99643be98c393310bc36d591262fcddd5f0227af
[urcu.git] / urcu / static / wfcqueue.h
1 #ifndef _URCU_WFCQUEUE_STATIC_H
2 #define _URCU_WFCQUEUE_STATIC_H
3
4 /*
5 * urcu/static/wfcqueue.h
6 *
7 * Userspace RCU library - Concurrent Queue with Wait-Free Enqueue/Blocking Dequeue
8 *
9 * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu/wfcqueue.h for
10 * linking dynamically with the userspace rcu library.
11 *
12 * Copyright 2010-2012 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
13 * Copyright 2011-2012 - Lai Jiangshan <laijs@cn.fujitsu.com>
14 *
15 * This library is free software; you can redistribute it and/or
16 * modify it under the terms of the GNU Lesser General Public
17 * License as published by the Free Software Foundation; either
18 * version 2.1 of the License, or (at your option) any later version.
19 *
20 * This library is distributed in the hope that it will be useful,
21 * but WITHOUT ANY WARRANTY; without even the implied warranty of
22 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
23 * Lesser General Public License for more details.
24 *
25 * You should have received a copy of the GNU Lesser General Public
26 * License along with this library; if not, write to the Free Software
27 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
28 */
29
30 #include <pthread.h>
31 #include <assert.h>
32 #include <poll.h>
33 #include <stdbool.h>
34 #include <urcu/compiler.h>
35 #include <urcu/uatomic.h>
36
37 #ifdef __cplusplus
38 extern "C" {
39 #endif
40
41 /*
42 * Concurrent queue with wait-free enqueue/blocking dequeue.
43 *
44 * This queue has been designed and implemented collaboratively by
45 * Mathieu Desnoyers and Lai Jiangshan. Inspired from
46 * half-wait-free/half-blocking queue implementation done by Paul E.
47 * McKenney.
48 *
49 * Mutual exclusion of cds_wfcq_* / __cds_wfcq_* API
50 *
51 * Synchronization table:
52 *
53 * External synchronization techniques described in the API below is
54 * required between pairs marked with "X". No external synchronization
55 * required between pairs marked with "-".
56 *
57 * Legend:
58 * [1] cds_wfcq_enqueue
59 * [2] __cds_wfcq_splice (destination queue)
60 * [3] __cds_wfcq_dequeue
61 * [4] __cds_wfcq_splice (source queue)
62 * [5] __cds_wfcq_first
63 * [6] __cds_wfcq_next
64 *
65 * [1] [2] [3] [4] [5] [6]
66 * [1] - - - - - -
67 * [2] - - - - - -
68 * [3] - - X X X X
69 * [4] - - X - X X
70 * [5] - - X X - -
71 * [6] - - X X - -
72 *
73 * Mutual exclusion can be ensured by holding cds_wfcq_dequeue_lock().
74 *
75 * For convenience, cds_wfcq_dequeue_blocking() and
76 * cds_wfcq_splice_blocking() hold the dequeue lock.
77 *
78 * Besides locking, mutual exclusion of dequeue, splice and iteration
79 * can be ensured by performing all of those operations from a single
80 * thread, without requiring any lock.
81 */
82
83 #define WFCQ_ADAPT_ATTEMPTS 10 /* Retry if being set */
84 #define WFCQ_WAIT 10 /* Wait 10 ms if being set */
85
86 /*
87 * cds_wfcq_node_init: initialize wait-free queue node.
88 */
89 static inline void _cds_wfcq_node_init(struct cds_wfcq_node *node)
90 {
91 node->next = NULL;
92 }
93
94 /*
95 * cds_wfcq_init: initialize wait-free queue.
96 */
97 static inline void _cds_wfcq_init(struct cds_wfcq_head *head,
98 struct cds_wfcq_tail *tail)
99 {
100 int ret;
101
102 /* Set queue head and tail */
103 _cds_wfcq_node_init(&head->node);
104 tail->p = &head->node;
105 ret = pthread_mutex_init(&head->lock, NULL);
106 assert(!ret);
107 }
108
109 /*
110 * cds_wfcq_empty: return whether wait-free queue is empty.
111 *
112 * No memory barrier is issued. No mutual exclusion is required.
113 *
114 * We perform the test on head->node.next to check if the queue is
115 * possibly empty, but we confirm this by checking if the tail pointer
116 * points to the head node because the tail pointer is the linearisation
117 * point of the enqueuers. Just checking the head next pointer could
118 * make a queue appear empty if an enqueuer is preempted for a long time
119 * between xchg() and setting the previous node's next pointer.
120 */
121 static inline bool _cds_wfcq_empty(struct cds_wfcq_head *head,
122 struct cds_wfcq_tail *tail)
123 {
124 /*
125 * Queue is empty if no node is pointed by head->node.next nor
126 * tail->p. Even though the tail->p check is sufficient to find
127 * out of the queue is empty, we first check head->node.next as a
128 * common case to ensure that dequeuers do not frequently access
129 * enqueuer's tail->p cache line.
130 */
131 return CMM_LOAD_SHARED(head->node.next) == NULL
132 && CMM_LOAD_SHARED(tail->p) == &head->node;
133 }
134
135 static inline void _cds_wfcq_dequeue_lock(struct cds_wfcq_head *head,
136 struct cds_wfcq_tail *tail)
137 {
138 int ret;
139
140 ret = pthread_mutex_lock(&head->lock);
141 assert(!ret);
142 }
143
144 static inline void _cds_wfcq_dequeue_unlock(struct cds_wfcq_head *head,
145 struct cds_wfcq_tail *tail)
146 {
147 int ret;
148
149 ret = pthread_mutex_unlock(&head->lock);
150 assert(!ret);
151 }
152
153 static inline bool ___cds_wfcq_append(struct cds_wfcq_head *head,
154 struct cds_wfcq_tail *tail,
155 struct cds_wfcq_node *new_head,
156 struct cds_wfcq_node *new_tail)
157 {
158 struct cds_wfcq_node *old_tail;
159
160 /*
161 * Implicit memory barrier before uatomic_xchg() orders earlier
162 * stores to data structure containing node and setting
163 * node->next to NULL before publication.
164 */
165 old_tail = uatomic_xchg(&tail->p, new_tail);
166
167 /*
168 * Implicit memory barrier after uatomic_xchg() orders store to
169 * q->tail before store to old_tail->next.
170 *
171 * At this point, dequeuers see a NULL tail->p->next, which
172 * indicates that the queue is being appended to. The following
173 * store will append "node" to the queue from a dequeuer
174 * perspective.
175 */
176 CMM_STORE_SHARED(old_tail->next, new_head);
177 /*
178 * Return false if queue was empty prior to adding the node,
179 * else return true.
180 */
181 return old_tail != &head->node;
182 }
183
184 /*
185 * cds_wfcq_enqueue: enqueue a node into a wait-free queue.
186 *
187 * Issues a full memory barrier before enqueue. No mutual exclusion is
188 * required.
189 *
190 * Returns false if the queue was empty prior to adding the node.
191 * Returns true otherwise.
192 */
193 static inline bool _cds_wfcq_enqueue(struct cds_wfcq_head *head,
194 struct cds_wfcq_tail *tail,
195 struct cds_wfcq_node *new_tail)
196 {
197 return ___cds_wfcq_append(head, tail, new_tail, new_tail);
198 }
199
200 /*
201 * ___cds_wfcq_busy_wait: adaptative busy-wait.
202 *
203 * Returns 1 if nonblocking and needs to block, 0 otherwise.
204 */
205 static inline bool
206 ___cds_wfcq_busy_wait(int *attempt, int blocking)
207 {
208 if (!blocking)
209 return 1;
210 if (++(*attempt) >= WFCQ_ADAPT_ATTEMPTS) {
211 poll(NULL, 0, WFCQ_WAIT); /* Wait for 10ms */
212 *attempt = 0;
213 } else {
214 caa_cpu_relax();
215 }
216 return 0;
217 }
218
219 /*
220 * Waiting for enqueuer to complete enqueue and return the next node.
221 */
222 static inline struct cds_wfcq_node *
223 ___cds_wfcq_node_sync_next(struct cds_wfcq_node *node, int blocking)
224 {
225 struct cds_wfcq_node *next;
226 int attempt = 0;
227
228 /*
229 * Adaptative busy-looping waiting for enqueuer to complete enqueue.
230 */
231 while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
232 if (___cds_wfcq_busy_wait(&attempt, blocking))
233 return CDS_WFCQ_WOULDBLOCK;
234 }
235
236 return next;
237 }
238
239 static inline struct cds_wfcq_node *
240 ___cds_wfcq_first(struct cds_wfcq_head *head,
241 struct cds_wfcq_tail *tail,
242 int blocking)
243 {
244 struct cds_wfcq_node *node;
245
246 if (_cds_wfcq_empty(head, tail))
247 return NULL;
248 node = ___cds_wfcq_node_sync_next(&head->node, blocking);
249 /* Load head->node.next before loading node's content */
250 cmm_smp_read_barrier_depends();
251 return node;
252 }
253
254 /*
255 * __cds_wfcq_first_blocking: get first node of a queue, without dequeuing.
256 *
257 * Content written into the node before enqueue is guaranteed to be
258 * consistent, but no other memory ordering is ensured.
259 * Dequeue/splice/iteration mutual exclusion should be ensured by the
260 * caller.
261 *
262 * Used by for-like iteration macros in urcu/wfqueue.h:
263 * __cds_wfcq_for_each_blocking()
264 * __cds_wfcq_for_each_blocking_safe()
265 */
266 static inline struct cds_wfcq_node *
267 ___cds_wfcq_first_blocking(struct cds_wfcq_head *head,
268 struct cds_wfcq_tail *tail)
269 {
270 return ___cds_wfcq_first(head, tail, 1);
271 }
272
273
274 /*
275 * __cds_wfcq_first_nonblocking: get first node of a queue, without dequeuing.
276 *
277 * Same as __cds_wfcq_first_blocking, but returns CDS_WFCQ_WOULDBLOCK if
278 * it needs to block.
279 */
280 static inline struct cds_wfcq_node *
281 ___cds_wfcq_first_nonblocking(struct cds_wfcq_head *head,
282 struct cds_wfcq_tail *tail)
283 {
284 return ___cds_wfcq_first(head, tail, 0);
285 }
286
287 static inline struct cds_wfcq_node *
288 ___cds_wfcq_next(struct cds_wfcq_head *head,
289 struct cds_wfcq_tail *tail,
290 struct cds_wfcq_node *node,
291 int blocking)
292 {
293 struct cds_wfcq_node *next;
294
295 /*
296 * Even though the following tail->p check is sufficient to find
297 * out if we reached the end of the queue, we first check
298 * node->next as a common case to ensure that iteration on nodes
299 * do not frequently access enqueuer's tail->p cache line.
300 */
301 if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
302 /* Load node->next before tail->p */
303 cmm_smp_rmb();
304 if (CMM_LOAD_SHARED(tail->p) == node)
305 return NULL;
306 next = ___cds_wfcq_node_sync_next(node, blocking);
307 }
308 /* Load node->next before loading next's content */
309 cmm_smp_read_barrier_depends();
310 return next;
311 }
312
313 /*
314 * __cds_wfcq_next_blocking: get next node of a queue, without dequeuing.
315 *
316 * Content written into the node before enqueue is guaranteed to be
317 * consistent, but no other memory ordering is ensured.
318 * Dequeue/splice/iteration mutual exclusion should be ensured by the
319 * caller.
320 *
321 * Used by for-like iteration macros in urcu/wfqueue.h:
322 * __cds_wfcq_for_each_blocking()
323 * __cds_wfcq_for_each_blocking_safe()
324 */
325 static inline struct cds_wfcq_node *
326 ___cds_wfcq_next_blocking(struct cds_wfcq_head *head,
327 struct cds_wfcq_tail *tail,
328 struct cds_wfcq_node *node)
329 {
330 return ___cds_wfcq_next(head, tail, node, 1);
331 }
332
333 /*
334 * __cds_wfcq_next_blocking: get next node of a queue, without dequeuing.
335 *
336 * Same as __cds_wfcq_next_blocking, but returns CDS_WFCQ_WOULDBLOCK if
337 * it needs to block.
338 */
339 static inline struct cds_wfcq_node *
340 ___cds_wfcq_next_nonblocking(struct cds_wfcq_head *head,
341 struct cds_wfcq_tail *tail,
342 struct cds_wfcq_node *node)
343 {
344 return ___cds_wfcq_next(head, tail, node, 0);
345 }
346
347 static inline struct cds_wfcq_node *
348 ___cds_wfcq_dequeue(struct cds_wfcq_head *head,
349 struct cds_wfcq_tail *tail,
350 int blocking)
351 {
352 struct cds_wfcq_node *node, *next;
353
354 if (_cds_wfcq_empty(head, tail))
355 return NULL;
356
357 node = ___cds_wfcq_node_sync_next(&head->node, blocking);
358 if (!blocking && node == CDS_WFCQ_WOULDBLOCK)
359 return CDS_WFCQ_WOULDBLOCK;
360
361 if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
362 /*
363 * @node is probably the only node in the queue.
364 * Try to move the tail to &q->head.
365 * q->head.next is set to NULL here, and stays
366 * NULL if the cmpxchg succeeds. Should the
367 * cmpxchg fail due to a concurrent enqueue, the
368 * q->head.next will be set to the next node.
369 * The implicit memory barrier before
370 * uatomic_cmpxchg() orders load node->next
371 * before loading q->tail.
372 * The implicit memory barrier before uatomic_cmpxchg
373 * orders load q->head.next before loading node's
374 * content.
375 */
376 _cds_wfcq_node_init(&head->node);
377 if (uatomic_cmpxchg(&tail->p, node, &head->node) == node)
378 return node;
379 next = ___cds_wfcq_node_sync_next(node, blocking);
380 /*
381 * In nonblocking mode, if we would need to block to
382 * get node's next, set the head next node pointer
383 * (currently NULL) back to its original value.
384 */
385 if (!blocking && next == CDS_WFCQ_WOULDBLOCK) {
386 head->node.next = node;
387 return CDS_WFCQ_WOULDBLOCK;
388 }
389 }
390
391 /*
392 * Move queue head forward.
393 */
394 head->node.next = next;
395
396 /* Load q->head.next before loading node's content */
397 cmm_smp_read_barrier_depends();
398 return node;
399 }
400
401 /*
402 * __cds_wfcq_dequeue_blocking: dequeue a node from the queue.
403 *
404 * Content written into the node before enqueue is guaranteed to be
405 * consistent, but no other memory ordering is ensured.
406 * It is valid to reuse and free a dequeued node immediately.
407 * Dequeue/splice/iteration mutual exclusion should be ensured by the
408 * caller.
409 */
410 static inline struct cds_wfcq_node *
411 ___cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
412 struct cds_wfcq_tail *tail)
413 {
414 return ___cds_wfcq_dequeue(head, tail, 1);
415 }
416
417 /*
418 * __cds_wfcq_dequeue_nonblocking: dequeue a node from a wait-free queue.
419 *
420 * Same as __cds_wfcq_dequeue_blocking, but returns CDS_WFCQ_WOULDBLOCK
421 * if it needs to block.
422 */
423 static inline struct cds_wfcq_node *
424 ___cds_wfcq_dequeue_nonblocking(struct cds_wfcq_head *head,
425 struct cds_wfcq_tail *tail)
426 {
427 return ___cds_wfcq_dequeue(head, tail, 0);
428 }
429
430 /*
431 * __cds_wfcq_splice: enqueue all src_q nodes at the end of dest_q.
432 *
433 * Dequeue all nodes from src_q.
434 * dest_q must be already initialized.
435 * Mutual exclusion for src_q should be ensured by the caller as
436 * specified in the "Synchronisation table".
437 * Returns enum cds_wfcq_ret which indicates the state of the src or
438 * dest queue.
439 */
440 static inline enum cds_wfcq_ret
441 ___cds_wfcq_splice(
442 struct cds_wfcq_head *dest_q_head,
443 struct cds_wfcq_tail *dest_q_tail,
444 struct cds_wfcq_head *src_q_head,
445 struct cds_wfcq_tail *src_q_tail,
446 int blocking)
447 {
448 struct cds_wfcq_node *head, *tail;
449 int attempt = 0;
450
451 /*
452 * Initial emptiness check to speed up cases where queue is
453 * empty: only require loads to check if queue is empty.
454 */
455 if (_cds_wfcq_empty(src_q_head, src_q_tail))
456 return CDS_WFCQ_RET_SRC_EMPTY;
457
458 for (;;) {
459 /*
460 * Open-coded _cds_wfcq_empty() by testing result of
461 * uatomic_xchg, as well as tail pointer vs head node
462 * address.
463 */
464 head = uatomic_xchg(&src_q_head->node.next, NULL);
465 if (head)
466 break; /* non-empty */
467 if (CMM_LOAD_SHARED(src_q_tail->p) == &src_q_head->node)
468 return CDS_WFCQ_RET_SRC_EMPTY;
469 if (___cds_wfcq_busy_wait(&attempt, blocking))
470 return CDS_WFCQ_RET_WOULDBLOCK;
471 }
472
473 /*
474 * Memory barrier implied before uatomic_xchg() orders store to
475 * src_q->head before store to src_q->tail. This is required by
476 * concurrent enqueue on src_q, which exchanges the tail before
477 * updating the previous tail's next pointer.
478 */
479 tail = uatomic_xchg(&src_q_tail->p, &src_q_head->node);
480
481 /*
482 * Append the spliced content of src_q into dest_q. Does not
483 * require mutual exclusion on dest_q (wait-free).
484 */
485 if (___cds_wfcq_append(dest_q_head, dest_q_tail, head, tail))
486 return CDS_WFCQ_RET_DEST_NON_EMPTY;
487 else
488 return CDS_WFCQ_RET_DEST_EMPTY;
489 }
490
491 /*
492 * __cds_wfcq_splice_blocking: enqueue all src_q nodes at the end of dest_q.
493 *
494 * Dequeue all nodes from src_q.
495 * dest_q must be already initialized.
496 * Mutual exclusion for src_q should be ensured by the caller as
497 * specified in the "Synchronisation table".
498 * Returns enum cds_wfcq_ret which indicates the state of the src or
499 * dest queue. Never returns CDS_WFCQ_RET_WOULDBLOCK.
500 */
501 static inline enum cds_wfcq_ret
502 ___cds_wfcq_splice_blocking(
503 struct cds_wfcq_head *dest_q_head,
504 struct cds_wfcq_tail *dest_q_tail,
505 struct cds_wfcq_head *src_q_head,
506 struct cds_wfcq_tail *src_q_tail)
507 {
508 return ___cds_wfcq_splice(dest_q_head, dest_q_tail,
509 src_q_head, src_q_tail, 1);
510 }
511
512 /*
513 * __cds_wfcq_splice_nonblocking: enqueue all src_q nodes at the end of dest_q.
514 *
515 * Same as __cds_wfcq_splice_blocking, but returns
516 * CDS_WFCQ_RET_WOULDBLOCK if it needs to block.
517 */
518 static inline enum cds_wfcq_ret
519 ___cds_wfcq_splice_nonblocking(
520 struct cds_wfcq_head *dest_q_head,
521 struct cds_wfcq_tail *dest_q_tail,
522 struct cds_wfcq_head *src_q_head,
523 struct cds_wfcq_tail *src_q_tail)
524 {
525 return ___cds_wfcq_splice(dest_q_head, dest_q_tail,
526 src_q_head, src_q_tail, 0);
527 }
528
529 /*
530 * cds_wfcq_dequeue_blocking: dequeue a node from a wait-free queue.
531 *
532 * Content written into the node before enqueue is guaranteed to be
533 * consistent, but no other memory ordering is ensured.
534 * Mutual exlusion with cds_wfcq_splice_blocking and dequeue lock is
535 * ensured.
536 * It is valid to reuse and free a dequeued node immediately.
537 */
538 static inline struct cds_wfcq_node *
539 _cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
540 struct cds_wfcq_tail *tail)
541 {
542 struct cds_wfcq_node *retval;
543
544 _cds_wfcq_dequeue_lock(head, tail);
545 retval = ___cds_wfcq_dequeue_blocking(head, tail);
546 _cds_wfcq_dequeue_unlock(head, tail);
547 return retval;
548 }
549
550 /*
551 * cds_wfcq_splice_blocking: enqueue all src_q nodes at the end of dest_q.
552 *
553 * Dequeue all nodes from src_q.
554 * dest_q must be already initialized.
555 * Content written into the node before enqueue is guaranteed to be
556 * consistent, but no other memory ordering is ensured.
557 * Mutual exlusion with cds_wfcq_dequeue_blocking and dequeue lock is
558 * ensured.
559 * Returns enum cds_wfcq_ret which indicates the state of the src or
560 * dest queue. Never returns CDS_WFCQ_RET_WOULDBLOCK.
561 */
562 static inline enum cds_wfcq_ret
563 _cds_wfcq_splice_blocking(
564 struct cds_wfcq_head *dest_q_head,
565 struct cds_wfcq_tail *dest_q_tail,
566 struct cds_wfcq_head *src_q_head,
567 struct cds_wfcq_tail *src_q_tail)
568 {
569 enum cds_wfcq_ret ret;
570
571 _cds_wfcq_dequeue_lock(src_q_head, src_q_tail);
572 ret = ___cds_wfcq_splice_blocking(dest_q_head, dest_q_tail,
573 src_q_head, src_q_tail);
574 _cds_wfcq_dequeue_unlock(src_q_head, src_q_tail);
575 return ret;
576 }
577
578 #ifdef __cplusplus
579 }
580 #endif
581
582 #endif /* _URCU_WFCQUEUE_STATIC_H */
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