4 * Userspace RCU QSBR 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/wfcqueue.h"
39 #include "urcu/map/urcu-qsbr.h"
40 #define BUILD_QSBR_LIB
41 #include "urcu/static/urcu-qsbr.h"
42 #include "urcu-pointer.h"
43 #include "urcu/tls-compat.h"
47 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
49 #include "urcu-qsbr.h"
52 void __attribute__((destructor
)) rcu_exit(void);
54 static pthread_mutex_t rcu_gp_lock
= PTHREAD_MUTEX_INITIALIZER
;
59 * Global grace period counter.
61 unsigned long rcu_gp_ctr
= RCU_GP_ONLINE
;
64 * Active attempts to check for reader Q.S. before calling futex().
66 #define RCU_QS_ACTIVE_ATTEMPTS 100
69 * Written to only by each individual reader. Read by both the reader and the
72 DEFINE_URCU_TLS(struct rcu_reader
, rcu_reader
);
75 unsigned int rcu_yield_active
;
76 DEFINE_URCU_TLS(unsigned int, rcu_rand_yield
);
79 static CDS_LIST_HEAD(registry
);
81 static void mutex_lock(pthread_mutex_t
*mutex
)
85 #ifndef DISTRUST_SIGNALS_EXTREME
86 ret
= pthread_mutex_lock(mutex
);
89 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
90 while ((ret
= pthread_mutex_trylock(mutex
)) != 0) {
91 if (ret
!= EBUSY
&& ret
!= EINTR
)
95 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
98 static void mutex_unlock(pthread_mutex_t
*mutex
)
102 ret
= pthread_mutex_unlock(mutex
);
108 * synchronize_rcu() waiting. Single thread.
110 static void wait_gp(void)
112 /* Read reader_gp before read futex */
114 if (uatomic_read(&rcu_gp_futex
) == -1)
115 futex_noasync(&rcu_gp_futex
, FUTEX_WAIT
, -1,
119 static void wait_for_readers(struct cds_list_head
*input_readers
,
120 struct cds_list_head
*cur_snap_readers
,
121 struct cds_list_head
*qsreaders
)
124 struct rcu_reader
*index
, *tmp
;
127 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
128 * indicate quiescence (offline), or for them to observe the
129 * current rcu_gp_ctr value.
133 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
134 uatomic_set(&rcu_gp_futex
, -1);
136 * Write futex before write waiting (the other side
137 * reads them in the opposite order).
140 cds_list_for_each_entry(index
, input_readers
, node
) {
141 _CMM_STORE_SHARED(index
->waiting
, 1);
143 /* Write futex before read reader_gp */
146 cds_list_for_each_entry_safe(index
, tmp
, input_readers
, node
) {
147 switch (rcu_reader_state(&index
->ctr
)) {
148 case RCU_READER_ACTIVE_CURRENT
:
149 if (cur_snap_readers
) {
150 cds_list_move(&index
->node
,
155 case RCU_READER_INACTIVE
:
156 cds_list_move(&index
->node
, qsreaders
);
158 case RCU_READER_ACTIVE_OLD
:
160 * Old snapshot. Leaving node in
161 * input_readers will make us busy-loop
162 * until the snapshot becomes current or
163 * the reader becomes inactive.
169 if (cds_list_empty(input_readers
)) {
170 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
171 /* Read reader_gp before write futex */
173 uatomic_set(&rcu_gp_futex
, 0);
177 if (wait_loops
>= RCU_QS_ACTIVE_ATTEMPTS
) {
180 #ifndef HAS_INCOHERENT_CACHES
182 #else /* #ifndef HAS_INCOHERENT_CACHES */
184 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
191 * Using a two-subphases algorithm for architectures with smaller than 64-bit
192 * long-size to ensure we do not encounter an overflow bug.
195 #if (CAA_BITS_PER_LONG < 64)
196 void synchronize_rcu(void)
198 CDS_LIST_HEAD(cur_snap_readers
);
199 CDS_LIST_HEAD(qsreaders
);
200 unsigned long was_online
;
202 was_online
= URCU_TLS(rcu_reader
).ctr
;
204 /* All threads should read qparity before accessing data structure
205 * where new ptr points to. In the "then" case, rcu_thread_offline
206 * includes a memory barrier.
208 * Mark the writer thread offline to make sure we don't wait for
209 * our own quiescent state. This allows using synchronize_rcu()
210 * in threads registered as readers.
213 rcu_thread_offline();
217 mutex_lock(&rcu_gp_lock
);
219 if (cds_list_empty(®istry
))
223 * Wait for readers to observe original parity or be quiescent.
225 wait_for_readers(®istry
, &cur_snap_readers
, &qsreaders
);
228 * Must finish waiting for quiescent state for original parity
229 * before committing next rcu_gp_ctr update to memory. Failure
230 * to do so could result in the writer waiting forever while new
231 * readers are always accessing data (no progress). Enforce
232 * compiler-order of load URCU_TLS(rcu_reader).ctr before store
238 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
239 * model easier to understand. It does not have a big performance impact
240 * anyway, given this is the write-side.
244 /* Switch parity: 0 -> 1, 1 -> 0 */
245 CMM_STORE_SHARED(rcu_gp_ctr
, rcu_gp_ctr
^ RCU_GP_CTR
);
248 * Must commit rcu_gp_ctr update to memory before waiting for
249 * quiescent state. Failure to do so could result in the writer
250 * waiting forever while new readers are always accessing data
251 * (no progress). Enforce compiler-order of store to rcu_gp_ctr
252 * before load URCU_TLS(rcu_reader).ctr.
257 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
258 * model easier to understand. It does not have a big performance impact
259 * anyway, given this is the write-side.
264 * Wait for readers to observe new parity or be quiescent.
266 wait_for_readers(&cur_snap_readers
, NULL
, &qsreaders
);
269 * Put quiescent reader list back into registry.
271 cds_list_splice(&qsreaders
, ®istry
);
273 mutex_unlock(&rcu_gp_lock
);
276 * Finish waiting for reader threads before letting the old ptr being
284 #else /* !(CAA_BITS_PER_LONG < 64) */
285 void synchronize_rcu(void)
287 CDS_LIST_HEAD(qsreaders
);
288 unsigned long was_online
;
290 was_online
= URCU_TLS(rcu_reader
).ctr
;
293 * Mark the writer thread offline to make sure we don't wait for
294 * our own quiescent state. This allows using synchronize_rcu()
295 * in threads registered as readers.
298 rcu_thread_offline();
302 mutex_lock(&rcu_gp_lock
);
303 if (cds_list_empty(®istry
))
306 /* Increment current G.P. */
307 CMM_STORE_SHARED(rcu_gp_ctr
, rcu_gp_ctr
+ RCU_GP_CTR
);
310 * Must commit rcu_gp_ctr update to memory before waiting for
311 * quiescent state. Failure to do so could result in the writer
312 * waiting forever while new readers are always accessing data
313 * (no progress). Enforce compiler-order of store to rcu_gp_ctr
314 * before load URCU_TLS(rcu_reader).ctr.
319 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
320 * model easier to understand. It does not have a big performance impact
321 * anyway, given this is the write-side.
326 * Wait for readers to observe new count of be quiescent.
328 wait_for_readers(®istry
, NULL
, &qsreaders
);
331 * Put quiescent reader list back into registry.
333 cds_list_splice(&qsreaders
, ®istry
);
335 mutex_unlock(&rcu_gp_lock
);
342 #endif /* !(CAA_BITS_PER_LONG < 64) */
345 * library wrappers to be used by non-LGPL compatible source code.
348 void rcu_read_lock(void)
353 void rcu_read_unlock(void)
358 void rcu_quiescent_state(void)
360 _rcu_quiescent_state();
363 void rcu_thread_offline(void)
365 _rcu_thread_offline();
368 void rcu_thread_online(void)
370 _rcu_thread_online();
373 void rcu_register_thread(void)
375 URCU_TLS(rcu_reader
).tid
= pthread_self();
376 assert(URCU_TLS(rcu_reader
).ctr
== 0);
378 mutex_lock(&rcu_gp_lock
);
379 cds_list_add(&URCU_TLS(rcu_reader
).node
, ®istry
);
380 mutex_unlock(&rcu_gp_lock
);
381 _rcu_thread_online();
384 void rcu_unregister_thread(void)
387 * We have to make the thread offline otherwise we end up dealocking
388 * with a waiting writer.
390 _rcu_thread_offline();
391 mutex_lock(&rcu_gp_lock
);
392 cds_list_del(&URCU_TLS(rcu_reader
).node
);
393 mutex_unlock(&rcu_gp_lock
);
399 * Assertion disabled because call_rcu threads are now rcu
400 * readers, and left running at exit.
401 * assert(cds_list_empty(®istry));
405 DEFINE_RCU_FLAVOR(rcu_flavor
);
407 #include "urcu-call-rcu-impl.h"
408 #include "urcu-defer-impl.h"