| 1 | /* |
| 2 | * rcuwfqueue.h |
| 3 | * |
| 4 | * Userspace RCU library - RCU Queue with Wait-Free Enqueue/Blocking Dequeue |
| 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 | * RCU queue with wait-free enqueue/blocking dequeue using reference counting. |
| 32 | * Enqueue and dequeue operations hold a RCU read lock to deal with cmpxchg ABA |
| 33 | * problem. This implementation keeps a dummy head node to ensure we can always |
| 34 | * update the queue locklessly. Given that this is a queue, the dummy head node |
| 35 | * must always advance as we dequeue entries. Therefore, we keep a reference |
| 36 | * count on each entry we are dequeueing, so they can be kept as dummy head node |
| 37 | * until the next dequeue, at which point their reference count will be |
| 38 | * decremented. |
| 39 | */ |
| 40 | |
| 41 | #define URCU_WFQ_PERMANENT_REF 128 |
| 42 | |
| 43 | struct rcu_wfq_node { |
| 44 | struct rcu_wfq_node *next; |
| 45 | struct urcu_ref ref; |
| 46 | }; |
| 47 | |
| 48 | struct rcu_wfq_queue { |
| 49 | struct rcu_wfq_node *head, *tail; |
| 50 | struct rcu_wfq_node init; /* Dummy initialization node */ |
| 51 | }; |
| 52 | |
| 53 | void rcu_wfq_node_init(struct rcu_wfq_node *node) |
| 54 | { |
| 55 | node->next = NULL; |
| 56 | urcu_ref_init(&node->ref); |
| 57 | } |
| 58 | |
| 59 | void rcu_wfq_init(struct rcu_wfq_queue *q) |
| 60 | { |
| 61 | rcu_wfq_node_init(&q->init); |
| 62 | /* Make sure the initial node is never freed. */ |
| 63 | urcu_ref_set(&q->init.ref, URCU_WFQ_PERMANENT_REF); |
| 64 | /* Set queue end */ |
| 65 | q->head = q->tail = &q->init; |
| 66 | } |
| 67 | |
| 68 | void rcu_wfq_enqueue(struct rcu_wfq_queue *q, struct rcu_wfq_node *node) |
| 69 | { |
| 70 | struct rcu_wfq_node *old_tail; |
| 71 | |
| 72 | urcu_ref_get(&node->ref); |
| 73 | /* |
| 74 | * uatomic_xchg() implicit memory barrier orders earlier stores to node |
| 75 | * (setting it to NULL and incrementing the refcount) before |
| 76 | * publication. |
| 77 | */ |
| 78 | old_tail = uatomic_xchg(&q->tail, node); |
| 79 | /* |
| 80 | * At this point, dequeuers see a NULL old_tail->next, which indicates |
| 81 | * end of queue. The following store will append "node" to the queue |
| 82 | * from a dequeuer perspective. |
| 83 | */ |
| 84 | STORE_SHARED(old_tail->next, node); |
| 85 | } |
| 86 | |
| 87 | /* |
| 88 | * The entry returned by dequeue must be taken care of by doing a urcu_ref_put, |
| 89 | * which calls the release primitive when the reference count drops to zero. A |
| 90 | * grace period must be waited before performing the actual memory reclamation |
| 91 | * in the release primitive. The wfq node returned by dequeue must not be |
| 92 | * modified/re-used/freed until the reference count reaches zero and a grace |
| 93 | * period has elapsed (after the refcount reached 0). |
| 94 | * |
| 95 | * TODO: implement adaptative busy-wait and wait/wakeup scheme rather than busy |
| 96 | * loops. Better for UP. |
| 97 | */ |
| 98 | struct rcu_wfq_node * |
| 99 | rcu_wfq_dequeue_blocking(struct rcu_wfq_queue *q, |
| 100 | void (*release)(struct urcu_ref *)) |
| 101 | { |
| 102 | for (;;) { |
| 103 | struct rcu_wfq_node *head, *next; |
| 104 | |
| 105 | rcu_read_lock(); |
| 106 | head = rcu_dereference(q->head); |
| 107 | next = rcu_dereference(head->next); |
| 108 | if (next) { |
| 109 | if (uatomic_cmpxchg(&q->head, head, next) == head) { |
| 110 | rcu_read_unlock(); |
| 111 | urcu_ref_put(&head->ref, release); |
| 112 | return next; |
| 113 | } else { |
| 114 | /* Concurrently pushed, retry */ |
| 115 | rcu_read_unlock(); |
| 116 | continue; |
| 117 | } |
| 118 | } else { |
| 119 | /* Empty */ |
| 120 | rcu_read_unlock(); |
| 121 | return NULL; |
| 122 | } |
| 123 | } |
| 124 | } |