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