| 1 | /* |
| 2 | * rculfqueue.h |
| 3 | * |
| 4 | * Userspace RCU library - Lock-Free RCU Queue |
| 5 | * |
| 6 | * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com> |
| 7 | * |
| 8 | * This library is free software; you can redistribute it and/or |
| 9 | * modify it under the terms of the GNU Lesser General Public |
| 10 | * License as published by the Free Software Foundation; either |
| 11 | * version 2.1 of the License, or (at your option) any later version. |
| 12 | * |
| 13 | * This library is distributed in the hope that it will be useful, |
| 14 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 15 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| 16 | * Lesser General Public License for more details. |
| 17 | * |
| 18 | * You should have received a copy of the GNU Lesser General Public |
| 19 | * License along with this library; if not, write to the Free Software |
| 20 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA |
| 21 | */ |
| 22 | |
| 23 | #include <urcu/urcu_ref.h> |
| 24 | #include <assert.h> |
| 25 | |
| 26 | #if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE)) |
| 27 | #error "Dynamic loader LGPL wrappers not implemented yet" |
| 28 | #endif |
| 29 | |
| 30 | /* |
| 31 | * Lock-free RCU queue using reference counting. Enqueue and dequeue operations |
| 32 | * hold a RCU read lock to deal with cmpxchg ABA problem. This implementation |
| 33 | * keeps a dummy head node to ensure we can always update the queue locklessly. |
| 34 | * Given that this is a queue, the dummy head node must always advance as we |
| 35 | * dequeue entries. Therefore, we keep a reference count on each entry we are |
| 36 | * dequeueing, so they can be kept as dummy head node until the next dequeue, at |
| 37 | * which point their reference count will be decremented. |
| 38 | */ |
| 39 | |
| 40 | #define URCU_LFQ_PERMANENT_REF 128 |
| 41 | |
| 42 | struct rcu_lfq_node { |
| 43 | struct rcu_lfq_node *next; |
| 44 | struct urcu_ref ref; |
| 45 | }; |
| 46 | |
| 47 | struct rcu_lfq_queue { |
| 48 | struct rcu_lfq_node *head, *tail; |
| 49 | struct rcu_lfq_node init; /* Dummy initialization node */ |
| 50 | }; |
| 51 | |
| 52 | void rcu_lfq_node_init(struct rcu_lfq_node *node) |
| 53 | { |
| 54 | node->next = NULL; |
| 55 | urcu_ref_init(&node->ref); |
| 56 | } |
| 57 | |
| 58 | void rcu_lfq_init(struct rcu_lfq_queue *q) |
| 59 | { |
| 60 | rcu_lfq_node_init(&q->init); |
| 61 | /* Make sure the initial node is never freed. */ |
| 62 | urcu_ref_set(&q->init.ref, URCU_LFQ_PERMANENT_REF); |
| 63 | q->head = q->tail = &q->init; |
| 64 | } |
| 65 | |
| 66 | void rcu_lfq_enqueue(struct rcu_lfq_queue *q, struct rcu_lfq_node *node) |
| 67 | { |
| 68 | urcu_ref_get(&node->ref); |
| 69 | |
| 70 | /* |
| 71 | * uatomic_cmpxchg() implicit memory barrier orders earlier stores to |
| 72 | * node before publication. |
| 73 | */ |
| 74 | |
| 75 | rcu_read_lock(); |
| 76 | for (;;) { |
| 77 | struct rcu_lfq_node *tail = rcu_dereference(q->tail); |
| 78 | struct rcu_lfq_node *next; |
| 79 | |
| 80 | /* |
| 81 | * Typically expect tail->next to be NULL. |
| 82 | */ |
| 83 | next = uatomic_cmpxchg(&tail->next, NULL, node); |
| 84 | if (next == NULL) { |
| 85 | /* |
| 86 | * Tail was at the end of queue, we successfully |
| 87 | * appended to it. |
| 88 | * Now move tail (another enqueue might beat |
| 89 | * us to it, that's fine). |
| 90 | */ |
| 91 | uatomic_cmpxchg(&q->tail, tail, node); |
| 92 | rcu_read_unlock(); |
| 93 | return; |
| 94 | } else { |
| 95 | /* |
| 96 | * Failure to append to current tail. Help moving tail |
| 97 | * further and retry. |
| 98 | */ |
| 99 | uatomic_cmpxchg(&q->tail, tail, next); |
| 100 | continue; |
| 101 | } |
| 102 | } |
| 103 | } |
| 104 | |
| 105 | /* |
| 106 | * The entry returned by dequeue must be taken care of by doing a urcu_ref_put, |
| 107 | * which calls the release primitive when the reference count drops to zero. A |
| 108 | * grace period must be waited before performing the actual memory reclamation |
| 109 | * in the release primitive. |
| 110 | * The entry lfq node returned by dequeue must no be re-used before the |
| 111 | * reference count reaches zero. |
| 112 | */ |
| 113 | struct rcu_lfq_node * |
| 114 | rcu_lfq_dequeue(struct rcu_lfq_queue *q, void (*release)(struct urcu_ref *)) |
| 115 | { |
| 116 | rcu_read_lock(); |
| 117 | for (;;) { |
| 118 | struct rcu_lfq_node *head = rcu_dereference(q->head); |
| 119 | struct rcu_lfq_node *next = rcu_dereference(head->next); |
| 120 | |
| 121 | if (next) { |
| 122 | if (uatomic_cmpxchg(&q->head, head, next) == head) { |
| 123 | rcu_read_unlock(); |
| 124 | urcu_ref_put(&head->ref, release); |
| 125 | return next; |
| 126 | } else { |
| 127 | /* Concurrently pushed, retry */ |
| 128 | continue; |
| 129 | } |
| 130 | } else { |
| 131 | /* Empty */ |
| 132 | rcu_read_unlock(); |
| 133 | return NULL; |
| 134 | } |
| 135 | } |
| 136 | } |