rculfhash: implement lock-free replacement
authorMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Wed, 21 Sep 2011 04:40:38 +0000 (00:40 -0400)
committerMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Wed, 21 Sep 2011 04:40:38 +0000 (00:40 -0400)
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
rculfhash.c
tests/test_urcu_hash.c
urcu/rculfhash.h

index 30c5c8a2e75390e72623e45af1f0889bbbcccd84..034c7f9053cfbd881496b592cbb3bcddf92a1d8f 100644 (file)
  * It indicates that no node must attach to the node scheduled for
  * removal. The gc flag also needs to be updated atomically with the
  * pointer. It indicates that node garbage collection must be performed.
- * "removed" and "gc" flags are separate for the benefit of replacement
- * operation.
  * The dummy flag does not require to be updated atomically with the
  * pointer, but it is added as a pointer low bit flag to save space.
  */
@@ -792,7 +790,7 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
                                enum add_mode mode, int dummy)
 {
        struct cds_lfht_node *iter_prev, *iter, *next, *new_node, *new_next,
-                       *dummy_node, *return_node, *replace_pinned = NULL;
+                       *dummy_node, *return_node;
        struct _cds_lfht_node *lookup;
        unsigned long hash, index, order;
 
@@ -806,10 +804,8 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
        }
        hash = bit_reverse_ulong(node->p.reverse_hash);
        for (;;) {
-               uint32_t chain_len;
+               uint32_t chain_len = 0;
 
-       retry:
-               chain_len = 0;
                /*
                 * iter_prev points to the non-removed node prior to the
                 * insert location.
@@ -829,25 +825,7 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
                        next = rcu_dereference(clear_flag(iter)->p.next);
                        if (unlikely(is_gc(next)))
                                goto gc_node;
-                       if (unlikely(replace_pinned)) {
-                               /*
-                                * We're in the retry of a node
-                                * replacement. Only get exact iter
-                                * pointer match. We own it, so it
-                                * _needs_ to be there at some point.
-                                */
-                               if (clear_flag(iter) == replace_pinned)
-                                       goto replace;
-                       }
-                       /*
-                        * Next is removed but not gc'd. We need to
-                        * busy-loop, because a concurrent replacement
-                        * is keeping it temporarily pinned there but we
-                        * cannot attach to it. The easiest solution is
-                        * to retry.
-                        */
-                       if (unlikely(is_removed(next)))
-                               goto retry;
+                       assert(!is_removed(next));
                        if ((mode == ADD_UNIQUE || mode == ADD_REPLACE)
                            && !is_dummy(next)
                            && !ht->compare_fct(node->key, node->key_len,
@@ -873,7 +851,6 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
                assert(!is_gc(iter_prev));
                assert(!is_gc(iter));
                assert(iter_prev != node);
-               assert(!replace_pinned);
                if (!dummy)
                        node->p.next = clear_flag(iter);
                else
@@ -894,6 +871,9 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
                }
 
        replace:
+               /* Insert after node to be replaced */
+               iter_prev = clear_flag(iter);
+               iter = next;
                assert(node != clear_flag(iter));
                assert(!is_removed(iter_prev));
                assert(!is_removed(iter));
@@ -901,53 +881,29 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
                assert(!is_gc(iter));
                assert(iter_prev != node);
                assert(!dummy);
-               node->p.next = clear_flag(next);
+               node->p.next = clear_flag(iter);
                if (is_dummy(iter))
                        new_node = flag_dummy(node);
                else
                        new_node = node;
                /*
-                * Try to delete to-be-replaced node. Don't gc yet. Not
-                * performing gc here is important, because this lets
-                * concurrent lookups see the old node until we
-                * atomically swap the new node into its place.
-                *
-                * This algorithm is _not_ strictly lock-free between
-                * _cds_lfht_del and the uatomic_cmpxchg of the
-                * replacement operation, so a replacement should _not_
-                * crash here (which means: don't do replacements if you
-                * need strict lock-free guarantees).
+                * Here is the whole trick for lock-free replace: we add
+                * the replacement node _after_ the node we want to
+                * replace by atomically setting its next pointer at the
+                * same time we set its removal and gc flags. Given that
+                * the lookups/get next use an iterator aware of the
+                * next pointer, they will either skip the old node due
+                * to the removal/gc flag and see the new node, or use
+                * the old new, but will not see the new one.
                 */
-               if (!replace_pinned) {
-                       if (_cds_lfht_del(ht, size, clear_flag(iter), 0, 0))
-                               continue;       /* concurrently removed. retry. */
-               }
-               /*
-                * After _cds_lfht_del succeeds, we have pinned the
-                * to-be-removed node in place by setting its removed
-                * flag, but not its gc flag. If we fail to cmpxchg our
-                * new node with this node, we need to retry everything
-                * from the initial lookup, and only stop when we reach
-                * the node we pinned into place.
-                */
-               return_node = uatomic_cmpxchg(&iter_prev->p.next,
-                                             iter, new_node);
-               if (return_node != iter) {
-                       /*
-                        * If cmpxchg fails, we need to do path
-                        * compression, but end it by placing our own
-                        * node into place.
-                        */
-                       replace_pinned = clear_flag(iter);
+               new_node = flag_removed(new_node);
+               new_node = flag_gc(new_node);
+               if (uatomic_cmpxchg(&iter_prev->p.next,
+                             iter, new_node) != iter) {
                        continue;       /* retry */
                } else {
-                       /*
-                        * cmpxchg succeeded. gc unnecessary, because we
-                        * unlinked the return_node ourself with the
-                        * cmpxchg.
-                        */
-                       return_node = clear_flag(return_node);
-                       goto end;
+                       return_node = iter_prev;
+                       goto gc_end;
                }
 
        gc_node:
@@ -967,7 +923,6 @@ gc_end:
        lookup = &ht->t.tbl[order]->nodes[index & (!order ? 0 : ((1UL << (order - 1)) - 1))];
        dummy_node = (struct cds_lfht_node *) lookup;
        _cds_lfht_gc_bucket(dummy_node, node);
-end:
        return return_node;
 }
 
@@ -1332,7 +1287,8 @@ struct cds_lfht *_cds_lfht_new(cds_lfht_hash_fct hash_fct,
        return ht;
 }
 
-struct cds_lfht_node *cds_lfht_lookup(struct cds_lfht *ht, void *key, size_t key_len)
+void cds_lfht_lookup(struct cds_lfht *ht, void *key, size_t key_len,
+               struct cds_lfht_iter *iter)
 {
        struct cds_lfht_node *node, *next, *dummy_node;
        struct _cds_lfht_node *lookup;
@@ -1361,11 +1317,7 @@ struct cds_lfht_node *cds_lfht_lookup(struct cds_lfht *ht, void *key, size_t key
                        break;
                }
                next = rcu_dereference(node->p.next);
-               /*
-                * We consider return nodes marked removed but not gc as
-                * hits for lookup vs replacement consistency.
-                */
-               if (likely(!is_gc(next))
+               if (likely(!is_removed(next))
                    && !is_dummy(next)
                    && likely(!ht->compare_fct(node->key, node->key_len, key, key_len))) {
                                break;
@@ -1373,21 +1325,22 @@ struct cds_lfht_node *cds_lfht_lookup(struct cds_lfht *ht, void *key, size_t key
                node = clear_flag(next);
        }
        assert(!node || !is_dummy(rcu_dereference(node->p.next)));
-       return node;
+       iter->node = node;
+       iter->next = next;
 }
 
-struct cds_lfht_node *cds_lfht_next(struct cds_lfht *ht,
-                               struct cds_lfht_node *node)
+void cds_lfht_next(struct cds_lfht *ht, struct cds_lfht_iter *iter)
 {
-       struct cds_lfht_node *next;
+       struct cds_lfht_node *node, *next;
        unsigned long reverse_hash;
        void *key;
        size_t key_len;
 
+       node = iter->node;
        reverse_hash = node->p.reverse_hash;
        key = node->key;
        key_len = node->key_len;
-       next = rcu_dereference(node->p.next);
+       next = iter->next;
        node = clear_flag(next);
 
        for (;;) {
@@ -1400,11 +1353,7 @@ struct cds_lfht_node *cds_lfht_next(struct cds_lfht *ht,
                        break;
                }
                next = rcu_dereference(node->p.next);
-               /*
-                * We consider return nodes marked removed but not gc as
-                * hits for lookup vs replacement consistency.
-                */
-               if (likely(!is_gc(next))
+               if (likely(!is_removed(next))
                    && !is_dummy(next)
                    && likely(!ht->compare_fct(node->key, node->key_len, key, key_len))) {
                                break;
@@ -1412,7 +1361,8 @@ struct cds_lfht_node *cds_lfht_next(struct cds_lfht *ht,
                node = clear_flag(next);
        }
        assert(!node || !is_dummy(rcu_dereference(node->p.next)));
-       return node;
+       iter->node = node;
+       iter->next = next;
 }
 
 void cds_lfht_add(struct cds_lfht *ht, struct cds_lfht_node *node)
index 89063b98626042262341fdcc27e5b6ee28c0277c..b68db492cc0a794e0503059de006109e41e4da02 100644 (file)
@@ -384,6 +384,7 @@ void *thr_reader(void *_count)
 {
        unsigned long long *count = _count;
        struct cds_lfht_node *node;
+       struct cds_lfht_iter iter;
 
        printf_verbose("thread_begin %s, thread id : %lx, tid %lu\n",
                        "reader", pthread_self(), (unsigned long)gettid());
@@ -399,9 +400,10 @@ void *thr_reader(void *_count)
 
        for (;;) {
                rcu_read_lock();
-               node = cds_lfht_lookup(test_ht,
+               cds_lfht_lookup(test_ht,
                        (void *)(((unsigned long) rand_r(&rand_lookup) % lookup_pool_size) + lookup_pool_offset),
-                       sizeof(void *));
+                       sizeof(void *), &iter);
+               node = cds_lfht_iter_get_node(&iter);
                if (node == NULL) {
                        if (validate_lookup) {
                                printf("[ERROR] Lookup cannot find initial node.\n");
@@ -444,6 +446,7 @@ void free_node_cb(struct rcu_head *head)
 void *thr_writer(void *_count)
 {
        struct cds_lfht_node *node, *ret_node;
+       struct cds_lfht_iter iter;
        struct wr_count *count = _count;
        int ret;
 
@@ -490,9 +493,10 @@ void *thr_writer(void *_count)
                } else {
                        /* May delete */
                        rcu_read_lock();
-                       node = cds_lfht_lookup(test_ht,
+                       cds_lfht_lookup(test_ht,
                                (void *)(((unsigned long) rand_r(&rand_lookup) % write_pool_size) + write_pool_offset),
-                               sizeof(void *));
+                               sizeof(void *), &iter);
+                       node = cds_lfht_iter_get_node(&iter);
                        if (node)
                                ret = cds_lfht_del(test_ht, node);
                        else
index 359edb574661400d53c3a2a94c79bf185fa67eb7..c3c1cbc2b797afbbc022f1bedbc146abd7cdb61e 100644 (file)
@@ -51,6 +51,16 @@ struct cds_lfht_node {
        struct rcu_head head;
 };
 
+struct cds_lfht_iter {
+       struct cds_lfht_node *node, *next;
+};
+
+static inline
+struct cds_lfht_node *cds_lfht_iter_get_node(struct cds_lfht_iter *iter)
+{
+       return iter->node;
+}
+
 struct cds_lfht;
 
 /*
@@ -155,26 +165,30 @@ int cds_lfht_destroy(struct cds_lfht *ht, pthread_attr_t **attr);
  * Call with rcu_read_lock held.
  */
 void cds_lfht_count_nodes(struct cds_lfht *ht,
-                       unsigned long *count,
-                       unsigned long *removed);
+               unsigned long *count,
+               unsigned long *removed);
 
 /*
  * cds_lfht_lookup - lookup a node by key.
  *
- * Return NULL if not found.
+ * Output in "*iter". *iter->node set to NULL if not found.
  * Call with rcu_read_lock held.
  */
-struct cds_lfht_node *cds_lfht_lookup(struct cds_lfht *ht, void *key, size_t key_len);
+void cds_lfht_lookup(struct cds_lfht *ht, void *key, size_t key_len,
+               struct cds_lfht_iter *iter);
 
 /*
  * cds_lfht_next - get the next item with same key (after a lookup).
  *
- * Return NULL if no following node exists with same key.
- * RCU read-side lock must be held across cds_lfht_lookup and cds_lfht_next calls, and also
- * between cds_lfht_next calls using the node returned by a previous
- * cds_lfht_next.  Call with rcu_read_lock held.
+ * Uses an iterator initialized by a lookup.
+ * Sets *iter-node to the following node with same key.
+ * Sets *iter->node to NULL if no following node exists with same key.
+ * RCU read-side lock must be held across cds_lfht_lookup and
+ * cds_lfht_next calls, and also between cds_lfht_next calls using the
+ * node returned by a previous cds_lfht_next.
+ * Call with rcu_read_lock held.
  */
-struct cds_lfht_node *cds_lfht_next(struct cds_lfht *ht, struct cds_lfht_node *node);
+void cds_lfht_next(struct cds_lfht *ht, struct cds_lfht_iter *iter);
 
 /*
  * cds_lfht_add - add a node to the hash table.
@@ -198,7 +212,8 @@ void cds_lfht_add(struct cds_lfht *ht, struct cds_lfht_node *node);
  * observable in the table. The same guarantee apply for combination of
  * add_unique and replace (see below).
  */
-struct cds_lfht_node *cds_lfht_add_unique(struct cds_lfht *ht, struct cds_lfht_node *node);
+struct cds_lfht_node *cds_lfht_add_unique(struct cds_lfht *ht,
+               struct cds_lfht_node *node);
 
 /*
  * cds_lfht_replace - replace a node within hash table.
@@ -213,19 +228,15 @@ struct cds_lfht_node *cds_lfht_add_unique(struct cds_lfht *ht, struct cds_lfht_n
  * The semantic of replacement vs lookups is the following: if lookups
  * are performed between a key insertion and its removal, we guarantee
  * that the lookups will always find the key if it is replaced
- * concurrently with the lookups. Providing this guarantee require us to
- * pin the node to remove in place (disallowing any insertion after this
- * node temporarily) before we can proceed to its exchange with the new
- * node atomically. This renders the "replace" operation not strictly
- * lock-free, because a thread crashing in the middle of the replace
- * operation could stop progress for other updaters.
+ * concurrently with the lookups.
  *
  * Providing this semantic allows us to ensure that replacement-only
  * schemes will never generate duplicated keys. It also allows us to
  * guarantee that a combination of replacement and add_unique updates
  * will never generate duplicated keys.
  */
-struct cds_lfht_node *cds_lfht_replace(struct cds_lfht *ht, struct cds_lfht_node *node);
+struct cds_lfht_node *cds_lfht_replace(struct cds_lfht *ht,
+               struct cds_lfht_node *node);
 
 /*
  * cds_lfht_del - remove node from hash table.
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