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.
38 #include "urcu/map/urcu.h"
40 #include "urcu/static/urcu.h"
41 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
45 * If a reader is really non-cooperative and refuses to commit its
46 * rcu_active_readers count to memory (there is no barrier in the reader
47 * per-se), kick it after a few loops waiting for it.
49 #define KICK_READER_LOOPS 10000
52 * Active attempts to check for reader Q.S. before calling futex().
54 #define RCU_QS_ACTIVE_ATTEMPTS 100
58 int has_sys_membarrier
;
60 void __attribute__((constructor
)) rcu_init(void);
72 void __attribute__((constructor
)) rcu_init(void);
73 void __attribute__((destructor
)) rcu_exit(void);
76 static pthread_mutex_t rcu_gp_lock
= PTHREAD_MUTEX_INITIALIZER
;
81 * Global grace period counter.
82 * Contains the current RCU_GP_CTR_PHASE.
83 * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
84 * Written to only by writer with mutex taken. Read by both writer and readers.
86 unsigned long rcu_gp_ctr
= RCU_GP_COUNT
;
89 * Written to only by each individual reader. Read by both the reader and the
92 struct rcu_reader __thread rcu_reader
;
95 unsigned int yield_active
;
96 unsigned int __thread rand_yield
;
99 static CDS_LIST_HEAD(registry
);
101 static void mutex_lock(pthread_mutex_t
*mutex
)
105 #ifndef DISTRUST_SIGNALS_EXTREME
106 ret
= pthread_mutex_lock(mutex
);
108 perror("Error in pthread mutex lock");
111 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
112 while ((ret
= pthread_mutex_trylock(mutex
)) != 0) {
113 if (ret
!= EBUSY
&& ret
!= EINTR
) {
114 printf("ret = %d, errno = %d\n", ret
, errno
);
115 perror("Error in pthread mutex lock");
118 if (CMM_LOAD_SHARED(rcu_reader
.need_mb
)) {
120 _CMM_STORE_SHARED(rcu_reader
.need_mb
, 0);
125 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
128 static void mutex_unlock(pthread_mutex_t
*mutex
)
132 ret
= pthread_mutex_unlock(mutex
);
134 perror("Error in pthread mutex unlock");
139 #ifdef RCU_MEMBARRIER
140 static void smp_mb_master(int group
)
142 if (likely(has_sys_membarrier
))
143 membarrier(MEMBARRIER_EXPEDITED
);
150 static void smp_mb_master(int group
)
157 static void force_mb_all_readers(void)
159 struct rcu_reader
*index
;
162 * Ask for each threads to execute a cmm_smp_mb() so we can consider the
163 * compiler barriers around rcu read lock as real memory barriers.
165 if (cds_list_empty(®istry
))
168 * pthread_kill has a cmm_smp_mb(). But beware, we assume it performs
169 * a cache flush on architectures with non-coherent cache. Let's play
170 * safe and don't assume anything : we use cmm_smp_mc() to make sure the
171 * cache flush is enforced.
173 cds_list_for_each_entry(index
, ®istry
, node
) {
174 CMM_STORE_SHARED(index
->need_mb
, 1);
175 pthread_kill(index
->tid
, SIGRCU
);
178 * Wait for sighandler (and thus mb()) to execute on every thread.
180 * Note that the pthread_kill() will never be executed on systems
181 * that correctly deliver signals in a timely manner. However, it
182 * is not uncommon for kernels to have bugs that can result in
183 * lost or unduly delayed signals.
185 * If you are seeing the below pthread_kill() executing much at
186 * all, we suggest testing the underlying kernel and filing the
187 * relevant bug report. For Linux kernels, we recommend getting
188 * the Linux Test Project (LTP).
190 cds_list_for_each_entry(index
, ®istry
, node
) {
191 while (CMM_LOAD_SHARED(index
->need_mb
)) {
192 pthread_kill(index
->tid
, SIGRCU
);
196 cmm_smp_mb(); /* read ->need_mb before ending the barrier */
199 static void smp_mb_master(int group
)
201 force_mb_all_readers();
203 #endif /* #ifdef RCU_SIGNAL */
206 * synchronize_rcu() waiting. Single thread.
208 static void wait_gp(void)
210 /* Read reader_gp before read futex */
211 smp_mb_master(RCU_MB_GROUP
);
212 if (uatomic_read(&gp_futex
) == -1)
213 futex_async(&gp_futex
, FUTEX_WAIT
, -1,
217 void update_counter_and_wait(void)
219 CDS_LIST_HEAD(qsreaders
);
221 struct rcu_reader
*index
, *tmp
;
223 /* Switch parity: 0 -> 1, 1 -> 0 */
224 CMM_STORE_SHARED(rcu_gp_ctr
, rcu_gp_ctr
^ RCU_GP_CTR_PHASE
);
227 * Must commit rcu_gp_ctr update to memory before waiting for quiescent
228 * state. Failure to do so could result in the writer waiting forever
229 * while new readers are always accessing data (no progress). Enforce
230 * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
236 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
237 * model easier to understand. It does not have a big performance impact
238 * anyway, given this is the write-side.
243 * Wait for each thread rcu_reader.ctr count to become 0.
247 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
248 uatomic_dec(&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
, ®istry
, node
) {
254 if (!rcu_gp_ongoing(&index
->ctr
))
255 cds_list_move(&index
->node
, &qsreaders
);
258 #ifndef HAS_INCOHERENT_CACHES
259 if (cds_list_empty(®istry
)) {
260 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
) {
261 /* Read reader_gp before write futex */
262 smp_mb_master(RCU_MB_GROUP
);
263 uatomic_set(&gp_futex
, 0);
267 if (wait_loops
== RCU_QS_ACTIVE_ATTEMPTS
)
272 #else /* #ifndef HAS_INCOHERENT_CACHES */
274 * BUSY-LOOP. Force the reader thread to commit its
275 * rcu_reader.ctr update to memory if we wait for too long.
277 if (cds_list_empty(®istry
)) {
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(&gp_futex
, 0);
285 switch (wait_loops
) {
286 case RCU_QS_ACTIVE_ATTEMPTS
:
288 break; /* only escape switch */
289 case KICK_READER_LOOPS
:
290 smp_mb_master(RCU_MB_GROUP
);
292 break; /* only escape switch */
297 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
299 /* put back the reader list in the registry */
300 cds_list_splice(&qsreaders
, ®istry
);
303 void synchronize_rcu(void)
305 mutex_lock(&rcu_gp_lock
);
307 if (cds_list_empty(®istry
))
310 /* All threads should read qparity before accessing data structure
311 * where new ptr points to. Must be done within rcu_gp_lock because it
312 * iterates on reader threads.*/
313 /* Write new ptr before changing the qparity */
314 smp_mb_master(RCU_MB_GROUP
);
317 * Wait for previous parity to be empty of readers.
319 update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */
322 * Must finish waiting for quiescent state for parity 0 before
323 * committing next rcu_gp_ctr update to memory. Failure to do so could
324 * result in the writer waiting forever while new readers are always
325 * accessing data (no progress). Enforce compiler-order of load
326 * rcu_reader ctr before store to rcu_gp_ctr.
331 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
332 * model easier to understand. It does not have a big performance impact
333 * anyway, given this is the write-side.
338 * Wait for previous parity to be empty of readers.
340 update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */
342 /* Finish waiting for reader threads before letting the old ptr being
343 * freed. Must be done within rcu_gp_lock because it iterates on reader
345 smp_mb_master(RCU_MB_GROUP
);
347 mutex_unlock(&rcu_gp_lock
);
351 * library wrappers to be used by non-LGPL compatible source code.
354 void rcu_read_lock(void)
359 void rcu_read_unlock(void)
364 void rcu_register_thread(void)
366 rcu_reader
.tid
= pthread_self();
367 assert(rcu_reader
.need_mb
== 0);
368 assert(!(rcu_reader
.ctr
& RCU_GP_CTR_NEST_MASK
));
370 mutex_lock(&rcu_gp_lock
);
371 rcu_init(); /* In case gcc does not support constructor attribute */
372 cds_list_add(&rcu_reader
.node
, ®istry
);
373 mutex_unlock(&rcu_gp_lock
);
376 void rcu_unregister_thread(void)
378 mutex_lock(&rcu_gp_lock
);
379 cds_list_del(&rcu_reader
.node
);
380 mutex_unlock(&rcu_gp_lock
);
383 #ifdef RCU_MEMBARRIER
389 if (!membarrier(MEMBARRIER_EXPEDITED
| MEMBARRIER_QUERY
))
390 has_sys_membarrier
= 1;
395 static void sigrcu_handler(int signo
, siginfo_t
*siginfo
, void *context
)
398 * Executing this cmm_smp_mb() is the only purpose of this signal handler.
399 * It punctually promotes cmm_barrier() into cmm_smp_mb() on every thread it is
403 _CMM_STORE_SHARED(rcu_reader
.need_mb
, 0);
408 * rcu_init constructor. Called when the library is linked, but also when
409 * reader threads are calling rcu_register_thread().
410 * Should only be called by a single thread at a given time. This is ensured by
411 * holing the rcu_gp_lock from rcu_register_thread() or by running at library
412 * load time, which should not be executed by multiple threads nor concurrently
413 * with rcu_register_thread() anyway.
417 struct sigaction act
;
424 act
.sa_sigaction
= sigrcu_handler
;
425 act
.sa_flags
= SA_SIGINFO
| SA_RESTART
;
426 sigemptyset(&act
.sa_mask
);
427 ret
= sigaction(SIGRCU
, &act
, NULL
);
429 perror("Error in sigaction");
436 struct sigaction act
;
439 ret
= sigaction(SIGRCU
, NULL
, &act
);
441 perror("Error in sigaction");
444 assert(act
.sa_sigaction
== sigrcu_handler
);
445 assert(cds_list_empty(®istry
));
448 #endif /* #ifdef RCU_SIGNAL */
450 #include "urcu-call-rcu-impl.h"
451 #include "urcu-defer-impl.h"