urcu/static/wfstack.h

   1 #ifndef _URCU_STATIC_WFSTACK_H
   2 #define _URCU_STATIC_WFSTACK_H
   3
   4 /*
   5  * urcu/static/wfstack.h
   6  *
   7  * Userspace RCU library - Stack with with wait-free push, blocking traversal.
   8  *
   9  * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu/wfstack.h for
  10  * linking dynamically with the userspace rcu library.
  11  *
  12  * Copyright 2010-2012 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  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 <pthread.h>
  30 #include <assert.h>
  31 #include <poll.h>
  32 #include <stdbool.h>
  33 #include <urcu/compiler.h>
  34 #include <urcu/uatomic.h>
  35
  36 #ifdef __cplusplus
  37 extern "C" {
  38 #endif
  39
  40 #define CDS_WFS_END                     ((void *) 0x1UL)
  41 #define CDS_WFS_ADAPT_ATTEMPTS          10      /* Retry if being set */
  42 #define CDS_WFS_WAIT                    10      /* Wait 10 ms if being set */
  43
  44 /*
  45  * Stack with wait-free push, blocking traversal.
  46  *
  47  * Stack implementing push, pop, pop_all operations, as well as iterator
  48  * on the stack head returned by pop_all.
  49  *
  50  * Wait-free operations: cds_wfs_push, __cds_wfs_pop_all, cds_wfs_empty,
  51  *                       cds_wfs_first.
  52  * Blocking operations: cds_wfs_pop, cds_wfs_pop_all, cds_wfs_next,
  53  *                      iteration on stack head returned by pop_all.
  54  *
  55  * Synchronization table:
  56  *
  57  * External synchronization techniques described in the API below is
  58  * required between pairs marked with "X". No external synchronization
  59  * required between pairs marked with "-".
  60  *
  61  *                      cds_wfs_push  __cds_wfs_pop  __cds_wfs_pop_all
  62  * cds_wfs_push               -              -                  -
  63  * __cds_wfs_pop              -              X                  X
  64  * __cds_wfs_pop_all          -              X                  -
  65  *
  66  * cds_wfs_pop and cds_wfs_pop_all use an internal mutex to provide
  67  * synchronization.
  68  */
  69
  70 /*
  71  * cds_wfs_node_init: initialize wait-free stack node.
  72  */
  73 static inline
  74 void _cds_wfs_node_init(struct cds_wfs_node *node)
  75 {
  76         node->next = NULL;
  77 }
  78
  79 /*
  80  * cds_wfs_init: initialize wait-free stack.
  81  */
  82 static inline
  83 void _cds_wfs_init(struct cds_wfs_stack *s)
  84 {
  85         int ret;
  86
  87         s->head = CDS_WFS_END;
  88         ret = pthread_mutex_init(&s->lock, NULL);
  89         assert(!ret);
  90 }
  91
  92 static inline bool ___cds_wfs_end(void *node)
  93 {
  94         return node == CDS_WFS_END;
  95 }
  96
  97 /*
  98  * cds_wfs_empty: return whether wait-free stack is empty.
  99  *
 100  * No memory barrier is issued. No mutual exclusion is required.
 101  */
 102 static inline bool _cds_wfs_empty(struct cds_wfs_stack *s)
 103 {
 104         return ___cds_wfs_end(CMM_LOAD_SHARED(s->head));
 105 }
 106
 107 /*
 108  * cds_wfs_push: push a node into the stack.
 109  *
 110  * Issues a full memory barrier before push. No mutual exclusion is
 111  * required.
 112  *
 113  * Returns 0 if the stack was empty prior to adding the node.
 114  * Returns non-zero otherwise.
 115  */
 116 static inline
 117 int _cds_wfs_push(struct cds_wfs_stack *s, struct cds_wfs_node *node)
 118 {
 119         struct cds_wfs_head *old_head, *new_head;
 120
 121         assert(node->next == NULL);
 122         new_head = caa_container_of(node, struct cds_wfs_head, node);
 123         /*
 124          * uatomic_xchg() implicit memory barrier orders earlier stores
 125          * to node (setting it to NULL) before publication.
 126          */
 127         old_head = uatomic_xchg(&s->head, new_head);
 128         /*
 129          * At this point, dequeuers see a NULL node->next, they should
 130          * busy-wait until node->next is set to old_head.
 131          */
 132         CMM_STORE_SHARED(node->next, &old_head->node);
 133         return !___cds_wfs_end(old_head);
 134 }
 135
 136 /*
 137  * Waiting for push to complete enqueue and return the next node.
 138  */
 139 static inline struct cds_wfs_node *
 140 ___cds_wfs_node_sync_next(struct cds_wfs_node *node)
 141 {
 142         struct cds_wfs_node *next;
 143         int attempt = 0;
 144
 145         /*
 146          * Adaptative busy-looping waiting for push to complete.
 147          */
 148         while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
 149                 if (++attempt >= CDS_WFS_ADAPT_ATTEMPTS) {
 150                         poll(NULL, 0, CDS_WFS_WAIT);    /* Wait for 10ms */
 151                         attempt = 0;
 152                 } else {
 153                         caa_cpu_relax();
 154                 }
 155         }
 156
 157         return next;
 158 }
 159
 160 /*
 161  * __cds_wfs_pop_blocking: pop a node from the stack.
 162  *
 163  * Returns NULL if stack is empty.
 164  *
 165  * __cds_wfs_pop_blocking needs to be synchronized using one of the
 166  * following techniques:
 167  *
 168  * 1) Calling __cds_wfs_pop_blocking under rcu read lock critical
 169  *    section. The caller must wait for a grace period to pass before
 170  *    freeing the returned node or modifying the cds_wfs_node structure.
 171  * 2) Using mutual exclusion (e.g. mutexes) to protect
 172  *     __cds_wfs_pop_blocking and __cds_wfs_pop_all callers.
 173  * 3) Ensuring that only ONE thread can call __cds_wfs_pop_blocking()
 174  *    and __cds_wfs_pop_all(). (multi-provider/single-consumer scheme).
 175  */
 176 static inline
 177 struct cds_wfs_node *
 178 ___cds_wfs_pop_blocking(struct cds_wfs_stack *s)
 179 {
 180         struct cds_wfs_head *head, *new_head;
 181         struct cds_wfs_node *next;
 182
 183         for (;;) {
 184                 head = CMM_LOAD_SHARED(s->head);
 185                 if (___cds_wfs_end(head))
 186                         return NULL;
 187                 next = ___cds_wfs_node_sync_next(&head->node);
 188                 new_head = caa_container_of(next, struct cds_wfs_head, node);
 189                 if (uatomic_cmpxchg(&s->head, head, new_head) == head)
 190                         return &head->node;
 191                 /* busy-loop if head changed under us */
 192         }
 193 }
 194
 195 /*
 196  * __cds_wfs_pop_all: pop all nodes from a stack.
 197  *
 198  * __cds_wfs_pop_all does not require any synchronization with other
 199  * push, nor with other __cds_wfs_pop_all, but requires synchronization
 200  * matching the technique used to synchronize __cds_wfs_pop_blocking:
 201  *
 202  * 1) If __cds_wfs_pop_blocking is called under rcu read lock critical
 203  *    section, both __cds_wfs_pop_blocking and cds_wfs_pop_all callers
 204  *    must wait for a grace period to pass before freeing the returned
 205  *    node or modifying the cds_wfs_node structure. However, no RCU
 206  *    read-side critical section is needed around __cds_wfs_pop_all.
 207  * 2) Using mutual exclusion (e.g. mutexes) to protect
 208  *     __cds_wfs_pop_blocking and __cds_wfs_pop_all callers.
 209  * 3) Ensuring that only ONE thread can call __cds_wfs_pop_blocking()
 210  *    and __cds_wfs_pop_all(). (multi-provider/single-consumer scheme).
 211  */
 212 static inline
 213 struct cds_wfs_head *
 214 ___cds_wfs_pop_all(struct cds_wfs_stack *s)
 215 {
 216         struct cds_wfs_head *head;
 217
 218         /*
 219          * Implicit memory barrier after uatomic_xchg() matches implicit
 220          * memory barrier before uatomic_xchg() in cds_wfs_push. It
 221          * ensures that all nodes of the returned list are consistent.
 222          * There is no need to issue memory barriers when iterating on
 223          * the returned list, because the full memory barrier issued
 224          * prior to each uatomic_cmpxchg, which each write to head, are
 225          * taking care to order writes to each node prior to the full
 226          * memory barrier after this uatomic_xchg().
 227          */
 228         head = uatomic_xchg(&s->head, CDS_WFS_END);
 229         if (___cds_wfs_end(head))
 230                 return NULL;
 231         return head;
 232 }
 233
 234 /*
 235  * cds_wfs_pop_lock: lock stack pop-protection mutex.
 236  */
 237 static inline void _cds_wfs_pop_lock(struct cds_wfs_stack *s)
 238 {
 239         int ret;
 240
 241         ret = pthread_mutex_lock(&s->lock);
 242         assert(!ret);
 243 }
 244
 245 /*
 246  * cds_wfs_pop_unlock: unlock stack pop-protection mutex.
 247  */
 248 static inline void _cds_wfs_pop_unlock(struct cds_wfs_stack *s)
 249 {
 250         int ret;
 251
 252         ret = pthread_mutex_unlock(&s->lock);
 253         assert(!ret);
 254 }
 255
 256 /*
 257  * Call __cds_wfs_pop_blocking with an internal pop mutex held.
 258  */
 259 static inline
 260 struct cds_wfs_node *
 261 _cds_wfs_pop_blocking(struct cds_wfs_stack *s)
 262 {
 263         struct cds_wfs_node *retnode;
 264
 265         _cds_wfs_pop_lock(s);
 266         retnode = ___cds_wfs_pop_blocking(s);
 267         _cds_wfs_pop_unlock(s);
 268         return retnode;
 269 }
 270
 271 /*
 272  * Call __cds_wfs_pop_all with an internal pop mutex held.
 273  */
 274 static inline
 275 struct cds_wfs_head *
 276 _cds_wfs_pop_all_blocking(struct cds_wfs_stack *s)
 277 {
 278         struct cds_wfs_head *rethead;
 279
 280         _cds_wfs_pop_lock(s);
 281         rethead = ___cds_wfs_pop_all(s);
 282         _cds_wfs_pop_unlock(s);
 283         return rethead;
 284 }
 285
 286 /*
 287  * cds_wfs_first: get first node of a popped stack.
 288  *
 289  * Content written into the node before enqueue is guaranteed to be
 290  * consistent, but no other memory ordering is ensured.
 291  *
 292  * Used by for-like iteration macros in urcu/wfstack.h:
 293  * cds_wfs_for_each_blocking()
 294  * cds_wfs_for_each_blocking_safe()
 295  *
 296  * Returns NULL if popped stack is empty, top stack node otherwise.
 297  */
 298 static inline struct cds_wfs_node *
 299 _cds_wfs_first(struct cds_wfs_head *head)
 300 {
 301         if (___cds_wfs_end(head))
 302                 return NULL;
 303         return &head->node;
 304 }
 305
 306 /*
 307  * cds_wfs_next_blocking: get next node of a popped stack.
 308  *
 309  * Content written into the node before enqueue is guaranteed to be
 310  * consistent, but no other memory ordering is ensured.
 311  *
 312  * Used by for-like iteration macros in urcu/wfstack.h:
 313  * cds_wfs_for_each_blocking()
 314  * cds_wfs_for_each_blocking_safe()
 315  *
 316  * Returns NULL if reached end of popped stack, non-NULL next stack
 317  * node otherwise.
 318  */
 319 static inline struct cds_wfs_node *
 320 _cds_wfs_next_blocking(struct cds_wfs_node *node)
 321 {
 322         struct cds_wfs_node *next;
 323
 324         next = ___cds_wfs_node_sync_next(node);
 325         if (___cds_wfs_end(next))
 326                 return NULL;
 327         return next;
 328 }
 329
 330 #ifdef __cplusplus
 331 }
 332 #endif
 333
 334 #endif /* _URCU_STATIC_WFSTACK_H */