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 static inline
  69 void _cds_lfs_push_rcu(struct cds_lfs_stack_rcu *s, struct cds_lfs_node_rcu *node)
  70 {
  71         struct cds_lfs_node_rcu *head = NULL;
  72
  73         for (;;) {
  74                 struct cds_lfs_node_rcu *old_head = head;
  75
  76                 node->next = head;
  77                 /*
  78                  * uatomic_cmpxchg() implicit memory barrier orders earlier
  79                  * stores to node before publication.
  80                  */
  81                 head = uatomic_cmpxchg(&s->head, old_head, node);
  82                 if (old_head == head)
  83                         break;
  84         }
  85 }
  86
  87 /*
  88  * Acts as a RCU reader.
  89  *
  90  * The caller must wait for a grace period to pass before freeing the returned
  91  * node or modifying the cds_lfs_node_rcu structure.
  92  * Returns NULL if stack is empty.
  93  */
  94 static inline
  95 struct cds_lfs_node_rcu *
  96 _cds_lfs_pop_rcu(struct cds_lfs_stack_rcu *s)
  97 {
  98         for (;;) {
  99                 struct cds_lfs_node_rcu *head;
 100
 101                 rcu_read_lock();
 102                 head = rcu_dereference(s->head);
 103                 if (head) {
 104                         struct cds_lfs_node_rcu *next = rcu_dereference(head->next);
 105
 106                         if (uatomic_cmpxchg(&s->head, head, next) == head) {
 107                                 rcu_read_unlock();
 108                                 return head;
 109                         } else {
 110                                 /* Concurrent modification. Retry. */
 111                                 rcu_read_unlock();
 112                                 continue;
 113                         }
 114                 } else {
 115                         /* Empty stack */
 116                         rcu_read_unlock();
 117                         return NULL;
 118                 }
 119         }
 120 }
 121
 122 #ifdef __cplusplus
 123 }
 124 #endif
 125
 126 #endif /* _URCU_RCULFSTACK_STATIC_H */