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, int blocking)
 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 (!blocking)
 150                         return CDS_WFS_WOULDBLOCK;
 151                 if (++attempt >= CDS_WFS_ADAPT_ATTEMPTS) {
 152                         poll(NULL, 0, CDS_WFS_WAIT);    /* Wait for 10ms */
 153                         attempt = 0;
 154                 } else {
 155                         caa_cpu_relax();
 156                 }
 157         }
 158
 159         return next;
 160 }
 161
 162 static inline
 163 struct cds_wfs_node *
 164 ___cds_wfs_pop(struct cds_wfs_stack *s, int blocking)
 165 {
 166         struct cds_wfs_head *head, *new_head;
 167         struct cds_wfs_node *next;
 168
 169         for (;;) {
 170                 head = CMM_LOAD_SHARED(s->head);
 171                 if (___cds_wfs_end(head))
 172                         return NULL;
 173                 next = ___cds_wfs_node_sync_next(&head->node, blocking);
 174                 if (!blocking && next == CDS_WFS_WOULDBLOCK)
 175                         return CDS_WFS_WOULDBLOCK;
 176                 new_head = caa_container_of(next, struct cds_wfs_head, node);
 177                 if (uatomic_cmpxchg(&s->head, head, new_head) == head)
 178                         return &head->node;
 179                 if (!blocking)
 180                         return CDS_WFS_WOULDBLOCK;
 181                 /* busy-loop if head changed under us */
 182         }
 183 }
 184
 185 /*
 186  * __cds_wfs_pop_blocking: pop a node from the stack.
 187  *
 188  * Returns NULL if stack is empty.
 189  *
 190  * __cds_wfs_pop_blocking needs to be synchronized using one of the
 191  * following techniques:
 192  *
 193  * 1) Calling __cds_wfs_pop_blocking under rcu read lock critical
 194  *    section. The caller must wait for a grace period to pass before
 195  *    freeing the returned node or modifying the cds_wfs_node structure.
 196  * 2) Using mutual exclusion (e.g. mutexes) to protect
 197  *     __cds_wfs_pop_blocking and __cds_wfs_pop_all callers.
 198  * 3) Ensuring that only ONE thread can call __cds_wfs_pop_blocking()
 199  *    and __cds_wfs_pop_all(). (multi-provider/single-consumer scheme).
 200  */
 201 static inline
 202 struct cds_wfs_node *
 203 ___cds_wfs_pop_blocking(struct cds_wfs_stack *s)
 204 {
 205         return ___cds_wfs_pop(s, 1);
 206 }
 207
 208 /*
 209  * __cds_wfs_pop_nonblocking: pop a node from the stack.
 210  *
 211  * Same as __cds_wfs_pop_blocking, but returns CDS_WFS_WOULDBLOCK if
 212  * it needs to block.
 213  */
 214 static inline
 215 struct cds_wfs_node *
 216 ___cds_wfs_pop_nonblocking(struct cds_wfs_stack *s)
 217 {
 218         return ___cds_wfs_pop(s, 0);
 219 }
 220
 221 /*
 222  * __cds_wfs_pop_all: pop all nodes from a stack.
 223  *
 224  * __cds_wfs_pop_all does not require any synchronization with other
 225  * push, nor with other __cds_wfs_pop_all, but requires synchronization
 226  * matching the technique used to synchronize __cds_wfs_pop_blocking:
 227  *
 228  * 1) If __cds_wfs_pop_blocking is called under rcu read lock critical
 229  *    section, both __cds_wfs_pop_blocking and cds_wfs_pop_all callers
 230  *    must wait for a grace period to pass before freeing the returned
 231  *    node or modifying the cds_wfs_node structure. However, no RCU
 232  *    read-side critical section is needed around __cds_wfs_pop_all.
 233  * 2) Using mutual exclusion (e.g. mutexes) to protect
 234  *     __cds_wfs_pop_blocking and __cds_wfs_pop_all callers.
 235  * 3) Ensuring that only ONE thread can call __cds_wfs_pop_blocking()
 236  *    and __cds_wfs_pop_all(). (multi-provider/single-consumer scheme).
 237  */
 238 static inline
 239 struct cds_wfs_head *
 240 ___cds_wfs_pop_all(struct cds_wfs_stack *s)
 241 {
 242         struct cds_wfs_head *head;
 243
 244         /*
 245          * Implicit memory barrier after uatomic_xchg() matches implicit
 246          * memory barrier before uatomic_xchg() in cds_wfs_push. It
 247          * ensures that all nodes of the returned list are consistent.
 248          * There is no need to issue memory barriers when iterating on
 249          * the returned list, because the full memory barrier issued
 250          * prior to each uatomic_cmpxchg, which each write to head, are
 251          * taking care to order writes to each node prior to the full
 252          * memory barrier after this uatomic_xchg().
 253          */
 254         head = uatomic_xchg(&s->head, CDS_WFS_END);
 255         if (___cds_wfs_end(head))
 256                 return NULL;
 257         return head;
 258 }
 259
 260 /*
 261  * cds_wfs_pop_lock: lock stack pop-protection mutex.
 262  */
 263 static inline void _cds_wfs_pop_lock(struct cds_wfs_stack *s)
 264 {
 265         int ret;
 266
 267         ret = pthread_mutex_lock(&s->lock);
 268         assert(!ret);
 269 }
 270
 271 /*
 272  * cds_wfs_pop_unlock: unlock stack pop-protection mutex.
 273  */
 274 static inline void _cds_wfs_pop_unlock(struct cds_wfs_stack *s)
 275 {
 276         int ret;
 277
 278         ret = pthread_mutex_unlock(&s->lock);
 279         assert(!ret);
 280 }
 281
 282 /*
 283  * Call __cds_wfs_pop_blocking with an internal pop mutex held.
 284  */
 285 static inline
 286 struct cds_wfs_node *
 287 _cds_wfs_pop_blocking(struct cds_wfs_stack *s)
 288 {
 289         struct cds_wfs_node *retnode;
 290
 291         _cds_wfs_pop_lock(s);
 292         retnode = ___cds_wfs_pop_blocking(s);
 293         _cds_wfs_pop_unlock(s);
 294         return retnode;
 295 }
 296
 297 /*
 298  * Call __cds_wfs_pop_all with an internal pop mutex held.
 299  */
 300 static inline
 301 struct cds_wfs_head *
 302 _cds_wfs_pop_all_blocking(struct cds_wfs_stack *s)
 303 {
 304         struct cds_wfs_head *rethead;
 305
 306         _cds_wfs_pop_lock(s);
 307         rethead = ___cds_wfs_pop_all(s);
 308         _cds_wfs_pop_unlock(s);
 309         return rethead;
 310 }
 311
 312 /*
 313  * cds_wfs_first: get first node of a popped stack.
 314  *
 315  * Content written into the node before enqueue is guaranteed to be
 316  * consistent, but no other memory ordering is ensured.
 317  *
 318  * Used by for-like iteration macros in urcu/wfstack.h:
 319  * cds_wfs_for_each_blocking()
 320  * cds_wfs_for_each_blocking_safe()
 321  *
 322  * Returns NULL if popped stack is empty, top stack node otherwise.
 323  */
 324 static inline struct cds_wfs_node *
 325 _cds_wfs_first(struct cds_wfs_head *head)
 326 {
 327         if (___cds_wfs_end(head))
 328                 return NULL;
 329         return &head->node;
 330 }
 331
 332 static inline struct cds_wfs_node *
 333 ___cds_wfs_next(struct cds_wfs_node *node, int blocking)
 334 {
 335         struct cds_wfs_node *next;
 336
 337         next = ___cds_wfs_node_sync_next(node, blocking);
 338         /*
 339          * CDS_WFS_WOULDBLOCK != CSD_WFS_END, so we can check for end
 340          * even if ___cds_wfs_node_sync_next returns CDS_WFS_WOULDBLOCK,
 341          * and still return CDS_WFS_WOULDBLOCK.
 342          */
 343         if (___cds_wfs_end(next))
 344                 return NULL;
 345         return next;
 346 }
 347
 348 /*
 349  * cds_wfs_next_blocking: get next node of a popped stack.
 350  *
 351  * Content written into the node before enqueue is guaranteed to be
 352  * consistent, but no other memory ordering is ensured.
 353  *
 354  * Used by for-like iteration macros in urcu/wfstack.h:
 355  * cds_wfs_for_each_blocking()
 356  * cds_wfs_for_each_blocking_safe()
 357  *
 358  * Returns NULL if reached end of popped stack, non-NULL next stack
 359  * node otherwise.
 360  */
 361 static inline struct cds_wfs_node *
 362 _cds_wfs_next_blocking(struct cds_wfs_node *node)
 363 {
 364         return ___cds_wfs_next(node, 1);
 365 }
 366
 367
 368 /*
 369  * cds_wfs_next_nonblocking: get next node of a popped stack.
 370  *
 371  * Same as cds_wfs_next_blocking, but returns CDS_WFS_WOULDBLOCK if it
 372  * needs to block.
 373  */
 374 static inline struct cds_wfs_node *
 375 _cds_wfs_next_nonblocking(struct cds_wfs_node *node)
 376 {
 377         return ___cds_wfs_next(node, 0);
 378 }
 379
 380 #ifdef __cplusplus
 381 }
 382 #endif
 383
 384 #endif /* _URCU_STATIC_WFSTACK_H */