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. Pair with
  81  * cds_wfs_destroy().
  82  */
  83 static inline
  84 void _cds_wfs_init(struct cds_wfs_stack *s)
  85 {
  86         int ret;
  87
  88         s->head = CDS_WFS_END;
  89         ret = pthread_mutex_init(&s->lock, NULL);
  90         assert(!ret);
  91 }
  92
  93 /*
  94  * cds_wfs_destroy: destroy wait-free stack. Pair with
  95  * cds_wfs_init().
  96  */
  97 static inline
  98 void _cds_wfs_destroy(struct cds_wfs_stack *s)
  99 {
 100         int ret = pthread_mutex_destroy(&s->lock);
 101         assert(!ret);
 102 }
 103
 104 static inline bool ___cds_wfs_end(void *node)
 105 {
 106         return node == CDS_WFS_END;
 107 }
 108
 109 /*
 110  * cds_wfs_empty: return whether wait-free stack is empty.
 111  *
 112  * No memory barrier is issued. No mutual exclusion is required.
 113  */
 114 static inline bool _cds_wfs_empty(struct cds_wfs_stack *s)
 115 {
 116         return ___cds_wfs_end(CMM_LOAD_SHARED(s->head));
 117 }
 118
 119 /*
 120  * cds_wfs_push: push a node into the stack.
 121  *
 122  * Issues a full memory barrier before push. No mutual exclusion is
 123  * required.
 124  *
 125  * Returns 0 if the stack was empty prior to adding the node.
 126  * Returns non-zero otherwise.
 127  */
 128 static inline
 129 int _cds_wfs_push(struct cds_wfs_stack *s, struct cds_wfs_node *node)
 130 {
 131         struct cds_wfs_head *old_head, *new_head;
 132
 133         assert(node->next == NULL);
 134         new_head = caa_container_of(node, struct cds_wfs_head, node);
 135         /*
 136          * uatomic_xchg() implicit memory barrier orders earlier stores
 137          * to node (setting it to NULL) before publication.
 138          */
 139         old_head = uatomic_xchg(&s->head, new_head);
 140         /*
 141          * At this point, dequeuers see a NULL node->next, they should
 142          * busy-wait until node->next is set to old_head.
 143          */
 144         CMM_STORE_SHARED(node->next, &old_head->node);
 145         return !___cds_wfs_end(old_head);
 146 }
 147
 148 /*
 149  * Waiting for push to complete enqueue and return the next node.
 150  */
 151 static inline struct cds_wfs_node *
 152 ___cds_wfs_node_sync_next(struct cds_wfs_node *node, int blocking)
 153 {
 154         struct cds_wfs_node *next;
 155         int attempt = 0;
 156
 157         /*
 158          * Adaptative busy-looping waiting for push to complete.
 159          */
 160         while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
 161                 if (!blocking)
 162                         return CDS_WFS_WOULDBLOCK;
 163                 if (++attempt >= CDS_WFS_ADAPT_ATTEMPTS) {
 164                         poll(NULL, 0, CDS_WFS_WAIT);    /* Wait for 10ms */
 165                         attempt = 0;
 166                 } else {
 167                         caa_cpu_relax();
 168                 }
 169         }
 170
 171         return next;
 172 }
 173
 174 static inline
 175 struct cds_wfs_node *
 176 ___cds_wfs_pop(struct cds_wfs_stack *s, int *state, int blocking)
 177 {
 178         struct cds_wfs_head *head, *new_head;
 179         struct cds_wfs_node *next;
 180
 181         if (state)
 182                 *state = 0;
 183         for (;;) {
 184                 head = CMM_LOAD_SHARED(s->head);
 185                 if (___cds_wfs_end(head)) {
 186                         return NULL;
 187                 }
 188                 next = ___cds_wfs_node_sync_next(&head->node, blocking);
 189                 if (!blocking && next == CDS_WFS_WOULDBLOCK) {
 190                         return CDS_WFS_WOULDBLOCK;
 191                 }
 192                 new_head = caa_container_of(next, struct cds_wfs_head, node);
 193                 if (uatomic_cmpxchg(&s->head, head, new_head) == head) {
 194                         if (state && ___cds_wfs_end(new_head))
 195                                 *state |= CDS_WFS_STATE_LAST;
 196                         return &head->node;
 197                 }
 198                 if (!blocking) {
 199                         return CDS_WFS_WOULDBLOCK;
 200                 }
 201                 /* busy-loop if head changed under us */
 202         }
 203 }
 204
 205 /*
 206  * __cds_wfs_pop_with_state_blocking: pop a node from the stack, with state.
 207  *
 208  * Returns NULL if stack is empty.
 209  *
 210  * __cds_wfs_pop_blocking needs to be synchronized using one of the
 211  * following techniques:
 212  *
 213  * 1) Calling __cds_wfs_pop_blocking under rcu read lock critical
 214  *    section. The caller must wait for a grace period to pass before
 215  *    freeing the returned node or modifying the cds_wfs_node structure.
 216  * 2) Using mutual exclusion (e.g. mutexes) to protect
 217  *     __cds_wfs_pop_blocking and __cds_wfs_pop_all callers.
 218  * 3) Ensuring that only ONE thread can call __cds_wfs_pop_blocking()
 219  *    and __cds_wfs_pop_all(). (multi-provider/single-consumer scheme).
 220  *
 221  * "state" saves state flags atomically sampled with pop operation.
 222  */
 223 static inline
 224 struct cds_wfs_node *
 225 ___cds_wfs_pop_with_state_blocking(struct cds_wfs_stack *s, int *state)
 226 {
 227         return ___cds_wfs_pop(s, state, 1);
 228 }
 229
 230 static inline
 231 struct cds_wfs_node *
 232 ___cds_wfs_pop_blocking(struct cds_wfs_stack *s)
 233 {
 234         return ___cds_wfs_pop_with_state_blocking(s, NULL);
 235 }
 236
 237 /*
 238  * __cds_wfs_pop_with_state_nonblocking: pop a node from the stack.
 239  *
 240  * Same as __cds_wfs_pop_with_state_blocking, but returns
 241  * CDS_WFS_WOULDBLOCK if it needs to block.
 242  *
 243  * "state" saves state flags atomically sampled with pop operation.
 244  */
 245 static inline
 246 struct cds_wfs_node *
 247 ___cds_wfs_pop_with_state_nonblocking(struct cds_wfs_stack *s, int *state)
 248 {
 249         return ___cds_wfs_pop(s, state, 0);
 250 }
 251
 252 /*
 253  * __cds_wfs_pop_nonblocking: pop a node from the stack.
 254  *
 255  * Same as __cds_wfs_pop_blocking, but returns CDS_WFS_WOULDBLOCK if
 256  * it needs to block.
 257  */
 258 static inline
 259 struct cds_wfs_node *
 260 ___cds_wfs_pop_nonblocking(struct cds_wfs_stack *s)
 261 {
 262         return ___cds_wfs_pop_with_state_nonblocking(s, NULL);
 263 }
 264
 265 /*
 266  * __cds_wfs_pop_all: pop all nodes from a stack.
 267  *
 268  * __cds_wfs_pop_all does not require any synchronization with other
 269  * push, nor with other __cds_wfs_pop_all, but requires synchronization
 270  * matching the technique used to synchronize __cds_wfs_pop_blocking:
 271  *
 272  * 1) If __cds_wfs_pop_blocking is called under rcu read lock critical
 273  *    section, both __cds_wfs_pop_blocking and cds_wfs_pop_all callers
 274  *    must wait for a grace period to pass before freeing the returned
 275  *    node or modifying the cds_wfs_node structure. However, no RCU
 276  *    read-side critical section is needed around __cds_wfs_pop_all.
 277  * 2) Using mutual exclusion (e.g. mutexes) to protect
 278  *     __cds_wfs_pop_blocking and __cds_wfs_pop_all callers.
 279  * 3) Ensuring that only ONE thread can call __cds_wfs_pop_blocking()
 280  *    and __cds_wfs_pop_all(). (multi-provider/single-consumer scheme).
 281  */
 282 static inline
 283 struct cds_wfs_head *
 284 ___cds_wfs_pop_all(struct cds_wfs_stack *s)
 285 {
 286         struct cds_wfs_head *head;
 287
 288         /*
 289          * Implicit memory barrier after uatomic_xchg() matches implicit
 290          * memory barrier before uatomic_xchg() in cds_wfs_push. It
 291          * ensures that all nodes of the returned list are consistent.
 292          * There is no need to issue memory barriers when iterating on
 293          * the returned list, because the full memory barrier issued
 294          * prior to each uatomic_cmpxchg, which each write to head, are
 295          * taking care to order writes to each node prior to the full
 296          * memory barrier after this uatomic_xchg().
 297          */
 298         head = uatomic_xchg(&s->head, CDS_WFS_END);
 299         if (___cds_wfs_end(head))
 300                 return NULL;
 301         return head;
 302 }
 303
 304 /*
 305  * cds_wfs_pop_lock: lock stack pop-protection mutex.
 306  */
 307 static inline void _cds_wfs_pop_lock(struct cds_wfs_stack *s)
 308 {
 309         int ret;
 310
 311         ret = pthread_mutex_lock(&s->lock);
 312         assert(!ret);
 313 }
 314
 315 /*
 316  * cds_wfs_pop_unlock: unlock stack pop-protection mutex.
 317  */
 318 static inline void _cds_wfs_pop_unlock(struct cds_wfs_stack *s)
 319 {
 320         int ret;
 321
 322         ret = pthread_mutex_unlock(&s->lock);
 323         assert(!ret);
 324 }
 325
 326 /*
 327  * Call __cds_wfs_pop_with_state_blocking with an internal pop mutex held.
 328  */
 329 static inline
 330 struct cds_wfs_node *
 331 _cds_wfs_pop_with_state_blocking(struct cds_wfs_stack *s, int *state)
 332 {
 333         struct cds_wfs_node *retnode;
 334
 335         _cds_wfs_pop_lock(s);
 336         retnode = ___cds_wfs_pop_with_state_blocking(s, state);
 337         _cds_wfs_pop_unlock(s);
 338         return retnode;
 339 }
 340
 341 /*
 342  * Call _cds_wfs_pop_with_state_blocking without saving any state.
 343  */
 344 static inline
 345 struct cds_wfs_node *
 346 _cds_wfs_pop_blocking(struct cds_wfs_stack *s)
 347 {
 348         return _cds_wfs_pop_with_state_blocking(s, NULL);
 349 }
 350
 351 /*
 352  * Call __cds_wfs_pop_all with an internal pop mutex held.
 353  */
 354 static inline
 355 struct cds_wfs_head *
 356 _cds_wfs_pop_all_blocking(struct cds_wfs_stack *s)
 357 {
 358         struct cds_wfs_head *rethead;
 359
 360         _cds_wfs_pop_lock(s);
 361         rethead = ___cds_wfs_pop_all(s);
 362         _cds_wfs_pop_unlock(s);
 363         return rethead;
 364 }
 365
 366 /*
 367  * cds_wfs_first: get first node of a popped stack.
 368  *
 369  * Content written into the node before enqueue is guaranteed to be
 370  * consistent, but no other memory ordering is ensured.
 371  *
 372  * Used by for-like iteration macros in urcu/wfstack.h:
 373  * cds_wfs_for_each_blocking()
 374  * cds_wfs_for_each_blocking_safe()
 375  *
 376  * Returns NULL if popped stack is empty, top stack node otherwise.
 377  */
 378 static inline struct cds_wfs_node *
 379 _cds_wfs_first(struct cds_wfs_head *head)
 380 {
 381         if (___cds_wfs_end(head))
 382                 return NULL;
 383         return &head->node;
 384 }
 385
 386 static inline struct cds_wfs_node *
 387 ___cds_wfs_next(struct cds_wfs_node *node, int blocking)
 388 {
 389         struct cds_wfs_node *next;
 390
 391         next = ___cds_wfs_node_sync_next(node, blocking);
 392         /*
 393          * CDS_WFS_WOULDBLOCK != CSD_WFS_END, so we can check for end
 394          * even if ___cds_wfs_node_sync_next returns CDS_WFS_WOULDBLOCK,
 395          * and still return CDS_WFS_WOULDBLOCK.
 396          */
 397         if (___cds_wfs_end(next))
 398                 return NULL;
 399         return next;
 400 }
 401
 402 /*
 403  * cds_wfs_next_blocking: get next node of a popped stack.
 404  *
 405  * Content written into the node before enqueue is guaranteed to be
 406  * consistent, but no other memory ordering is ensured.
 407  *
 408  * Used by for-like iteration macros in urcu/wfstack.h:
 409  * cds_wfs_for_each_blocking()
 410  * cds_wfs_for_each_blocking_safe()
 411  *
 412  * Returns NULL if reached end of popped stack, non-NULL next stack
 413  * node otherwise.
 414  */
 415 static inline struct cds_wfs_node *
 416 _cds_wfs_next_blocking(struct cds_wfs_node *node)
 417 {
 418         return ___cds_wfs_next(node, 1);
 419 }
 420
 421
 422 /*
 423  * cds_wfs_next_nonblocking: get next node of a popped stack.
 424  *
 425  * Same as cds_wfs_next_blocking, but returns CDS_WFS_WOULDBLOCK if it
 426  * needs to block.
 427  */
 428 static inline struct cds_wfs_node *
 429 _cds_wfs_next_nonblocking(struct cds_wfs_node *node)
 430 {
 431         return ___cds_wfs_next(node, 0);
 432 }
 433
 434 #ifdef __cplusplus
 435 }
 436 #endif
 437
 438 #endif /* _URCU_STATIC_WFSTACK_H */