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