rculfhash: remove helper scheme
authorMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Mon, 19 Sep 2011 16:45:26 +0000 (12:45 -0400)
committerMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Mon, 19 Sep 2011 16:48:25 +0000 (12:48 -0400)
There is a trade-off to consider here:
- The helper scheme would require the helpers to allocate the memory for
  dummy nodes themself for correctness (the current implementation is
  buggy because lookups consider an half-linked dummy node to be ready for
  use), which does not work with a cache-efficient per-level array of
  dummy nodes.
- We want cache-efficiency, so we want to keep the per-level array
  allocation.

Therefore, letting insert/removal/lookups help expand is not the way to
go here. This patch removes the helping scheme.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
rculfhash.c

index 824a940391461a1a3b7f1db0a847be38e84dd741..08d024c10996833462c94e1ea96162f609502195 100644 (file)
 /*
  * Define the minimum table size.
  */
-//#define MIN_TABLE_SIZE                       128
 #define MIN_TABLE_SIZE                 1
 
 #if (CAA_BITS_PER_LONG == 32)
 #define FLAGS_MASK             ((1UL << 2) - 1)
 
 /* Value of the end pointer. Should not interact with flags. */
-#define END_VALUE              0x4
+#define END_VALUE              NULL
 
 struct ht_items_count {
        unsigned long add, remove;
@@ -695,7 +694,7 @@ void cds_lfht_free_level(struct rcu_head *head)
  * Remove all logically deleted nodes from a bucket up to a certain node key.
  */
 static
-int _cds_lfht_gc_bucket(struct cds_lfht_node *dummy, struct cds_lfht_node *node)
+void _cds_lfht_gc_bucket(struct cds_lfht_node *dummy, struct cds_lfht_node *node)
 {
        struct cds_lfht_node *iter_prev, *iter, *next, *new_next;
 
@@ -707,15 +706,6 @@ int _cds_lfht_gc_bucket(struct cds_lfht_node *dummy, struct cds_lfht_node *node)
                iter_prev = dummy;
                /* We can always skip the dummy node initially */
                iter = rcu_dereference(iter_prev->p.next);
-               if (unlikely(iter == NULL)) {
-                       /*
-                        * We are executing concurrently with a hash table
-                        * expand, so we see a dummy node with NULL next value.
-                        * Help expand by linking this node into the list and
-                        * retry.
-                        */
-                       return 1;
-               }
                assert(iter_prev->p.reverse_hash <= node->p.reverse_hash);
                /*
                 * We should never be called with dummy (start of chain)
@@ -725,11 +715,10 @@ int _cds_lfht_gc_bucket(struct cds_lfht_node *dummy, struct cds_lfht_node *node)
                 */
                assert(dummy != node);
                for (;;) {
-                       assert(iter != NULL);
                        if (unlikely(is_end(iter)))
-                               return 0;
+                               return;
                        if (likely(clear_flag(iter)->p.reverse_hash > node->p.reverse_hash))
-                               return 0;
+                               return;
                        next = rcu_dereference(clear_flag(iter)->p.next);
                        if (likely(is_removed(next)))
                                break;
@@ -741,10 +730,9 @@ int _cds_lfht_gc_bucket(struct cds_lfht_node *dummy, struct cds_lfht_node *node)
                        new_next = flag_dummy(clear_flag(next));
                else
                        new_next = clear_flag(next);
-               assert(new_next != NULL);
                (void) uatomic_cmpxchg(&iter_prev->p.next, iter, new_next);
        }
-       return 0;
+       return;
 }
 
 static
@@ -757,7 +745,6 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
                        *dummy_node;
        struct _cds_lfht_node *lookup;
        unsigned long hash, index, order;
-       int force_dummy = 0;
 
        assert(!is_dummy(node));
        assert(!is_removed(node));
@@ -780,26 +767,8 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
                iter_prev = (struct cds_lfht_node *) lookup;
                /* We can always skip the dummy node initially */
                iter = rcu_dereference(iter_prev->p.next);
-               if (unlikely(iter == NULL)) {
-                       /*
-                        * We are executing concurrently with a hash table
-                        * expand, so we see a dummy node with NULL next value.
-                        * Help expand by linking this node into the list and
-                        * retry.
-                        */
-                       (void) _cds_lfht_add(ht, size >> 1, iter_prev, 0, 1);
-                       continue;       /* retry */
-               }
                assert(iter_prev->p.reverse_hash <= node->p.reverse_hash);
                for (;;) {
-                       assert(iter != NULL);
-                       /*
-                        * When adding a dummy node, we allow concurrent
-                        * add/removal to help. If we find the dummy node in
-                        * place, skip its insertion.
-                        */
-                       if (unlikely(dummy && clear_flag(iter) == node))
-                               return node;
                        if (unlikely(is_end(iter)))
                                goto insert;
                        if (likely(clear_flag(iter)->p.reverse_hash > node->p.reverse_hash))
@@ -825,31 +794,14 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
                assert(!is_removed(iter_prev));
                assert(!is_removed(iter));
                assert(iter_prev != node);
-               if (!dummy) {
+               if (!dummy)
                        node->p.next = clear_flag(iter);
-               } else {
-                       /*
-                        * Dummy node insertion is performed concurrently (help
-                        * scheme). We try to link its next node, and if this
-                        * succeeds, it _means_ it's us who link this dummy node
-                        * into the table. force_dummy is set as soon as we
-                        * succeed this cmpxchg within this function.
-                        */
-                       if (!force_dummy) {
-                               if (uatomic_cmpxchg(&node->p.next, NULL,
-                                               flag_dummy(clear_flag(iter))) != NULL) {
-                                       return NULL;
-                               }
-                               force_dummy = 1;
-                       } else {
-                               node->p.next = flag_dummy(clear_flag(iter));
-                       }
-               }
+               else
+                       node->p.next = flag_dummy(clear_flag(iter));
                if (is_dummy(iter))
                        new_node = flag_dummy(node);
                else
                        new_node = node;
-               assert(new_node != NULL);
                if (uatomic_cmpxchg(&iter_prev->p.next, iter,
                                    new_node) != iter)
                        continue;       /* retry */
@@ -871,11 +823,7 @@ gc_end:
        order = get_count_order_ulong(index + 1);
        lookup = &ht->t.tbl[order]->nodes[index & (!order ? 0 : ((1UL << (order - 1)) - 1))];
        dummy_node = (struct cds_lfht_node *) lookup;
-       if (_cds_lfht_gc_bucket(dummy_node, node)) {
-               /* Help expand */
-               (void) _cds_lfht_add(ht, size >> 1, dummy_node, 0, 1);
-               goto gc_end;    /* retry */
-       }
+       _cds_lfht_gc_bucket(dummy_node, node);
        return node;
 }
 
@@ -914,18 +862,13 @@ int _cds_lfht_remove(struct cds_lfht *ht, unsigned long size,
         * the node, and remove it (along with any other logically removed node)
         * if found.
         */
-gc_retry:
        hash = bit_reverse_ulong(node->p.reverse_hash);
        assert(size > 0);
        index = hash & (size - 1);
        order = get_count_order_ulong(index + 1);
        lookup = &ht->t.tbl[order]->nodes[index & (!order ? 0 : ((1UL << (order - 1)) - 1))];
        dummy = (struct cds_lfht_node *) lookup;
-       if (_cds_lfht_gc_bucket(dummy, node)) {
-               /* Help expand */
-               (void) _cds_lfht_add(ht, size >> 1, dummy, 0, 1);
-               goto gc_retry;  /* retry */
-       }
+       _cds_lfht_gc_bucket(dummy, node);
 end:
        /*
         * Only the flagging action indicated that we (and no other)
@@ -1009,21 +952,18 @@ void init_table(struct cds_lfht *ht,
                /* Set all dummy nodes reverse hash values for a level */
                init_table_hash(ht, i, len);
 
-               /*
-                * Update table size. At this point, concurrent add/remove see
-                * dummy nodes with correctly initialized reverse hash value,
-                * but with NULL next pointers. If they do, they can help us
-                * link the dummy nodes into the list and retry.
-                */
-               cmm_smp_wmb();  /* populate data before RCU size */
-               CMM_STORE_SHARED(ht->t.size, !i ? 1 : (1UL << i));
-
                /*
                 * Link all dummy nodes into the table. Concurrent
                 * add/remove are helping us.
                 */
                init_table_link(ht, i, len);
 
+               /*
+                * Update table size.
+                */
+               cmm_smp_wmb();  /* populate data before RCU size */
+               CMM_STORE_SHARED(ht->t.size, !i ? 1 : (1UL << i));
+
                dbg_printf("init new size: %lu\n", !i ? 1 : (1UL << i));
                if (CMM_LOAD_SHARED(ht->in_progress_destroy))
                        break;
@@ -1173,7 +1113,6 @@ struct cds_lfht_node *cds_lfht_lookup(struct cds_lfht *ht, void *key, size_t key
        hash = ht->hash_fct(key, key_len, ht->hash_seed);
        reverse_hash = bit_reverse_ulong(hash);
 
-restart:
        size = rcu_dereference(ht->t.size);
        index = hash & (size - 1);
        order = get_count_order_ulong(index + 1);
@@ -1183,16 +1122,6 @@ restart:
        dummy_node = (struct cds_lfht_node *) lookup;
        /* We can always skip the dummy node initially */
        node = rcu_dereference(dummy_node->p.next);
-       if (unlikely(node == NULL)) {
-               /*
-                * We are executing concurrently with a hash table
-                * expand, so we see a dummy node with NULL next value.
-                * Help expand by linking this node into the list and
-                * retry.
-                */
-               (void) _cds_lfht_add(ht, size >> 1, dummy_node, 0, 1);
-               goto restart;   /* retry */
-       }
        node = clear_flag(node);
        for (;;) {
                if (unlikely(is_end(node))) {
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