urcu/static/rculfqueue.h

   1 #ifndef _URCU_RCULFQUEUE_STATIC_H
   2 #define _URCU_RCULFQUEUE_STATIC_H
   3
   4 /*
   5  * rculfqueue-static.h
   6  *
   7  * Userspace RCU library - Lock-Free RCU Queue
   8  *
   9  * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  10  *
  11  * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See rculfqueue.h for linking
  12  * dynamically with the userspace rcu library.
  13  *
  14  * This library is free software; you can redistribute it and/or
  15  * modify it under the terms of the GNU Lesser General Public
  16  * License as published by the Free Software Foundation; either
  17  * version 2.1 of the License, or (at your option) any later version.
  18  *
  19  * This library is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  22  * Lesser General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU Lesser General Public
  25  * License along with this library; if not, write to the Free Software
  26  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  27  */
  28
  29 #include <urcu-call-rcu.h>
  30 #include <urcu/uatomic.h>
  31 #include <urcu-pointer.h>
  32 #include <assert.h>
  33 #include <errno.h>
  34
  35 #ifdef __cplusplus
  36 extern "C" {
  37 #endif
  38
  39 struct cds_lfq_node_rcu_dummy {
  40         struct cds_lfq_node_rcu parent;
  41         struct rcu_head head;
  42         struct cds_lfq_queue_rcu *q;
  43 };
  44
  45 /*
  46  * Lock-free RCU queue. Enqueue and dequeue operations hold a RCU read
  47  * lock to deal with cmpxchg ABA problem. This queue is *not* circular:
  48  * head points to the oldest node, tail points to the newest node.
  49  * A dummy node is kept to ensure enqueue and dequeue can always proceed
  50  * concurrently. Keeping a separate head and tail helps with large
  51  * queues: enqueue and dequeue can proceed concurrently without
  52  * wrestling for exclusive access to the same variables.
  53  *
  54  * Dequeue retry if it detects that it would be dequeueing the last node
  55  * (it means a dummy node dequeue-requeue is in progress). This ensures
  56  * that there is always at least one node in the queue.
  57  *
  58  * In the dequeue operation, we internally reallocate the dummy node
  59  * upon dequeue/requeue and use call_rcu to free the old one after a
  60  * grace period.
  61  */
  62
  63 static inline
  64 struct cds_lfq_node_rcu *make_dummy(struct cds_lfq_queue_rcu *q,
  65                                     struct cds_lfq_node_rcu *next)
  66 {
  67         struct cds_lfq_node_rcu_dummy *dummy;
  68
  69         dummy = malloc(sizeof(struct cds_lfq_node_rcu_dummy));
  70         assert(dummy);
  71         dummy->parent.next = next;
  72         dummy->parent.dummy = 1;
  73         dummy->q = q;
  74         return &dummy->parent;
  75 }
  76
  77 static inline
  78 void free_dummy_cb(struct rcu_head *head)
  79 {
  80         struct cds_lfq_node_rcu_dummy *dummy =
  81                 caa_container_of(head, struct cds_lfq_node_rcu_dummy, head);
  82         free(dummy);
  83 }
  84
  85 static inline
  86 void rcu_free_dummy(struct cds_lfq_node_rcu *node)
  87 {
  88         struct cds_lfq_node_rcu_dummy *dummy;
  89
  90         assert(node->dummy);
  91         dummy = caa_container_of(node, struct cds_lfq_node_rcu_dummy, parent);
  92         dummy->q->queue_call_rcu(&dummy->head, free_dummy_cb);
  93 }
  94
  95 static inline
  96 void free_dummy(struct cds_lfq_node_rcu *node)
  97 {
  98         struct cds_lfq_node_rcu_dummy *dummy;
  99
 100         assert(node->dummy);
 101         dummy = caa_container_of(node, struct cds_lfq_node_rcu_dummy, parent);
 102         free(dummy);
 103 }
 104
 105 static inline
 106 void _cds_lfq_node_init_rcu(struct cds_lfq_node_rcu *node)
 107 {
 108         node->next = NULL;
 109         node->dummy = 0;
 110 }
 111
 112 static inline
 113 void _cds_lfq_init_rcu(struct cds_lfq_queue_rcu *q,
 114                        void queue_call_rcu(struct rcu_head *head,
 115                                 void (*func)(struct rcu_head *head)))
 116 {
 117         q->tail = make_dummy(q, NULL);
 118         q->head = q->tail;
 119         q->queue_call_rcu = queue_call_rcu;
 120 }
 121
 122 /*
 123  * The queue should be emptied before calling destroy.
 124  *
 125  * Return 0 on success, -EPERM if queue is not empty.
 126  */
 127 static inline
 128 int _cds_lfq_destroy_rcu(struct cds_lfq_queue_rcu *q)
 129 {
 130         struct cds_lfq_node_rcu *head;
 131
 132         head = rcu_dereference(q->head);
 133         if (!(head->dummy && head->next == NULL))
 134                 return -EPERM;  /* not empty */
 135         free_dummy(head);
 136         return 0;
 137 }
 138
 139 /*
 140  * Should be called under rcu read lock critical section.
 141  */
 142 static inline
 143 void _cds_lfq_enqueue_rcu(struct cds_lfq_queue_rcu *q,
 144                           struct cds_lfq_node_rcu *node)
 145 {
 146         /*
 147          * uatomic_cmpxchg() implicit memory barrier orders earlier stores to
 148          * node before publication.
 149          */
 150
 151         for (;;) {
 152                 struct cds_lfq_node_rcu *tail, *next;
 153
 154                 tail = rcu_dereference(q->tail);
 155                 next = uatomic_cmpxchg(&tail->next, NULL, node);
 156                 if (next == NULL) {
 157                         /*
 158                          * Tail was at the end of queue, we successfully
 159                          * appended to it. Now move tail (another
 160                          * enqueue might beat us to it, that's fine).
 161                          */
 162                         (void) uatomic_cmpxchg(&q->tail, tail, node);
 163                         return;
 164                 } else {
 165                         /*
 166                          * Failure to append to current tail.
 167                          * Help moving tail further and retry.
 168                          */
 169                         (void) uatomic_cmpxchg(&q->tail, tail, next);
 170                         continue;
 171                 }
 172         }
 173 }
 174
 175 static inline
 176 void enqueue_dummy(struct cds_lfq_queue_rcu *q)
 177 {
 178         struct cds_lfq_node_rcu *node;
 179
 180         /* We need to reallocate to protect from ABA. */
 181         node = make_dummy(q, NULL);
 182         _cds_lfq_enqueue_rcu(q, node);
 183 }
 184
 185 /*
 186  * Should be called under rcu read lock critical section.
 187  *
 188  * The caller must wait for a grace period to pass before freeing the returned
 189  * node or modifying the cds_lfq_node_rcu structure.
 190  * Returns NULL if queue is empty.
 191  */
 192 static inline
 193 struct cds_lfq_node_rcu *_cds_lfq_dequeue_rcu(struct cds_lfq_queue_rcu *q)
 194 {
 195         for (;;) {
 196                 struct cds_lfq_node_rcu *head, *next;
 197
 198                 head = rcu_dereference(q->head);
 199                 next = rcu_dereference(head->next);
 200                 if (head->dummy && next == NULL)
 201                         return NULL;    /* empty */
 202                 /*
 203                  * We never, ever allow dequeue to get to a state where
 204                  * the queue is empty (we need at least one node in the
 205                  * queue). This is ensured by checking if the head next
 206                  * is NULL, which means we need to enqueue a dummy node
 207                  * before we can hope dequeuing anything.
 208                  */
 209                 if (!next) {
 210                         enqueue_dummy(q);
 211                         next = rcu_dereference(head->next);
 212                 }
 213                 if (uatomic_cmpxchg(&q->head, head, next) != head)
 214                         continue;       /* Concurrently pushed. */
 215                 if (head->dummy) {
 216                         /* Free dummy after grace period. */
 217                         rcu_free_dummy(head);
 218                         continue;       /* try again */
 219                 }
 220                 return head;
 221         }
 222 }
 223
 224 #ifdef __cplusplus
 225 }
 226 #endif
 227
 228 #endif /* _URCU_RCULFQUEUE_STATIC_H */