urcu/static/rculfstack.h

   1 #ifndef _URCU_RCULFSTACK_STATIC_H
   2 #define _URCU_RCULFSTACK_STATIC_H
   3
   4 /*
   5  * rculfstack-static.h
   6  *
   7  * Userspace RCU library - Lock-Free RCU Stack
   8  *
   9  * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  10  *
  11  * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See rculfstack.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/uatomic.h>
  30
  31 #ifdef __cplusplus
  32 extern "C" {
  33 #endif
  34
  35 static inline
  36 void _cds_lfs_node_init_rcu(struct cds_lfs_node_rcu *node)
  37 {
  38 }
  39
  40 static inline
  41 void _cds_lfs_init_rcu(struct cds_lfs_stack_rcu *s)
  42 {
  43         s->head = NULL;
  44 }
  45
  46 /*
  47  * Lock-free stack push is not subject to ABA problem, so no need to
  48  * take the RCU read-side lock. Even if "head" changes between two
  49  * uatomic_cmpxchg() invocations here (being popped, and then pushed
  50  * again by one or more concurrent threads), the second
  51  * uatomic_cmpxchg() invocation only cares about pushing a new entry at
  52  * the head of the stack, ensuring consistency by making sure the new
  53  * node->next is the same pointer value as the value replaced as head.
  54  * It does not care about the content of the actual next node, so it can
  55  * very well be reallocated between the two uatomic_cmpxchg().
  56  *
  57  * We take the approach of expecting the stack to be usually empty, so
  58  * we first try an initial uatomic_cmpxchg() on a NULL old_head, and
  59  * retry if the old head was non-NULL (the value read by the first
  60  * uatomic_cmpxchg() is used as old head for the following loop). The
  61  * upside of this scheme is to minimize the amount of cacheline traffic,
  62  * always performing an exclusive cacheline access, rather than doing
  63  * non-exclusive followed by exclusive cacheline access (which would be
  64  * required if we first read the old head value). This design decision
  65  * might be revisited after more throrough benchmarking on various
  66  * platforms.
  67  *
  68  * Returns 0 if the stack was empty prior to adding the node.
  69  * Returns non-zero otherwise.
  70  */
  71 static inline
  72 int _cds_lfs_push_rcu(struct cds_lfs_stack_rcu *s,
  73                           struct cds_lfs_node_rcu *node)
  74 {
  75         struct cds_lfs_node_rcu *head = NULL;
  76
  77         for (;;) {
  78                 struct cds_lfs_node_rcu *old_head = head;
  79
  80                 node->next = head;
  81                 /*
  82                  * uatomic_cmpxchg() implicit memory barrier orders earlier
  83                  * stores to node before publication.
  84                  */
  85                 head = uatomic_cmpxchg(&s->head, old_head, node);
  86                 if (old_head == head)
  87                         break;
  88         }
  89         return (int) !!((unsigned long) head);
  90 }
  91
  92 /*
  93  * Should be called under rcu read-side lock.
  94  *
  95  * The caller must wait for a grace period to pass before freeing the returned
  96  * node or modifying the cds_lfs_node_rcu structure.
  97  * Returns NULL if stack is empty.
  98  */
  99 static inline
 100 struct cds_lfs_node_rcu *
 101 _cds_lfs_pop_rcu(struct cds_lfs_stack_rcu *s)
 102 {
 103         for (;;) {
 104                 struct cds_lfs_node_rcu *head;
 105
 106                 head = rcu_dereference(s->head);
 107                 if (head) {
 108                         struct cds_lfs_node_rcu *next = rcu_dereference(head->next);
 109
 110                         if (uatomic_cmpxchg(&s->head, head, next) == head) {
 111                                 return head;
 112                         } else {
 113                                 /* Concurrent modification. Retry. */
 114                                 continue;
 115                         }
 116                 } else {
 117                         /* Empty stack */
 118                         return NULL;
 119                 }
 120         }
 121 }
 122
 123 #ifdef __cplusplus
 124 }
 125 #endif
 126
 127 #endif /* _URCU_RCULFSTACK_STATIC_H */