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 *state, int blocking)
 165 {
 166         struct cds_wfs_head *head, *new_head;
 167         struct cds_wfs_node *next;
 168
 169         if (state)
 170                 *state = 0;
 171         for (;;) {
 172                 head = CMM_LOAD_SHARED(s->head);
 173                 if (___cds_wfs_end(head)) {
 174                         return NULL;
 175                 }
 176                 next = ___cds_wfs_node_sync_next(&head->node, blocking);
 177                 if (!blocking && next == CDS_WFS_WOULDBLOCK) {
 178                         return CDS_WFS_WOULDBLOCK;
 179                 }
 180                 new_head = caa_container_of(next, struct cds_wfs_head, node);
 181                 if (uatomic_cmpxchg(&s->head, head, new_head) == head) {
 182                         if (state && ___cds_wfs_end(new_head))
 183                                 *state |= CDS_WFS_STATE_LAST;
 184                         return &head->node;
 185                 }
 186                 if (!blocking) {
 187                         return CDS_WFS_WOULDBLOCK;
 188                 }
 189                 /* busy-loop if head changed under us */
 190         }
 191 }
 192
 193 /*
 194  * __cds_wfs_pop_with_state_blocking: pop a node from the stack, with state.
 195  *
 196  * Returns NULL if stack is empty.
 197  *
 198  * __cds_wfs_pop_blocking needs to be synchronized using one of the
 199  * following techniques:
 200  *
 201  * 1) Calling __cds_wfs_pop_blocking under rcu read lock critical
 202  *    section. The caller must wait for a grace period to pass before
 203  *    freeing the returned node or modifying the cds_wfs_node structure.
 204  * 2) Using mutual exclusion (e.g. mutexes) to protect
 205  *     __cds_wfs_pop_blocking and __cds_wfs_pop_all callers.
 206  * 3) Ensuring that only ONE thread can call __cds_wfs_pop_blocking()
 207  *    and __cds_wfs_pop_all(). (multi-provider/single-consumer scheme).
 208  *
 209  * "state" saves state flags atomically sampled with pop operation.
 210  */
 211 static inline
 212 struct cds_wfs_node *
 213 ___cds_wfs_pop_with_state_blocking(struct cds_wfs_stack *s, int *state)
 214 {
 215         return ___cds_wfs_pop(s, state, 1);
 216 }
 217
 218 static inline
 219 struct cds_wfs_node *
 220 ___cds_wfs_pop_blocking(struct cds_wfs_stack *s)
 221 {
 222         return ___cds_wfs_pop_with_state_blocking(s, NULL);
 223 }
 224
 225 /*
 226  * __cds_wfs_pop_with_state_nonblocking: pop a node from the stack.
 227  *
 228  * Same as __cds_wfs_pop_with_state_blocking, but returns
 229  * CDS_WFS_WOULDBLOCK if it needs to block.
 230  *
 231  * "state" saves state flags atomically sampled with pop operation.
 232  */
 233 static inline
 234 struct cds_wfs_node *
 235 ___cds_wfs_pop_with_state_nonblocking(struct cds_wfs_stack *s, int *state)
 236 {
 237         return ___cds_wfs_pop(s, state, 0);
 238 }
 239
 240 /*
 241  * __cds_wfs_pop_nonblocking: pop a node from the stack.
 242  *
 243  * Same as __cds_wfs_pop_blocking, but returns CDS_WFS_WOULDBLOCK if
 244  * it needs to block.
 245  */
 246 static inline
 247 struct cds_wfs_node *
 248 ___cds_wfs_pop_nonblocking(struct cds_wfs_stack *s)
 249 {
 250         return ___cds_wfs_pop_with_state_nonblocking(s, NULL);
 251 }
 252
 253 /*
 254  * __cds_wfs_pop_all: pop all nodes from a stack.
 255  *
 256  * __cds_wfs_pop_all does not require any synchronization with other
 257  * push, nor with other __cds_wfs_pop_all, but requires synchronization
 258  * matching the technique used to synchronize __cds_wfs_pop_blocking:
 259  *
 260  * 1) If __cds_wfs_pop_blocking is called under rcu read lock critical
 261  *    section, both __cds_wfs_pop_blocking and cds_wfs_pop_all callers
 262  *    must wait for a grace period to pass before freeing the returned
 263  *    node or modifying the cds_wfs_node structure. However, no RCU
 264  *    read-side critical section is needed around __cds_wfs_pop_all.
 265  * 2) Using mutual exclusion (e.g. mutexes) to protect
 266  *     __cds_wfs_pop_blocking and __cds_wfs_pop_all callers.
 267  * 3) Ensuring that only ONE thread can call __cds_wfs_pop_blocking()
 268  *    and __cds_wfs_pop_all(). (multi-provider/single-consumer scheme).
 269  */
 270 static inline
 271 struct cds_wfs_head *
 272 ___cds_wfs_pop_all(struct cds_wfs_stack *s)
 273 {
 274         struct cds_wfs_head *head;
 275
 276         /*
 277          * Implicit memory barrier after uatomic_xchg() matches implicit
 278          * memory barrier before uatomic_xchg() in cds_wfs_push. It
 279          * ensures that all nodes of the returned list are consistent.
 280          * There is no need to issue memory barriers when iterating on
 281          * the returned list, because the full memory barrier issued
 282          * prior to each uatomic_cmpxchg, which each write to head, are
 283          * taking care to order writes to each node prior to the full
 284          * memory barrier after this uatomic_xchg().
 285          */
 286         head = uatomic_xchg(&s->head, CDS_WFS_END);
 287         if (___cds_wfs_end(head))
 288                 return NULL;
 289         return head;
 290 }
 291
 292 /*
 293  * cds_wfs_pop_lock: lock stack pop-protection mutex.
 294  */
 295 static inline void _cds_wfs_pop_lock(struct cds_wfs_stack *s)
 296 {
 297         int ret;
 298
 299         ret = pthread_mutex_lock(&s->lock);
 300         assert(!ret);
 301 }
 302
 303 /*
 304  * cds_wfs_pop_unlock: unlock stack pop-protection mutex.
 305  */
 306 static inline void _cds_wfs_pop_unlock(struct cds_wfs_stack *s)
 307 {
 308         int ret;
 309
 310         ret = pthread_mutex_unlock(&s->lock);
 311         assert(!ret);
 312 }
 313
 314 /*
 315  * Call __cds_wfs_pop_with_state_blocking with an internal pop mutex held.
 316  */
 317 static inline
 318 struct cds_wfs_node *
 319 _cds_wfs_pop_with_state_blocking(struct cds_wfs_stack *s, int *state)
 320 {
 321         struct cds_wfs_node *retnode;
 322
 323         _cds_wfs_pop_lock(s);
 324         retnode = ___cds_wfs_pop_with_state_blocking(s, state);
 325         _cds_wfs_pop_unlock(s);
 326         return retnode;
 327 }
 328
 329 /*
 330  * Call _cds_wfs_pop_with_state_blocking without saving any state.
 331  */
 332 static inline
 333 struct cds_wfs_node *
 334 _cds_wfs_pop_blocking(struct cds_wfs_stack *s)
 335 {
 336         return _cds_wfs_pop_with_state_blocking(s, NULL);
 337 }
 338
 339 /*
 340  * Call __cds_wfs_pop_all with an internal pop mutex held.
 341  */
 342 static inline
 343 struct cds_wfs_head *
 344 _cds_wfs_pop_all_blocking(struct cds_wfs_stack *s)
 345 {
 346         struct cds_wfs_head *rethead;
 347
 348         _cds_wfs_pop_lock(s);
 349         rethead = ___cds_wfs_pop_all(s);
 350         _cds_wfs_pop_unlock(s);
 351         return rethead;
 352 }
 353
 354 /*
 355  * cds_wfs_first: get first node of a popped stack.
 356  *
 357  * Content written into the node before enqueue is guaranteed to be
 358  * consistent, but no other memory ordering is ensured.
 359  *
 360  * Used by for-like iteration macros in urcu/wfstack.h:
 361  * cds_wfs_for_each_blocking()
 362  * cds_wfs_for_each_blocking_safe()
 363  *
 364  * Returns NULL if popped stack is empty, top stack node otherwise.
 365  */
 366 static inline struct cds_wfs_node *
 367 _cds_wfs_first(struct cds_wfs_head *head)
 368 {
 369         if (___cds_wfs_end(head))
 370                 return NULL;
 371         return &head->node;
 372 }
 373
 374 static inline struct cds_wfs_node *
 375 ___cds_wfs_next(struct cds_wfs_node *node, int blocking)
 376 {
 377         struct cds_wfs_node *next;
 378
 379         next = ___cds_wfs_node_sync_next(node, blocking);
 380         /*
 381          * CDS_WFS_WOULDBLOCK != CSD_WFS_END, so we can check for end
 382          * even if ___cds_wfs_node_sync_next returns CDS_WFS_WOULDBLOCK,
 383          * and still return CDS_WFS_WOULDBLOCK.
 384          */
 385         if (___cds_wfs_end(next))
 386                 return NULL;
 387         return next;
 388 }
 389
 390 /*
 391  * cds_wfs_next_blocking: get next node of a popped stack.
 392  *
 393  * Content written into the node before enqueue is guaranteed to be
 394  * consistent, but no other memory ordering is ensured.
 395  *
 396  * Used by for-like iteration macros in urcu/wfstack.h:
 397  * cds_wfs_for_each_blocking()
 398  * cds_wfs_for_each_blocking_safe()
 399  *
 400  * Returns NULL if reached end of popped stack, non-NULL next stack
 401  * node otherwise.
 402  */
 403 static inline struct cds_wfs_node *
 404 _cds_wfs_next_blocking(struct cds_wfs_node *node)
 405 {
 406         return ___cds_wfs_next(node, 1);
 407 }
 408
 409
 410 /*
 411  * cds_wfs_next_nonblocking: get next node of a popped stack.
 412  *
 413  * Same as cds_wfs_next_blocking, but returns CDS_WFS_WOULDBLOCK if it
 414  * needs to block.
 415  */
 416 static inline struct cds_wfs_node *
 417 _cds_wfs_next_nonblocking(struct cds_wfs_node *node)
 418 {
 419         return ___cds_wfs_next(node, 0);
 420 }
 421
 422 #ifdef __cplusplus
 423 }
 424 #endif
 425
 426 #endif /* _URCU_STATIC_WFSTACK_H */