8774c0375eff1aa6cd458f39697cd1b7a39ef173
[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_* API
50 *
51 * Unless otherwise stated, the caller must ensure mutual exclusion of
52 * queue update operations "dequeue" and "splice" (for source queue).
53 * Queue read operations "first" and "next", which are used by
54 * "for_each" iterations, need to be protected against concurrent
55 * "dequeue" and "splice" (for source queue) by the caller.
56 * "enqueue", "splice" (for destination queue), and "empty" are the only
57 * operations that can be used without any mutual exclusion.
58 * Mutual exclusion can be ensured by holding cds_wfcq_dequeue_lock().
59 *
60 * For convenience, cds_wfcq_dequeue_blocking() and
61 * cds_wfcq_splice_blocking() hold the dequeue lock.
62 *
63 * Besides locking, mutual exclusion of dequeue, splice and iteration
64 * can be ensured by performing all of those operations from a single
65 * thread, without requiring any lock.
66 */
67
68 #define WFCQ_ADAPT_ATTEMPTS 10 /* Retry if being set */
69 #define WFCQ_WAIT 10 /* Wait 10 ms if being set */
70
71 /*
72 * cds_wfcq_node_init: initialize wait-free queue node.
73 */
74 static inline void _cds_wfcq_node_init(struct cds_wfcq_node *node)
75 {
76 node->next = NULL;
77 }
78
79 /*
80 * cds_wfcq_init: initialize wait-free queue.
81 */
82 static inline void _cds_wfcq_init(struct cds_wfcq_head *head,
83 struct cds_wfcq_tail *tail)
84 {
85 int ret;
86
87 /* Set queue head and tail */
88 _cds_wfcq_node_init(&head->node);
89 tail->p = &head->node;
90 ret = pthread_mutex_init(&head->lock, NULL);
91 assert(!ret);
92 }
93
94 /*
95 * cds_wfcq_empty: return whether wait-free queue is empty.
96 *
97 * No memory barrier is issued. No mutual exclusion is required.
98 *
99 * We perform the test on head->node.next to check if the queue is
100 * possibly empty, but we confirm this by checking if the tail pointer
101 * points to the head node because the tail pointer is the linearisation
102 * point of the enqueuers. Just checking the head next pointer could
103 * make a queue appear empty if an enqueuer is preempted for a long time
104 * between xchg() and setting the previous node's next pointer.
105 */
106 static inline bool _cds_wfcq_empty(struct cds_wfcq_head *head,
107 struct cds_wfcq_tail *tail)
108 {
109 /*
110 * Queue is empty if no node is pointed by head->node.next nor
111 * tail->p. Even though the tail->p check is sufficient to find
112 * out of the queue is empty, we first check head->node.next as a
113 * common case to ensure that dequeuers do not frequently access
114 * enqueuer's tail->p cache line.
115 */
116 return CMM_LOAD_SHARED(head->node.next) == NULL
117 && CMM_LOAD_SHARED(tail->p) == &head->node;
118 }
119
120 static inline void _cds_wfcq_dequeue_lock(struct cds_wfcq_head *head,
121 struct cds_wfcq_tail *tail)
122 {
123 int ret;
124
125 ret = pthread_mutex_lock(&head->lock);
126 assert(!ret);
127 }
128
129 static inline void _cds_wfcq_dequeue_unlock(struct cds_wfcq_head *head,
130 struct cds_wfcq_tail *tail)
131 {
132 int ret;
133
134 ret = pthread_mutex_unlock(&head->lock);
135 assert(!ret);
136 }
137
138 static inline bool ___cds_wfcq_append(struct cds_wfcq_head *head,
139 struct cds_wfcq_tail *tail,
140 struct cds_wfcq_node *new_head,
141 struct cds_wfcq_node *new_tail)
142 {
143 struct cds_wfcq_node *old_tail;
144
145 /*
146 * Implicit memory barrier before uatomic_xchg() orders earlier
147 * stores to data structure containing node and setting
148 * node->next to NULL before publication.
149 */
150 old_tail = uatomic_xchg(&tail->p, new_tail);
151
152 /*
153 * Implicit memory barrier after uatomic_xchg() orders store to
154 * q->tail before store to old_tail->next.
155 *
156 * At this point, dequeuers see a NULL tail->p->next, which
157 * indicates that the queue is being appended to. The following
158 * store will append "node" to the queue from a dequeuer
159 * perspective.
160 */
161 CMM_STORE_SHARED(old_tail->next, new_head);
162 /*
163 * Return false if queue was empty prior to adding the node,
164 * else return true.
165 */
166 return old_tail != &head->node;
167 }
168
169 /*
170 * cds_wfcq_enqueue: enqueue a node into a wait-free queue.
171 *
172 * Issues a full memory barrier before enqueue. No mutual exclusion is
173 * required.
174 *
175 * Returns false if the queue was empty prior to adding the node.
176 * Returns true otherwise.
177 */
178 static inline bool _cds_wfcq_enqueue(struct cds_wfcq_head *head,
179 struct cds_wfcq_tail *tail,
180 struct cds_wfcq_node *new_tail)
181 {
182 return ___cds_wfcq_append(head, tail, new_tail, new_tail);
183 }
184
185 /*
186 * Waiting for enqueuer to complete enqueue and return the next node.
187 */
188 static inline struct cds_wfcq_node *
189 ___cds_wfcq_node_sync_next(struct cds_wfcq_node *node, int blocking)
190 {
191 struct cds_wfcq_node *next;
192 int attempt = 0;
193
194 /*
195 * Adaptative busy-looping waiting for enqueuer to complete enqueue.
196 */
197 while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
198 if (!blocking)
199 return CDS_WFCQ_WOULDBLOCK;
200 if (++attempt >= WFCQ_ADAPT_ATTEMPTS) {
201 poll(NULL, 0, WFCQ_WAIT); /* Wait for 10ms */
202 attempt = 0;
203 } else {
204 caa_cpu_relax();
205 }
206 }
207
208 return next;
209 }
210
211 static inline struct cds_wfcq_node *
212 ___cds_wfcq_first(struct cds_wfcq_head *head,
213 struct cds_wfcq_tail *tail,
214 int blocking)
215 {
216 struct cds_wfcq_node *node;
217
218 if (_cds_wfcq_empty(head, tail))
219 return NULL;
220 node = ___cds_wfcq_node_sync_next(&head->node, blocking);
221 /* Load head->node.next before loading node's content */
222 cmm_smp_read_barrier_depends();
223 return node;
224 }
225
226 /*
227 * __cds_wfcq_first_blocking: get first node of a queue, without dequeuing.
228 *
229 * Content written into the node before enqueue is guaranteed to be
230 * consistent, but no other memory ordering is ensured.
231 * Dequeue/splice/iteration mutual exclusion should be ensured by the
232 * caller.
233 *
234 * Used by for-like iteration macros in urcu/wfqueue.h:
235 * __cds_wfcq_for_each_blocking()
236 * __cds_wfcq_for_each_blocking_safe()
237 */
238 static inline struct cds_wfcq_node *
239 ___cds_wfcq_first_blocking(struct cds_wfcq_head *head,
240 struct cds_wfcq_tail *tail)
241 {
242 return ___cds_wfcq_first(head, tail, 1);
243 }
244
245
246 /*
247 * __cds_wfcq_first_nonblocking: get first node of a queue, without dequeuing.
248 *
249 * Same as __cds_wfcq_first_blocking, but returns CDS_WFCQ_WOULDBLOCK if
250 * it needs to block.
251 */
252 static inline struct cds_wfcq_node *
253 ___cds_wfcq_first_nonblocking(struct cds_wfcq_head *head,
254 struct cds_wfcq_tail *tail)
255 {
256 return ___cds_wfcq_first(head, tail, 0);
257 }
258
259 static inline struct cds_wfcq_node *
260 ___cds_wfcq_next(struct cds_wfcq_head *head,
261 struct cds_wfcq_tail *tail,
262 struct cds_wfcq_node *node,
263 int blocking)
264 {
265 struct cds_wfcq_node *next;
266
267 /*
268 * Even though the following tail->p check is sufficient to find
269 * out if we reached the end of the queue, we first check
270 * node->next as a common case to ensure that iteration on nodes
271 * do not frequently access enqueuer's tail->p cache line.
272 */
273 if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
274 /* Load node->next before tail->p */
275 cmm_smp_rmb();
276 if (CMM_LOAD_SHARED(tail->p) == node)
277 return NULL;
278 next = ___cds_wfcq_node_sync_next(node, blocking);
279 }
280 /* Load node->next before loading next's content */
281 cmm_smp_read_barrier_depends();
282 return next;
283 }
284
285 /*
286 * __cds_wfcq_next_blocking: get next node of a queue, without dequeuing.
287 *
288 * Content written into the node before enqueue is guaranteed to be
289 * consistent, but no other memory ordering is ensured.
290 * Dequeue/splice/iteration mutual exclusion should be ensured by the
291 * caller.
292 *
293 * Used by for-like iteration macros in urcu/wfqueue.h:
294 * __cds_wfcq_for_each_blocking()
295 * __cds_wfcq_for_each_blocking_safe()
296 */
297 static inline struct cds_wfcq_node *
298 ___cds_wfcq_next_blocking(struct cds_wfcq_head *head,
299 struct cds_wfcq_tail *tail,
300 struct cds_wfcq_node *node)
301 {
302 return ___cds_wfcq_next(head, tail, node, 1);
303 }
304
305 /*
306 * __cds_wfcq_next_blocking: get next node of a queue, without dequeuing.
307 *
308 * Same as __cds_wfcq_next_blocking, but returns CDS_WFCQ_WOULDBLOCK if
309 * it needs to block.
310 */
311 static inline struct cds_wfcq_node *
312 ___cds_wfcq_next_nonblocking(struct cds_wfcq_head *head,
313 struct cds_wfcq_tail *tail,
314 struct cds_wfcq_node *node)
315 {
316 return ___cds_wfcq_next(head, tail, node, 0);
317 }
318
319 static inline struct cds_wfcq_node *
320 ___cds_wfcq_dequeue(struct cds_wfcq_head *head,
321 struct cds_wfcq_tail *tail,
322 int blocking)
323 {
324 struct cds_wfcq_node *node, *next;
325
326 if (_cds_wfcq_empty(head, tail))
327 return NULL;
328
329 node = ___cds_wfcq_node_sync_next(&head->node, blocking);
330 if (!blocking && node == CDS_WFCQ_WOULDBLOCK)
331 return CDS_WFCQ_WOULDBLOCK;
332
333 if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
334 /*
335 * @node is probably the only node in the queue.
336 * Try to move the tail to &q->head.
337 * q->head.next is set to NULL here, and stays
338 * NULL if the cmpxchg succeeds. Should the
339 * cmpxchg fail due to a concurrent enqueue, the
340 * q->head.next will be set to the next node.
341 * The implicit memory barrier before
342 * uatomic_cmpxchg() orders load node->next
343 * before loading q->tail.
344 * The implicit memory barrier before uatomic_cmpxchg
345 * orders load q->head.next before loading node's
346 * content.
347 */
348 _cds_wfcq_node_init(&head->node);
349 if (uatomic_cmpxchg(&tail->p, node, &head->node) == node)
350 return node;
351 next = ___cds_wfcq_node_sync_next(node, blocking);
352 /*
353 * In nonblocking mode, if we would need to block to
354 * get node's next, set the head next node pointer
355 * (currently NULL) back to its original value.
356 */
357 if (!blocking && next == CDS_WFCQ_WOULDBLOCK) {
358 head->node.next = node;
359 return CDS_WFCQ_WOULDBLOCK;
360 }
361 }
362
363 /*
364 * Move queue head forward.
365 */
366 head->node.next = next;
367
368 /* Load q->head.next before loading node's content */
369 cmm_smp_read_barrier_depends();
370 return node;
371 }
372
373 /*
374 * __cds_wfcq_dequeue_blocking: dequeue a node from the queue.
375 *
376 * Content written into the node before enqueue is guaranteed to be
377 * consistent, but no other memory ordering is ensured.
378 * It is valid to reuse and free a dequeued node immediately.
379 * Dequeue/splice/iteration mutual exclusion should be ensured by the
380 * caller.
381 */
382 static inline struct cds_wfcq_node *
383 ___cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
384 struct cds_wfcq_tail *tail)
385 {
386 return ___cds_wfcq_dequeue(head, tail, 1);
387 }
388
389 /*
390 * __cds_wfcq_dequeue_nonblocking: dequeue a node from a wait-free queue.
391 *
392 * Same as __cds_wfcq_dequeue_blocking, but returns CDS_WFCQ_WOULDBLOCK
393 * if it needs to block.
394 */
395 static inline struct cds_wfcq_node *
396 ___cds_wfcq_dequeue_nonblocking(struct cds_wfcq_head *head,
397 struct cds_wfcq_tail *tail)
398 {
399 return ___cds_wfcq_dequeue(head, tail, 0);
400 }
401
402 static inline enum cds_wfcq_ret
403 ___cds_wfcq_splice(
404 struct cds_wfcq_head *dest_q_head,
405 struct cds_wfcq_tail *dest_q_tail,
406 struct cds_wfcq_head *src_q_head,
407 struct cds_wfcq_tail *src_q_tail,
408 int blocking)
409 {
410 struct cds_wfcq_node *head, *tail;
411
412 if (_cds_wfcq_empty(src_q_head, src_q_tail))
413 return CDS_WFCQ_RET_SRC_EMPTY;
414
415 head = ___cds_wfcq_node_sync_next(&src_q_head->node, blocking);
416 if (head == CDS_WFCQ_WOULDBLOCK)
417 return CDS_WFCQ_RET_WOULDBLOCK;
418 _cds_wfcq_node_init(&src_q_head->node);
419
420 /*
421 * Memory barrier implied before uatomic_xchg() orders store to
422 * src_q->head before store to src_q->tail. This is required by
423 * concurrent enqueue on src_q, which exchanges the tail before
424 * updating the previous tail's next pointer.
425 */
426 tail = uatomic_xchg(&src_q_tail->p, &src_q_head->node);
427
428 /*
429 * Append the spliced content of src_q into dest_q. Does not
430 * require mutual exclusion on dest_q (wait-free).
431 */
432 if (___cds_wfcq_append(dest_q_head, dest_q_tail, head, tail))
433 return CDS_WFCQ_RET_DEST_NON_EMPTY;
434 else
435 return CDS_WFCQ_RET_DEST_EMPTY;
436 }
437
438
439 /*
440 * __cds_wfcq_splice_blocking: enqueue all src_q nodes at the end of dest_q.
441 *
442 * Dequeue all nodes from src_q.
443 * dest_q must be already initialized.
444 * Dequeue/splice/iteration mutual exclusion for src_q should be ensured
445 * by the caller.
446 * Returns enum cds_wfcq_ret which indicates the state of the src or
447 * dest queue. Never returns CDS_WFCQ_RET_WOULDBLOCK.
448 */
449 static inline enum cds_wfcq_ret
450 ___cds_wfcq_splice_blocking(
451 struct cds_wfcq_head *dest_q_head,
452 struct cds_wfcq_tail *dest_q_tail,
453 struct cds_wfcq_head *src_q_head,
454 struct cds_wfcq_tail *src_q_tail)
455 {
456 return ___cds_wfcq_splice(dest_q_head, dest_q_tail,
457 src_q_head, src_q_tail, 1);
458 }
459
460 /*
461 * __cds_wfcq_splice_nonblocking: enqueue all src_q nodes at the end of dest_q.
462 *
463 * Same as __cds_wfcq_splice_blocking, but returns
464 * CDS_WFCQ_RET_WOULDBLOCK if it needs to block.
465 */
466 static inline enum cds_wfcq_ret
467 ___cds_wfcq_splice_nonblocking(
468 struct cds_wfcq_head *dest_q_head,
469 struct cds_wfcq_tail *dest_q_tail,
470 struct cds_wfcq_head *src_q_head,
471 struct cds_wfcq_tail *src_q_tail)
472 {
473 return ___cds_wfcq_splice(dest_q_head, dest_q_tail,
474 src_q_head, src_q_tail, 0);
475 }
476
477 /*
478 * cds_wfcq_dequeue_blocking: dequeue a node from a wait-free queue.
479 *
480 * Content written into the node before enqueue is guaranteed to be
481 * consistent, but no other memory ordering is ensured.
482 * Mutual exlusion with cds_wfcq_splice_blocking and dequeue lock is
483 * ensured.
484 * It is valid to reuse and free a dequeued node immediately.
485 */
486 static inline struct cds_wfcq_node *
487 _cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
488 struct cds_wfcq_tail *tail)
489 {
490 struct cds_wfcq_node *retval;
491
492 _cds_wfcq_dequeue_lock(head, tail);
493 retval = ___cds_wfcq_dequeue_blocking(head, tail);
494 _cds_wfcq_dequeue_unlock(head, tail);
495 return retval;
496 }
497
498 /*
499 * cds_wfcq_splice_blocking: enqueue all src_q nodes at the end of dest_q.
500 *
501 * Dequeue all nodes from src_q.
502 * dest_q must be already initialized.
503 * Content written into the node before enqueue is guaranteed to be
504 * consistent, but no other memory ordering is ensured.
505 * Mutual exlusion with cds_wfcq_dequeue_blocking and dequeue lock is
506 * ensured.
507 * Returns enum cds_wfcq_ret which indicates the state of the src or
508 * dest queue. Never returns CDS_WFCQ_RET_WOULDBLOCK.
509 */
510 static inline enum cds_wfcq_ret
511 _cds_wfcq_splice_blocking(
512 struct cds_wfcq_head *dest_q_head,
513 struct cds_wfcq_tail *dest_q_tail,
514 struct cds_wfcq_head *src_q_head,
515 struct cds_wfcq_tail *src_q_tail)
516 {
517 enum cds_wfcq_ret ret;
518
519 _cds_wfcq_dequeue_lock(src_q_head, src_q_tail);
520 ret = ___cds_wfcq_splice_blocking(dest_q_head, dest_q_tail,
521 src_q_head, src_q_tail);
522 _cds_wfcq_dequeue_unlock(src_q_head, src_q_tail);
523 return ret;
524 }
525
526 #ifdef __cplusplus
527 }
528 #endif
529
530 #endif /* _URCU_WFCQUEUE_STATIC_H */
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