4 * Userspace RCU library
6 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
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.
35 #include "urcu-static.h"
36 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
42 void __attribute__((constructor
)) urcu_init(void);
43 void __attribute__((destructor
)) urcu_exit(void);
50 static pthread_mutex_t urcu_mutex
= PTHREAD_MUTEX_INITIALIZER
;
55 * Global grace period counter.
56 * Contains the current RCU_GP_CTR_BIT.
57 * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
58 * Written to only by writer with mutex taken. Read by both writer and readers.
60 long urcu_gp_ctr
= RCU_GP_COUNT
;
63 * Written to only by each individual reader. Read by both the reader and the
66 struct urcu_reader __thread urcu_reader
;
68 /* Thread IDs of registered readers */
69 #define INIT_NUM_THREADS 4
72 unsigned int yield_active
;
73 unsigned int __thread rand_yield
;
76 static LIST_HEAD(registry
);
78 static void internal_urcu_lock(void)
82 #ifndef DISTRUST_SIGNALS_EXTREME
83 ret
= pthread_mutex_lock(&urcu_mutex
);
85 perror("Error in pthread mutex lock");
88 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
89 while ((ret
= pthread_mutex_trylock(&urcu_mutex
)) != 0) {
90 if (ret
!= EBUSY
&& ret
!= EINTR
) {
91 printf("ret = %d, errno = %d\n", ret
, errno
);
92 perror("Error in pthread mutex lock");
95 if (urcu_reader
.need_mb
) {
97 urcu_reader
.need_mb
= 0;
102 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
105 static void internal_urcu_unlock(void)
109 ret
= pthread_mutex_unlock(&urcu_mutex
);
111 perror("Error in pthread mutex unlock");
117 * called with urcu_mutex held.
119 static void switch_next_urcu_qparity(void)
121 STORE_SHARED(urcu_gp_ctr
, urcu_gp_ctr
^ RCU_GP_CTR_BIT
);
125 static void force_mb_single_thread(struct urcu_reader
*index
)
130 static void force_mb_all_threads(void)
134 #else /* #ifdef URCU_MB */
135 static void force_mb_single_thread(struct urcu_reader
*index
)
137 assert(!list_empty(®istry
));
139 * pthread_kill has a smp_mb(). But beware, we assume it performs
140 * a cache flush on architectures with non-coherent cache. Let's play
141 * safe and don't assume anything : we use smp_mc() to make sure the
142 * cache flush is enforced.
145 smp_mc(); /* write ->need_mb before sending the signals */
146 pthread_kill(index
->tid
, SIGURCU
);
149 * Wait for sighandler (and thus mb()) to execute on every thread.
152 while (index
->need_mb
) {
155 smp_mb(); /* read ->need_mb before ending the barrier */
158 static void force_mb_all_threads(void)
160 struct urcu_reader
*index
;
163 * Ask for each threads to execute a smp_mb() so we can consider the
164 * compiler barriers around rcu read lock as real memory barriers.
166 if (list_empty(®istry
))
169 * pthread_kill has a smp_mb(). But beware, we assume it performs
170 * a cache flush on architectures with non-coherent cache. Let's play
171 * safe and don't assume anything : we use smp_mc() to make sure the
172 * cache flush is enforced.
174 list_for_each_entry(index
, ®istry
, head
) {
176 smp_mc(); /* write need_mb before sending the signal */
177 pthread_kill(index
->tid
, SIGURCU
);
180 * Wait for sighandler (and thus mb()) to execute on every thread.
182 * Note that the pthread_kill() will never be executed on systems
183 * that correctly deliver signals in a timely manner. However, it
184 * is not uncommon for kernels to have bugs that can result in
185 * lost or unduly delayed signals.
187 * If you are seeing the below pthread_kill() executing much at
188 * all, we suggest testing the underlying kernel and filing the
189 * relevant bug report. For Linux kernels, we recommend getting
190 * the Linux Test Project (LTP).
192 list_for_each_entry(index
, ®istry
, head
) {
193 while (index
->need_mb
) {
194 pthread_kill(index
->tid
, SIGURCU
);
198 smp_mb(); /* read ->need_mb before ending the barrier */
200 #endif /* #else #ifdef URCU_MB */
203 * synchronize_rcu() waiting. Single thread.
205 static void wait_gp(struct urcu_reader
*index
)
207 uatomic_dec(&gp_futex
);
208 force_mb_single_thread(index
); /* Write futex before read reader_gp */
209 if (!rcu_old_gp_ongoing(&index
->ctr
)) {
210 /* Read reader_gp before write futex */
211 force_mb_single_thread(index
);
212 /* Callbacks are queued, don't wait. */
213 uatomic_set(&gp_futex
, 0);
215 /* Read reader_gp before read futex */
216 force_mb_single_thread(index
);
217 if (uatomic_read(&gp_futex
) == -1)
218 futex(&gp_futex
, FUTEX_WAIT
, -1,
223 void wait_for_quiescent_state(void)
225 struct urcu_reader
*index
;
227 if (list_empty(®istry
))
230 * Wait for each thread urcu_reader.ctr count to become 0.
232 list_for_each_entry(index
, ®istry
, head
) {
234 #ifndef HAS_INCOHERENT_CACHES
235 while (rcu_old_gp_ongoing(&index
->ctr
)) {
236 if (wait_loops
++ == RCU_QS_ACTIVE_ATTEMPTS
) {
242 #else /* #ifndef HAS_INCOHERENT_CACHES */
244 * BUSY-LOOP. Force the reader thread to commit its
245 * urcu_reader.ctr update to memory if we wait for too long.
247 while (rcu_old_gp_ongoing(&index
->ctr
)) {
248 switch (wait_loops
++) {
249 case RCU_QS_ACTIVE_ATTEMPTS
:
252 case KICK_READER_LOOPS
:
253 force_mb_single_thread(index
);
260 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
264 void synchronize_rcu(void)
266 internal_urcu_lock();
268 /* All threads should read qparity before accessing data structure
269 * where new ptr points to. Must be done within internal_urcu_lock
270 * because it iterates on reader threads.*/
271 /* Write new ptr before changing the qparity */
272 force_mb_all_threads();
274 switch_next_urcu_qparity(); /* 0 -> 1 */
277 * Must commit qparity update to memory before waiting for parity
278 * 0 quiescent state. Failure to do so could result in the writer
279 * waiting forever while new readers are always accessing data (no
281 * Ensured by STORE_SHARED and LOAD_SHARED.
285 * Adding a smp_mb() which is _not_ formally required, but makes the
286 * model easier to understand. It does not have a big performance impact
287 * anyway, given this is the write-side.
292 * Wait for previous parity to be empty of readers.
294 wait_for_quiescent_state(); /* Wait readers in parity 0 */
297 * Must finish waiting for quiescent state for parity 0 before
298 * committing qparity update to memory. Failure to do so could result in
299 * the writer waiting forever while new readers are always accessing
300 * data (no progress).
301 * Ensured by STORE_SHARED and LOAD_SHARED.
305 * Adding a smp_mb() which is _not_ formally required, but makes the
306 * model easier to understand. It does not have a big performance impact
307 * anyway, given this is the write-side.
311 switch_next_urcu_qparity(); /* 1 -> 0 */
314 * Must commit qparity update to memory before waiting for parity
315 * 1 quiescent state. Failure to do so could result in the writer
316 * waiting forever while new readers are always accessing data (no
318 * Ensured by STORE_SHARED and LOAD_SHARED.
322 * Adding a smp_mb() which is _not_ formally required, but makes the
323 * model easier to understand. It does not have a big performance impact
324 * anyway, given this is the write-side.
329 * Wait for previous parity to be empty of readers.
331 wait_for_quiescent_state(); /* Wait readers in parity 1 */
333 /* Finish waiting for reader threads before letting the old ptr being
334 * freed. Must be done within internal_urcu_lock because it iterates on
336 force_mb_all_threads();
338 internal_urcu_unlock();
342 * library wrappers to be used by non-LGPL compatible source code.
345 void rcu_read_lock(void)
350 void rcu_read_unlock(void)
355 void *rcu_dereference(void *p
)
357 return _rcu_dereference(p
);
360 void *rcu_assign_pointer_sym(void **p
, void *v
)
363 return STORE_SHARED(p
, v
);
366 void *rcu_xchg_pointer_sym(void **p
, void *v
)
369 return uatomic_xchg(p
, v
);
372 void *rcu_cmpxchg_pointer_sym(void **p
, void *old
, void *_new
)
375 return uatomic_cmpxchg(p
, old
, _new
);
378 void *rcu_publish_content_sym(void **p
, void *v
)
382 oldptr
= _rcu_xchg_pointer(p
, v
);
387 void rcu_register_thread(void)
389 internal_urcu_lock();
390 urcu_init(); /* In case gcc does not support constructor attribute */
391 urcu_reader
.tid
= pthread_self();
392 assert(urcu_reader
.need_mb
== 0);
393 assert(urcu_reader
.ctr
== 0);
394 list_add(&urcu_reader
.head
, ®istry
);
395 internal_urcu_unlock();
398 void rcu_unregister_thread(void)
400 internal_urcu_lock();
401 list_del(&urcu_reader
.head
);
402 internal_urcu_unlock();
406 static void sigurcu_handler(int signo
, siginfo_t
*siginfo
, void *context
)
409 * Executing this smp_mb() is the only purpose of this signal handler.
410 * It punctually promotes barrier() into smp_mb() on every thread it is
414 urcu_reader
.need_mb
= 0;
419 * urcu_init constructor. Called when the library is linked, but also when
420 * reader threads are calling rcu_register_thread().
421 * Should only be called by a single thread at a given time. This is ensured by
422 * holing the internal_urcu_lock() from rcu_register_thread() or by running at
423 * library load time, which should not be executed by multiple threads nor
424 * concurrently with rcu_register_thread() anyway.
428 struct sigaction act
;
435 act
.sa_sigaction
= sigurcu_handler
;
436 act
.sa_flags
= SA_SIGINFO
| SA_RESTART
;
437 sigemptyset(&act
.sa_mask
);
438 ret
= sigaction(SIGURCU
, &act
, NULL
);
440 perror("Error in sigaction");
447 struct sigaction act
;
450 ret
= sigaction(SIGURCU
, NULL
, &act
);
452 perror("Error in sigaction");
455 assert(act
.sa_sigaction
== sigurcu_handler
);
456 assert(list_empty(®istry
));
458 #endif /* #ifndef URCU_MB */