urcu/static/lfstack.h

   1 #ifndef _URCU_STATIC_LFSTACK_H
   2 #define _URCU_STATIC_LFSTACK_H
   3
   4 /*
   5  * urcu/static/lfstack.h
   6  *
   7  * Userspace RCU library - Lock-Free Stack
   8  *
   9  * Copyright 2010-2012 - 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(struct cds_lfs_node *node)
  38 {
  39 }
  40
  41 static inline
  42 void _cds_lfs_init(struct cds_lfs_stack *s)
  43 {
  44         s->head = NULL;
  45 }
  46
  47 /*
  48  * cds_lfs_push: push a node into the stack.
  49  *
  50  * Does not require any synchronization with other push nor pop.
  51  *
  52  * Lock-free stack push is not subject to ABA problem, so no need to
  53  * take the RCU read-side lock. Even if "head" changes between two
  54  * uatomic_cmpxchg() invocations here (being popped, and then pushed
  55  * again by one or more concurrent threads), the second
  56  * uatomic_cmpxchg() invocation only cares about pushing a new entry at
  57  * the head of the stack, ensuring consistency by making sure the new
  58  * node->next is the same pointer value as the value replaced as head.
  59  * It does not care about the content of the actual next node, so it can
  60  * very well be reallocated between the two uatomic_cmpxchg().
  61  *
  62  * We take the approach of expecting the stack to be usually empty, so
  63  * we first try an initial uatomic_cmpxchg() on a NULL old_head, and
  64  * retry if the old head was non-NULL (the value read by the first
  65  * uatomic_cmpxchg() is used as old head for the following loop). The
  66  * upside of this scheme is to minimize the amount of cacheline traffic,
  67  * always performing an exclusive cacheline access, rather than doing
  68  * non-exclusive followed by exclusive cacheline access (which would be
  69  * required if we first read the old head value). This design decision
  70  * might be revisited after more throrough benchmarking on various
  71  * platforms.
  72  *
  73  * Returns 0 if the stack was empty prior to adding the node.
  74  * Returns non-zero otherwise.
  75  */
  76 static inline
  77 int _cds_lfs_push(struct cds_lfs_stack *s,
  78                   struct cds_lfs_node *node)
  79 {
  80         struct cds_lfs_node *head = NULL;
  81
  82         for (;;) {
  83                 struct cds_lfs_node *old_head = head;
  84
  85                 /*
  86                  * node->next is still private at this point, no need to
  87                  * perform a _CMM_STORE_SHARED().
  88                  */
  89                 node->next = head;
  90                 /*
  91                  * uatomic_cmpxchg() implicit memory barrier orders earlier
  92                  * stores to node before publication.
  93                  */
  94                 head = uatomic_cmpxchg(&s->head, old_head, node);
  95                 if (old_head == head)
  96                         break;
  97         }
  98         return (int) !!((unsigned long) head);
  99 }
 100
 101 /*
 102  * cds_lfs_pop: pop a node from the stack.
 103  *
 104  * Returns NULL if stack is empty.
 105  *
 106  * cds_lfs_pop needs to be synchronized using one of the following
 107  * techniques:
 108  *
 109  * 1) Calling cds_lfs_pop under rcu read lock critical section. The
 110  *    caller must wait for a grace period to pass before freeing the
 111  *    returned node or modifying the cds_lfs_node structure.
 112  * 2) Using mutual exclusion (e.g. mutexes) to protect cds_lfs_pop
 113  *    callers.
 114  * 3) Ensuring that only ONE thread can call cds_lfs_pop().
 115  *    (multi-provider/single-consumer scheme).
 116  */
 117 static inline
 118 struct cds_lfs_node *_cds_lfs_pop(struct cds_lfs_stack *s)
 119 {
 120         for (;;) {
 121                 struct cds_lfs_node *head;
 122
 123                 head = _CMM_LOAD_SHARED(s->head);
 124                 if (head) {
 125                         struct cds_lfs_node *next;
 126
 127                         /*
 128                          * Read head before head->next. Matches the
 129                          * implicit memory barrier before
 130                          * uatomic_cmpxchg() in cds_lfs_push.
 131                          */
 132                         cmm_smp_read_barrier_depends();
 133                         next = _CMM_LOAD_SHARED(head->next);
 134                         if (uatomic_cmpxchg(&s->head, head, next) == head) {
 135                                 return head;
 136                         } else {
 137                                 /* Concurrent modification. Retry. */
 138                                 continue;
 139                         }
 140                 } else {
 141                         /* Empty stack */
 142                         return NULL;
 143                 }
 144         }
 145 }
 146
 147 #ifdef __cplusplus
 148 }
 149 #endif
 150
 151 #endif /* _URCU_STATIC_LFSTACK_H */