urcu/static/lfstack.h

   1 #ifndef _URCU_STATIC_LFSTACK_H
   2 #define _URCU_STATIC_LFSTACK_H
   3
   4 /*
   5  * urcu/static/lfstack.h
   6  *
   7  * Userspace RCU library - Lock-Free Stack
   8  *
   9  * Copyright 2010-2012 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  10  *
  11  * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu/lfstack.h for
  12  * linking dynamically with the userspace rcu library.
  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 <stdbool.h>
  30 #include <pthread.h>
  31 #include <assert.h>
  32 #include <urcu/uatomic.h>
  33 #include <urcu-pointer.h>
  34
  35 #ifdef __cplusplus
  36 extern "C" {
  37 #endif
  38
  39 /*
  40  * Lock-free stack.
  41  *
  42  * Stack implementing push, pop, pop_all operations, as well as iterator
  43  * on the stack head returned by pop_all.
  44  *
  45  * Synchronization table:
  46  *
  47  * External synchronization techniques described in the API below is
  48  * required between pairs marked with "X". No external synchronization
  49  * required between pairs marked with "-".
  50  *
  51  *                      cds_lfs_push  __cds_lfs_pop  __cds_lfs_pop_all
  52  * cds_lfs_push               -              -                  -
  53  * __cds_lfs_pop              -              X                  X
  54  * __cds_lfs_pop_all          -              X                  -
  55  *
  56  * cds_lfs_pop_blocking and cds_lfs_pop_all_blocking use an internal
  57  * mutex to provide synchronization.
  58  */
  59
  60 /*
  61  * cds_lfs_node_init: initialize lock-free stack node.
  62  */
  63 static inline
  64 void _cds_lfs_node_init(struct cds_lfs_node *node)
  65 {
  66 }
  67
  68 /*
  69  * cds_lfs_init: initialize lock-free stack.
  70  */
  71 static inline
  72 void _cds_lfs_init(struct cds_lfs_stack *s)
  73 {
  74         int ret;
  75
  76         s->head = NULL;
  77         ret = pthread_mutex_init(&s->lock, NULL);
  78         assert(!ret);
  79 }
  80
  81 /*
  82  * ___cds_lfs_init: initialize lock-free stack.
  83  */
  84 static inline
  85 void ___cds_lfs_init(struct __cds_lfs_stack *s)
  86 {
  87         s->head = NULL;
  88 }
  89
  90 static inline
  91 bool ___cds_lfs_empty_head(struct cds_lfs_head *head)
  92 {
  93         return head == NULL;
  94 }
  95
  96 /*
  97  * cds_lfs_empty: return whether lock-free stack is empty.
  98  *
  99  * No memory barrier is issued. No mutual exclusion is required.
 100  */
 101 static inline
 102 bool _cds_lfs_empty(cds_lfs_stack_ptr_t s)
 103 {
 104         return ___cds_lfs_empty_head(CMM_LOAD_SHARED(s._s->head));
 105 }
 106
 107 /*
 108  * cds_lfs_push: push a node into the stack.
 109  *
 110  * Does not require any synchronization with other push nor pop.
 111  *
 112  * Lock-free stack push is not subject to ABA problem, so no need to
 113  * take the RCU read-side lock. Even if "head" changes between two
 114  * uatomic_cmpxchg() invocations here (being popped, and then pushed
 115  * again by one or more concurrent threads), the second
 116  * uatomic_cmpxchg() invocation only cares about pushing a new entry at
 117  * the head of the stack, ensuring consistency by making sure the new
 118  * node->next is the same pointer value as the value replaced as head.
 119  * It does not care about the content of the actual next node, so it can
 120  * very well be reallocated between the two uatomic_cmpxchg().
 121  *
 122  * We take the approach of expecting the stack to be usually empty, so
 123  * we first try an initial uatomic_cmpxchg() on a NULL old_head, and
 124  * retry if the old head was non-NULL (the value read by the first
 125  * uatomic_cmpxchg() is used as old head for the following loop). The
 126  * upside of this scheme is to minimize the amount of cacheline traffic,
 127  * always performing an exclusive cacheline access, rather than doing
 128  * non-exclusive followed by exclusive cacheline access (which would be
 129  * required if we first read the old head value). This design decision
 130  * might be revisited after more thorough benchmarking on various
 131  * platforms.
 132  *
 133  * Returns 0 if the stack was empty prior to adding the node.
 134  * Returns non-zero otherwise.
 135  */
 136 static inline
 137 bool _cds_lfs_push(cds_lfs_stack_ptr_t u_s,
 138                   struct cds_lfs_node *node)
 139 {
 140         struct __cds_lfs_stack *s = u_s._s;
 141         struct cds_lfs_head *head = NULL;
 142         struct cds_lfs_head *new_head =
 143                 caa_container_of(node, struct cds_lfs_head, node);
 144
 145         for (;;) {
 146                 struct cds_lfs_head *old_head = head;
 147
 148                 /*
 149                  * node->next is still private at this point, no need to
 150                  * perform a _CMM_STORE_SHARED().
 151                  */
 152                 node->next = &head->node;
 153                 /*
 154                  * uatomic_cmpxchg() implicit memory barrier orders earlier
 155                  * stores to node before publication.
 156                  */
 157                 head = uatomic_cmpxchg(&s->head, old_head, new_head);
 158                 if (old_head == head)
 159                         break;
 160         }
 161         return !___cds_lfs_empty_head(head);
 162 }
 163
 164 /*
 165  * __cds_lfs_pop: pop a node from the stack.
 166  *
 167  * Returns NULL if stack is empty.
 168  *
 169  * __cds_lfs_pop needs to be synchronized using one of the following
 170  * techniques:
 171  *
 172  * 1) Calling __cds_lfs_pop under rcu read lock critical section.
 173  *    Both __cds_lfs_pop and __cds_lfs_pop_all callers must wait for a
 174  *    grace period to pass before freeing the returned node or pushing
 175  *    the node back into the stack. It is valid to overwrite the content
 176  *    of cds_lfs_node immediately after __cds_lfs_pop and
 177  *    __cds_lfs_pop_all. No RCU read-side critical section is needed
 178  *    around __cds_lfs_pop_all.
 179  * 2) Using mutual exclusion (e.g. mutexes) to protect __cds_lfs_pop
 180  *    and __cds_lfs_pop_all callers.
 181  * 3) Ensuring that only ONE thread can call __cds_lfs_pop() and
 182  *    __cds_lfs_pop_all(). (multi-provider/single-consumer scheme).
 183  */
 184 static inline
 185 struct cds_lfs_node *___cds_lfs_pop(cds_lfs_stack_ptr_t u_s)
 186 {
 187         struct __cds_lfs_stack *s = u_s._s;
 188
 189         for (;;) {
 190                 struct cds_lfs_head *head, *next_head;
 191                 struct cds_lfs_node *next;
 192
 193                 head = _CMM_LOAD_SHARED(s->head);
 194                 if (___cds_lfs_empty_head(head))
 195                         return NULL;    /* Empty stack */
 196
 197                 /*
 198                  * Read head before head->next. Matches the implicit
 199                  * memory barrier before uatomic_cmpxchg() in
 200                  * cds_lfs_push.
 201                  */
 202                 cmm_smp_read_barrier_depends();
 203                 next = _CMM_LOAD_SHARED(head->node.next);
 204                 next_head = caa_container_of(next,
 205                                 struct cds_lfs_head, node);
 206                 if (uatomic_cmpxchg(&s->head, head, next_head) == head)
 207                         return &head->node;
 208                 /* busy-loop if head changed under us */
 209         }
 210 }
 211
 212 /*
 213  * __cds_lfs_pop_all: pop all nodes from a stack.
 214  *
 215  * __cds_lfs_pop_all does not require any synchronization with other
 216  * push, nor with other __cds_lfs_pop_all, but requires synchronization
 217  * matching the technique used to synchronize __cds_lfs_pop:
 218  *
 219  * 1) If __cds_lfs_pop is called under rcu read lock critical section,
 220  *    both __cds_lfs_pop and __cds_lfs_pop_all callers must wait for a
 221  *    grace period to pass before freeing the returned node or pushing
 222  *    the node back into the stack. It is valid to overwrite the content
 223  *    of cds_lfs_node immediately after __cds_lfs_pop and
 224  *    __cds_lfs_pop_all. No RCU read-side critical section is needed
 225  *    around __cds_lfs_pop_all.
 226  * 2) Using mutual exclusion (e.g. mutexes) to protect __cds_lfs_pop and
 227  *    __cds_lfs_pop_all callers.
 228  * 3) Ensuring that only ONE thread can call __cds_lfs_pop() and
 229  *    __cds_lfs_pop_all(). (multi-provider/single-consumer scheme).
 230  */
 231 static inline
 232 struct cds_lfs_head *___cds_lfs_pop_all(cds_lfs_stack_ptr_t u_s)
 233 {
 234         struct __cds_lfs_stack *s = u_s._s;
 235
 236         /*
 237          * Implicit memory barrier after uatomic_xchg() matches implicit
 238          * memory barrier before uatomic_cmpxchg() in cds_lfs_push. It
 239          * ensures that all nodes of the returned list are consistent.
 240          * There is no need to issue memory barriers when iterating on
 241          * the returned list, because the full memory barrier issued
 242          * prior to each uatomic_cmpxchg, which each write to head, are
 243          * taking care to order writes to each node prior to the full
 244          * memory barrier after this uatomic_xchg().
 245          */
 246         return uatomic_xchg(&s->head, NULL);
 247 }
 248
 249 /*
 250  * cds_lfs_pop_lock: lock stack pop-protection mutex.
 251  */
 252 static inline void _cds_lfs_pop_lock(struct cds_lfs_stack *s)
 253 {
 254         int ret;
 255
 256         ret = pthread_mutex_lock(&s->lock);
 257         assert(!ret);
 258 }
 259
 260 /*
 261  * cds_lfs_pop_unlock: unlock stack pop-protection mutex.
 262  */
 263 static inline void _cds_lfs_pop_unlock(struct cds_lfs_stack *s)
 264 {
 265         int ret;
 266
 267         ret = pthread_mutex_unlock(&s->lock);
 268         assert(!ret);
 269 }
 270
 271 /*
 272  * Call __cds_lfs_pop with an internal pop mutex held.
 273  */
 274 static inline
 275 struct cds_lfs_node *
 276 _cds_lfs_pop_blocking(struct cds_lfs_stack *s)
 277 {
 278         struct cds_lfs_node *retnode;
 279
 280         _cds_lfs_pop_lock(s);
 281         retnode = ___cds_lfs_pop(s);
 282         _cds_lfs_pop_unlock(s);
 283         return retnode;
 284 }
 285
 286 /*
 287  * Call __cds_lfs_pop_all with an internal pop mutex held.
 288  */
 289 static inline
 290 struct cds_lfs_head *
 291 _cds_lfs_pop_all_blocking(struct cds_lfs_stack *s)
 292 {
 293         struct cds_lfs_head *rethead;
 294
 295         _cds_lfs_pop_lock(s);
 296         rethead = ___cds_lfs_pop_all(s);
 297         _cds_lfs_pop_unlock(s);
 298         return rethead;
 299 }
 300
 301 #ifdef __cplusplus
 302 }
 303 #endif
 304
 305 #endif /* _URCU_STATIC_LFSTACK_H */