| 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 <assert.h> |
| 32 | #include <errno.h> |
| 33 | /* A urcu implementation header should be already included. */ |
| 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 | * Dummy nodes are 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 | * We keep two dummy nodes in the queue to distinguish between empty queue |
| 55 | * state and intermediate state while a dummy node dequeue/requeue is |
| 56 | * being performed. Dequeue retry if it detects that it would be |
| 57 | * dequeueing the last node (it means a dummy node dequeue-requeue is in |
| 58 | * progress). This ensures that there is always at least one node in |
| 59 | * the queue. In a situation where the two dummy nodes are being |
| 60 | * requeued (they therefore don't appear in the queue at a given |
| 61 | * moment), we are certain that there is at least one non-dummy node in |
| 62 | * the queue (ensured by the test for NULL next node upon dequeue). |
| 63 | * |
| 64 | * In the dequeue operation, we internally reallocate the dummy nodes |
| 65 | * upon dequeue/requeue and use call_rcu to free them after a grace |
| 66 | * period. |
| 67 | */ |
| 68 | |
| 69 | static inline |
| 70 | int is_dummy(struct cds_lfq_node_rcu *node) |
| 71 | { |
| 72 | return ((unsigned long) node) & 0x1UL; |
| 73 | } |
| 74 | |
| 75 | static inline |
| 76 | struct cds_lfq_node_rcu *make_dummy(struct cds_lfq_queue_rcu *q, |
| 77 | struct cds_lfq_node_rcu *next) |
| 78 | { |
| 79 | struct cds_lfq_node_rcu_dummy *dummy; |
| 80 | |
| 81 | dummy = malloc(sizeof(struct cds_lfq_node_rcu_dummy)); |
| 82 | assert(dummy); |
| 83 | dummy->parent.next = next; |
| 84 | dummy->q = q; |
| 85 | return (struct cds_lfq_node_rcu *) (((unsigned long) &dummy->parent) | 0x1UL); |
| 86 | } |
| 87 | |
| 88 | static inline |
| 89 | struct cds_lfq_node_rcu *get_node(struct cds_lfq_node_rcu *node) |
| 90 | { |
| 91 | return (struct cds_lfq_node_rcu *) (((unsigned long )node) & ~0x1UL); |
| 92 | } |
| 93 | |
| 94 | static inline |
| 95 | void free_dummy(struct rcu_head *head) |
| 96 | { |
| 97 | struct cds_lfq_node_rcu_dummy *dummy = |
| 98 | caa_container_of(head, struct cds_lfq_node_rcu_dummy, head); |
| 99 | free(dummy); |
| 100 | } |
| 101 | |
| 102 | static inline |
| 103 | void rcu_free_dummy(struct cds_lfq_node_rcu *node) |
| 104 | { |
| 105 | struct cds_lfq_node_rcu_dummy *dummy; |
| 106 | |
| 107 | dummy = caa_container_of(get_node(node), struct cds_lfq_node_rcu_dummy, |
| 108 | parent); |
| 109 | dummy->q->queue_call_rcu(&dummy->head, free_dummy); |
| 110 | } |
| 111 | |
| 112 | static inline |
| 113 | void _cds_lfq_node_init_rcu(struct cds_lfq_node_rcu *node) |
| 114 | { |
| 115 | node->next = NULL; |
| 116 | } |
| 117 | |
| 118 | static inline |
| 119 | void _cds_lfq_init_rcu(struct cds_lfq_queue_rcu *q, |
| 120 | void queue_call_rcu(struct rcu_head *head, |
| 121 | void (*func)(struct rcu_head *head))) |
| 122 | { |
| 123 | q->tail = make_dummy(q, NULL); |
| 124 | q->head = make_dummy(q, q->tail); |
| 125 | q->queue_call_rcu = queue_call_rcu; |
| 126 | } |
| 127 | |
| 128 | /* |
| 129 | * The queue should be emptied before calling destroy. |
| 130 | * |
| 131 | * Return 0 on success, -EPERM if queue is not empty. |
| 132 | */ |
| 133 | static inline |
| 134 | int _cds_lfq_destroy_rcu(struct cds_lfq_queue_rcu *q) |
| 135 | { |
| 136 | struct cds_lfq_node_rcu *head, *next; |
| 137 | |
| 138 | head = rcu_dereference(q->head); |
| 139 | next = rcu_dereference(get_node(head)->next); |
| 140 | if (!(is_dummy(head) && is_dummy(next) && get_node(next)->next == NULL)) |
| 141 | return -EPERM; /* not empty */ |
| 142 | rcu_free_dummy(head); |
| 143 | rcu_free_dummy(next); |
| 144 | return 0; |
| 145 | } |
| 146 | |
| 147 | /* |
| 148 | * Should be called under rcu read lock critical section. |
| 149 | */ |
| 150 | static inline |
| 151 | void _cds_lfq_enqueue_rcu(struct cds_lfq_queue_rcu *q, |
| 152 | struct cds_lfq_node_rcu *node) |
| 153 | { |
| 154 | /* |
| 155 | * uatomic_cmpxchg() implicit memory barrier orders earlier stores to |
| 156 | * node before publication. |
| 157 | */ |
| 158 | |
| 159 | for (;;) { |
| 160 | struct cds_lfq_node_rcu *tail, *next; |
| 161 | |
| 162 | tail = rcu_dereference(q->tail); |
| 163 | next = uatomic_cmpxchg(&get_node(tail)->next, NULL, node); |
| 164 | if (next == NULL) { |
| 165 | /* |
| 166 | * Tail was at the end of queue, we successfully |
| 167 | * appended to it. Now move tail (another |
| 168 | * enqueue might beat us to it, that's fine). |
| 169 | */ |
| 170 | (void) uatomic_cmpxchg(&q->tail, tail, node); |
| 171 | return; |
| 172 | } else { |
| 173 | /* |
| 174 | * Failure to append to current tail. |
| 175 | * Help moving tail further and retry. |
| 176 | */ |
| 177 | (void) uatomic_cmpxchg(&q->tail, tail, next); |
| 178 | continue; |
| 179 | } |
| 180 | } |
| 181 | } |
| 182 | |
| 183 | /* |
| 184 | * Should be called under rcu read lock critical section. |
| 185 | * |
| 186 | * The caller must wait for a grace period to pass before freeing the returned |
| 187 | * node or modifying the cds_lfq_node_rcu structure. |
| 188 | * Returns NULL if queue is empty. |
| 189 | */ |
| 190 | static inline |
| 191 | struct cds_lfq_node_rcu *_cds_lfq_dequeue_rcu(struct cds_lfq_queue_rcu *q) |
| 192 | { |
| 193 | for (;;) { |
| 194 | struct cds_lfq_node_rcu *head, *next; |
| 195 | |
| 196 | head = rcu_dereference(q->head); |
| 197 | next = rcu_dereference(get_node(head)->next); |
| 198 | if (is_dummy(head) && is_dummy(next) && get_node(next)->next == NULL) |
| 199 | return NULL; /* empty */ |
| 200 | /* |
| 201 | * We never, ever allow dequeue to get to a state where |
| 202 | * the queue is empty (we need at least one node in the |
| 203 | * queue). This is ensured by checking if the head next |
| 204 | * is NULL and retry in that case (this means a |
| 205 | * concurrent dummy node re-enqueue is in progress). |
| 206 | */ |
| 207 | if (next) { |
| 208 | if (uatomic_cmpxchg(&q->head, head, next) == head) { |
| 209 | if (is_dummy(head)) { |
| 210 | struct cds_lfq_node_rcu *node; |
| 211 | /* |
| 212 | * Requeue dummy. We need to |
| 213 | * reallocate to protect from |
| 214 | * ABA. |
| 215 | */ |
| 216 | rcu_free_dummy(head); |
| 217 | node = make_dummy(q, NULL); |
| 218 | _cds_lfq_enqueue_rcu(q, node); |
| 219 | continue; /* try again */ |
| 220 | } |
| 221 | return head; |
| 222 | } else { |
| 223 | /* Concurrently pushed, retry */ |
| 224 | continue; |
| 225 | } |
| 226 | } else { |
| 227 | /* Dummy node re-enqueue is in progress, retry. */ |
| 228 | continue; |
| 229 | } |
| 230 | } |
| 231 | } |
| 232 | |
| 233 | #ifdef __cplusplus |
| 234 | } |
| 235 | #endif |
| 236 | |
| 237 | #endif /* _URCU_RCULFQUEUE_STATIC_H */ |