| 1 | #ifndef _URCU_WFQUEUE_H |
| 2 | #define _URCU_WFQUEUE_H |
| 3 | |
| 4 | /* |
| 5 | * wfqueue.h |
| 6 | * |
| 7 | * Userspace RCU library - Queue with Wait-Free Enqueue/Blocking Dequeue |
| 8 | * |
| 9 | * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com> |
| 10 | * |
| 11 | * This library is free software; you can redistribute it and/or |
| 12 | * modify it under the terms of the GNU Lesser General Public |
| 13 | * License as published by the Free Software Foundation; either |
| 14 | * version 2.1 of the License, or (at your option) any later version. |
| 15 | * |
| 16 | * This library is distributed in the hope that it will be useful, |
| 17 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 18 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| 19 | * Lesser General Public License for more details. |
| 20 | * |
| 21 | * You should have received a copy of the GNU Lesser General Public |
| 22 | * License along with this library; if not, write to the Free Software |
| 23 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA |
| 24 | */ |
| 25 | |
| 26 | #include <pthread.h> |
| 27 | #include <assert.h> |
| 28 | #include <urcu/compiler.h> |
| 29 | |
| 30 | #ifdef __cplusplus |
| 31 | extern "C" { |
| 32 | #endif |
| 33 | |
| 34 | #if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE)) |
| 35 | #error "Dynamic loader LGPL wrappers not implemented yet" |
| 36 | #endif |
| 37 | |
| 38 | /* |
| 39 | * Queue with wait-free enqueue/blocking dequeue. |
| 40 | * This implementation adds a dummy head node when the queue is empty to ensure |
| 41 | * we can always update the queue locklessly. |
| 42 | * |
| 43 | * Inspired from half-wait-free/half-blocking queue implementation done by |
| 44 | * Paul E. McKenney. |
| 45 | */ |
| 46 | |
| 47 | struct wfq_node { |
| 48 | struct wfq_node *next; |
| 49 | }; |
| 50 | |
| 51 | struct wfq_queue { |
| 52 | struct wfq_node *head, **tail; |
| 53 | struct wfq_node dummy; /* Dummy node */ |
| 54 | pthread_mutex_t lock; |
| 55 | }; |
| 56 | |
| 57 | void wfq_node_init(struct wfq_node *node) |
| 58 | { |
| 59 | node->next = NULL; |
| 60 | } |
| 61 | |
| 62 | void wfq_init(struct wfq_queue *q) |
| 63 | { |
| 64 | int ret; |
| 65 | |
| 66 | wfq_node_init(&q->dummy); |
| 67 | /* Set queue head and tail */ |
| 68 | q->head = &q->dummy; |
| 69 | q->tail = &q->dummy.next; |
| 70 | ret = pthread_mutex_init(&q->lock, NULL); |
| 71 | assert(!ret); |
| 72 | } |
| 73 | |
| 74 | void wfq_enqueue(struct wfq_queue *q, struct wfq_node *node) |
| 75 | { |
| 76 | struct wfq_node **old_tail; |
| 77 | |
| 78 | /* |
| 79 | * uatomic_xchg() implicit memory barrier orders earlier stores to data |
| 80 | * structure containing node and setting node->next to NULL before |
| 81 | * publication. |
| 82 | */ |
| 83 | old_tail = uatomic_xchg(&q->tail, node); |
| 84 | /* |
| 85 | * At this point, dequeuers see a NULL old_tail->next, which indicates |
| 86 | * that the queue is being appended to. The following store will append |
| 87 | * "node" to the queue from a dequeuer perspective. |
| 88 | */ |
| 89 | STORE_SHARED(*old_tail, node); |
| 90 | } |
| 91 | |
| 92 | /* |
| 93 | * It is valid to reuse and free a dequeued node immediately. |
| 94 | * |
| 95 | * No need to go on a waitqueue here, as there is no possible state in which the |
| 96 | * list could cause dequeue to busy-loop needlessly while waiting for another |
| 97 | * thread to be scheduled. The queue appears empty until tail->next is set by |
| 98 | * enqueue. |
| 99 | */ |
| 100 | struct wfq_node * |
| 101 | __wfq_dequeue_blocking(struct wfq_queue *q) |
| 102 | { |
| 103 | struct wfq_node *node, *next; |
| 104 | int busy_wait = 16; |
| 105 | |
| 106 | /* |
| 107 | * Queue is empty if it only contains the dummy node. |
| 108 | */ |
| 109 | if (q->head == &q->dummy && LOAD_SHARED(q->tail) == &q->dummy.next) |
| 110 | return NULL; |
| 111 | node = q->head; |
| 112 | |
| 113 | /* |
| 114 | * Adaptative busy-looping waiting for enqueuer to complete enqueue. |
| 115 | */ |
| 116 | while ((next = LOAD_SHARED(node->next)) == NULL) { |
| 117 | if (busy_wait > 0) { |
| 118 | cpu_relax(); |
| 119 | busy_wait--; |
| 120 | } else |
| 121 | poll(NULL, 0, 1); /* Wait for 1ms */ |
| 122 | } |
| 123 | /* |
| 124 | * Move queue head forward. |
| 125 | */ |
| 126 | q->head = next; |
| 127 | /* |
| 128 | * Requeue dummy node if we just dequeued it. |
| 129 | */ |
| 130 | if (node == &q->dummy) { |
| 131 | wfq_node_init(node); |
| 132 | wfq_enqueue(q, node); |
| 133 | return __wfq_dequeue_blocking(q); |
| 134 | } |
| 135 | return node; |
| 136 | } |
| 137 | |
| 138 | struct wfq_node * |
| 139 | wfq_dequeue_blocking(struct wfq_queue *q) |
| 140 | { |
| 141 | struct wfq_node *retnode; |
| 142 | int ret; |
| 143 | |
| 144 | ret = pthread_mutex_lock(&q->lock); |
| 145 | assert(!ret); |
| 146 | retnode = __wfq_dequeue_blocking(q); |
| 147 | ret = pthread_mutex_unlock(&q->lock); |
| 148 | assert(!ret); |
| 149 | return retnode; |
| 150 | } |
| 151 | |
| 152 | #ifdef __cplusplus |
| 153 | } |
| 154 | #endif |
| 155 | |
| 156 | #endif /* _URCU_WFQUEUE_H */ |