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 urcu/lfstack.h for
  12  * linking 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 <stdbool.h>
  30 #include <pthread.h>
  31 #include <assert.h>
  32 #include <urcu/uatomic.h>
  33 #include <urcu-pointer.h>
  34
  35 #ifdef __cplusplus
  36 extern "C" {
  37 #endif
  38
  39 /*
  40  * Lock-free stack.
  41  *
  42  * Stack implementing push, pop, pop_all operations, as well as iterator
  43  * on the stack head returned by pop_all.
  44  *
  45  * Synchronization table:
  46  *
  47  * External synchronization techniques described in the API below is
  48  * required between pairs marked with "X". No external synchronization
  49  * required between pairs marked with "-".
  50  *
  51  *                      cds_lfs_push  __cds_lfs_pop  __cds_lfs_pop_all
  52  * cds_lfs_push               -              -                  -
  53  * __cds_lfs_pop              -              X                  X
  54  * __cds_lfs_pop_all          -              X                  -
  55  *
  56  * cds_lfs_pop_blocking and cds_lfs_pop_all_blocking use an internal
  57  * mutex to provide synchronization.
  58  */
  59
  60 /*
  61  * cds_lfs_node_init: initialize lock-free stack node.
  62  */
  63 static inline
  64 void _cds_lfs_node_init(struct cds_lfs_node *node)
  65 {
  66 }
  67
  68 /*
  69  * cds_lfs_init: initialize lock-free stack.
  70  */
  71 static inline
  72 void _cds_lfs_init(struct cds_lfs_stack *s)
  73 {
  74         int ret;
  75
  76         s->head = NULL;
  77         ret = pthread_mutex_init(&s->lock, NULL);
  78         assert(!ret);
  79 }
  80
  81 /*
  82  * ___cds_lfs_init: initialize lock-free stack.
  83  */
  84 static inline
  85 void ___cds_lfs_init(struct __cds_lfs_stack *s)
  86 {
  87         s->head = NULL;
  88 }
  89
  90 static inline
  91 bool ___cds_lfs_empty_head(struct cds_lfs_head *head)
  92 {
  93         return head == NULL;
  94 }
  95
  96 /*
  97  * cds_lfs_empty: return whether lock-free stack is empty.
  98  *
  99  * No memory barrier is issued. No mutual exclusion is required.
 100  */
 101 static inline
 102 bool _cds_lfs_empty(cds_lfs_stack_ptr_t s)
 103 {
 104         return ___cds_lfs_empty_head(CMM_LOAD_SHARED(s._s->head));
 105 }
 106
 107 /*
 108  * cds_lfs_push: push a node into the stack.
 109  *
 110  * Does not require any synchronization with other push nor pop.
 111  *
 112  * Lock-free stack push is not subject to ABA problem, so no need to
 113  * take the RCU read-side lock. Even if "head" changes between two
 114  * uatomic_cmpxchg() invocations here (being popped, and then pushed
 115  * again by one or more concurrent threads), the second
 116  * uatomic_cmpxchg() invocation only cares about pushing a new entry at
 117  * the head of the stack, ensuring consistency by making sure the new
 118  * node->next is the same pointer value as the value replaced as head.
 119  * It does not care about the content of the actual next node, so it can
 120  * very well be reallocated between the two uatomic_cmpxchg().
 121  *
 122  * We take the approach of expecting the stack to be usually empty, so
 123  * we first try an initial uatomic_cmpxchg() on a NULL old_head, and
 124  * retry if the old head was non-NULL (the value read by the first
 125  * uatomic_cmpxchg() is used as old head for the following loop). The
 126  * upside of this scheme is to minimize the amount of cacheline traffic,
 127  * always performing an exclusive cacheline access, rather than doing
 128  * non-exclusive followed by exclusive cacheline access (which would be
 129  * required if we first read the old head value). This design decision
 130  * might be revisited after more thorough benchmarking on various
 131  * platforms.
 132  *
 133  * Returns 0 if the stack was empty prior to adding the node.
 134  * Returns non-zero otherwise.
 135  */
 136 static inline
 137 bool _cds_lfs_push(cds_lfs_stack_ptr_t u_s,
 138                   struct cds_lfs_node *node)
 139 {
 140         struct __cds_lfs_stack *s = u_s._s;
 141         struct cds_lfs_head *head = NULL;
 142         struct cds_lfs_head *new_head =
 143                 caa_container_of(node, struct cds_lfs_head, node);
 144
 145         for (;;) {
 146                 struct cds_lfs_head *old_head = head;
 147
 148                 /*
 149                  * node->next is still private at this point, no need to
 150                  * perform a _CMM_STORE_SHARED().
 151                  */
 152                 node->next = &head->node;
 153                 /*
 154                  * uatomic_cmpxchg() implicit memory barrier orders earlier
 155                  * stores to node before publication.
 156                  */
 157                 head = uatomic_cmpxchg(&s->head, old_head, new_head);
 158                 if (old_head == head)
 159                         break;
 160         }
 161         return !___cds_lfs_empty_head(head);
 162 }
 163
 164 /*
 165  * __cds_lfs_pop: pop a node from the stack.
 166  *
 167  * Returns NULL if stack is empty.
 168  *
 169  * __cds_lfs_pop needs to be synchronized using one of the following
 170  * techniques:
 171  *
 172  * 1) Calling __cds_lfs_pop under rcu read lock critical section. The
 173  *    caller must wait for a grace period to pass before freeing the
 174  *    returned node or modifying the cds_lfs_node structure.
 175  * 2) Using mutual exclusion (e.g. mutexes) to protect __cds_lfs_pop
 176  *    and __cds_lfs_pop_all callers.
 177  * 3) Ensuring that only ONE thread can call __cds_lfs_pop() and
 178  *    __cds_lfs_pop_all(). (multi-provider/single-consumer scheme).
 179  */
 180 static inline
 181 struct cds_lfs_node *___cds_lfs_pop(cds_lfs_stack_ptr_t u_s)
 182 {
 183         struct __cds_lfs_stack *s = u_s._s;
 184
 185         for (;;) {
 186                 struct cds_lfs_head *head, *next_head;
 187                 struct cds_lfs_node *next;
 188
 189                 head = _CMM_LOAD_SHARED(s->head);
 190                 if (___cds_lfs_empty_head(head))
 191                         return NULL;    /* Empty stack */
 192
 193                 /*
 194                  * Read head before head->next. Matches the implicit
 195                  * memory barrier before uatomic_cmpxchg() in
 196                  * cds_lfs_push.
 197                  */
 198                 cmm_smp_read_barrier_depends();
 199                 next = _CMM_LOAD_SHARED(head->node.next);
 200                 next_head = caa_container_of(next,
 201                                 struct cds_lfs_head, node);
 202                 if (uatomic_cmpxchg(&s->head, head, next_head) == head)
 203                         return &head->node;
 204                 /* busy-loop if head changed under us */
 205         }
 206 }
 207
 208 /*
 209  * __cds_lfs_pop_all: pop all nodes from a stack.
 210  *
 211  * __cds_lfs_pop_all does not require any synchronization with other
 212  * push, nor with other __cds_lfs_pop_all, but requires synchronization
 213  * matching the technique used to synchronize __cds_lfs_pop:
 214  *
 215  * 1) If __cds_lfs_pop is called under rcu read lock critical section,
 216  *    both __cds_lfs_pop and cds_lfs_pop_all callers must wait for a
 217  *    grace period to pass before freeing the returned node or modifying
 218  *    the cds_lfs_node structure. However, no RCU read-side critical
 219  *    section is needed around __cds_lfs_pop_all.
 220  * 2) Using mutual exclusion (e.g. mutexes) to protect __cds_lfs_pop and
 221  *    __cds_lfs_pop_all callers.
 222  * 3) Ensuring that only ONE thread can call __cds_lfs_pop() and
 223  *    __cds_lfs_pop_all(). (multi-provider/single-consumer scheme).
 224  */
 225 static inline
 226 struct cds_lfs_head *___cds_lfs_pop_all(cds_lfs_stack_ptr_t u_s)
 227 {
 228         struct __cds_lfs_stack *s = u_s._s;
 229
 230         /*
 231          * Implicit memory barrier after uatomic_xchg() matches implicit
 232          * memory barrier before uatomic_cmpxchg() in cds_lfs_push. It
 233          * ensures that all nodes of the returned list are consistent.
 234          * There is no need to issue memory barriers when iterating on
 235          * the returned list, because the full memory barrier issued
 236          * prior to each uatomic_cmpxchg, which each write to head, are
 237          * taking care to order writes to each node prior to the full
 238          * memory barrier after this uatomic_xchg().
 239          */
 240         return uatomic_xchg(&s->head, NULL);
 241 }
 242
 243 /*
 244  * cds_lfs_pop_lock: lock stack pop-protection mutex.
 245  */
 246 static inline void _cds_lfs_pop_lock(struct cds_lfs_stack *s)
 247 {
 248         int ret;
 249
 250         ret = pthread_mutex_lock(&s->lock);
 251         assert(!ret);
 252 }
 253
 254 /*
 255  * cds_lfs_pop_unlock: unlock stack pop-protection mutex.
 256  */
 257 static inline void _cds_lfs_pop_unlock(struct cds_lfs_stack *s)
 258 {
 259         int ret;
 260
 261         ret = pthread_mutex_unlock(&s->lock);
 262         assert(!ret);
 263 }
 264
 265 /*
 266  * Call __cds_lfs_pop with an internal pop mutex held.
 267  */
 268 static inline
 269 struct cds_lfs_node *
 270 _cds_lfs_pop_blocking(struct cds_lfs_stack *s)
 271 {
 272         struct cds_lfs_node *retnode;
 273
 274         _cds_lfs_pop_lock(s);
 275         retnode = ___cds_lfs_pop(s);
 276         _cds_lfs_pop_unlock(s);
 277         return retnode;
 278 }
 279
 280 /*
 281  * Call __cds_lfs_pop_all with an internal pop mutex held.
 282  */
 283 static inline
 284 struct cds_lfs_head *
 285 _cds_lfs_pop_all_blocking(struct cds_lfs_stack *s)
 286 {
 287         struct cds_lfs_head *rethead;
 288
 289         _cds_lfs_pop_lock(s);
 290         rethead = ___cds_lfs_pop_all(s);
 291         _cds_lfs_pop_unlock(s);
 292         return rethead;
 293 }
 294
 295 #ifdef __cplusplus
 296 }
 297 #endif
 298
 299 #endif /* _URCU_STATIC_LFSTACK_H */