CACHES */
340 * BUSY-LOOP. Force the reader thread to commit its
341 * URCU_TLS(rcu_reader).ctr update to memory if we wait
344 if (cds_list_empty(input_readers
)) {
345 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
346 /* Read reader_gp before write futex */
348 uatomic_set(&rcu_gp
.futex
, 0);
352 if (wait_gp_loops
== KICK_READER_LOOPS
) {
356 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
357 /* wait_gp unlocks/locks registry lock. */
361 /* Temporarily unlock the registry lock. */
362 mutex_unlock(&rcu_registry_lock
);
365 * Re-lock the registry lock before the
368 mutex_lock(&rcu_registry_lock
);
371 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
375 void synchronize_rcu(void)
377 CDS_LIST_HEAD(cur_snap_readers
);
378 CDS_LIST_HEAD(qsreaders
);
379 DEFINE_URCU_WAIT_NODE(wait
, URCU_WAIT_WAITING
);
380 struct urcu_waiters waiters
;
383 * Add ourself to gp_waiters queue of threads awaiting to wait
384 * for a grace period. Proceed to perform the grace period only
385 * if we are the first thread added into the queue.
386 * The implicit memory barrier before urcu_wait_add()
387 * orders prior memory accesses of threads put into the wait
388 * queue before their insertion into the wait queue.
390 if (urcu_wait_add(&gp_waiters
, &wait
) != 0) {
391 /* Not first in queue: will be awakened by another thread. */
392 urcu_adaptative_busy_wait(&wait
);
393 /* Order following memory accesses after grace period. */
397 /* We won't need to wake ourself up */
398 urcu_wait_set_state(&wait
, URCU_WAIT_RUNNING
);
400 mutex_lock(&rcu_gp_lock
);
403 * Move all waiters into our local queue.
405 urcu_move_waiters(&waiters
, &gp_waiters
);
407 mutex_lock(&rcu_registry_lock
);
409 if (cds_list_empty(®istry
))
413 * All threads should read qparity before accessing data structure
414 * where new ptr points to. Must be done within rcu_registry_lock
415 * because it iterates on reader threads.
417 /* Write new ptr before changing the qparity */
421 * Wait for readers to observe original parity or be quiescent.
422 * wait_for_readers() can release and grab again rcu_registry_lock
425 wait_for_readers(®istry
, &cur_snap_readers
, &qsreaders
);
428 * Must finish waiting for quiescent state for original parity before
429 * committing next rcu_gp.ctr update to memory. Failure to do so could
430 * result in the writer waiting forever while new readers are always
431 * accessing data (no progress). Enforce compiler-order of load
432 * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr.
437 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
438 * model easier to understand. It does not have a big performance impact
439 * anyway, given this is the write-side.
443 /* Switch parity: 0 -> 1, 1 -> 0 */
444 CMM_STORE_SHARED(rcu_gp
.ctr
, rcu_gp
.ctr
^ RCU_GP_CTR_PHASE
);
447 * Must commit rcu_gp.ctr update to memory before waiting for quiescent
448 * state. Failure to do so could result in the writer waiting forever
449 * while new readers are always accessing data (no progress). Enforce
450 * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr.
456 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
457 * model easier to understand. It does not have a big performance impact
458 * anyway, given this is the write-side.
463 * Wait for readers to observe new parity or be quiescent.
464 * wait_for_readers() can release and grab again rcu_registry_lock
467 wait_for_readers(&cur_snap_readers
, NULL
, &qsreaders
);
470 * Put quiescent reader list back into registry.
472 cds_list_splice(&qsreaders
, ®istry
);
475 * Finish waiting for reader threads before letting the old ptr
476 * being freed. Must be done within rcu_registry_lock because it
477 * iterates on reader threads.
481 mutex_unlock(&rcu_registry_lock
);
482 mutex_unlock(&rcu_gp_lock
);
485 * Wakeup waiters only after we have completed the grace period
486 * and have ensured the memory barriers at the end of the grace
487 * period have been issued.
489 urcu_wake_all_waiters(&waiters
);
493 * library wrappers to be used by non-LGPL compatible source code.
496 void rcu_read_lock(void)
501 void rcu_read_unlock(void)
506 int rcu_read_ongoing(void)
508 return _rcu_read_ongoing();
511 void rcu_register_thread(void)
513 URCU_TLS(rcu_reader
).tid
= pthread_self();
514 assert(URCU_TLS(rcu_reader
).need_mb
== 0);
515 assert(!(URCU_TLS(rcu_reader
).ctr
& RCU_GP_CTR_NEST_MASK
));
517 mutex_lock(&rcu_registry_lock
);
518 assert(!URCU_TLS(rcu_reader
).registered
);
519 URCU_TLS(rcu_reader
).registered
= 1;
520 rcu_init(); /* In case gcc does not support constructor attribute */
521 cds_list_add(&URCU_TLS(rcu_reader
).node
, ®istry
);
522 mutex_unlock(&rcu_registry_lock
);
525 void rcu_unregister_thread(void)
527 mutex_lock(&rcu_registry_lock
);
528 assert(URCU_TLS(rcu_reader
).registered
);
529 URCU_TLS(rcu_reader
).registered
= 0;
530 cds_list_del(&URCU_TLS(rcu_reader
).node
);
531 mutex_unlock(&rcu_registry_lock
);
534 #ifdef RCU_MEMBARRIER
542 ret
= membarrier(MEMBARRIER_CMD_QUERY
, 0);
543 if (ret
>= 0 && (ret
& MEMBARRIER_CMD_SHARED
)) {
544 rcu_has_sys_membarrier
= 1;
550 static void sigrcu_handler(int signo
, siginfo_t
*siginfo
, void *context
)
553 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
554 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
558 _CMM_STORE_SHARED(URCU_TLS(rcu_reader
).need_mb
, 0);
563 * rcu_init constructor. Called when the library is linked, but also when
564 * reader threads are calling rcu_register_thread().
565 * Should only be called by a single thread at a given time. This is ensured by
566 * holing the rcu_registry_lock from rcu_register_thread() or by running
567 * at library load time, which should not be executed by multiple
568 * threads nor concurrently with rcu_register_thread() anyway.
572 struct sigaction act
;
579 act
.sa_sigaction
= sigrcu_handler
;
580 act
.sa_flags
= SA_SIGINFO
| SA_RESTART
;
581 sigemptyset(&act
.sa_mask
);
582 ret
= sigaction(SIGRCU
, &act
, NULL
);
590 * Don't unregister the SIGRCU signal handler anymore, because
591 * call_rcu threads could still be using it shortly before the
593 * Assertion disabled because call_rcu threads are now rcu
594 * readers, and left running at exit.
595 * assert(cds_list_empty(®istry));
599 #endif /* #ifdef RCU_SIGNAL */
601 DEFINE_RCU_FLAVOR(rcu_flavor
);
603 #include "urcu-call-rcu-impl.h"
604 #include "urcu-defer-impl.h"