rculfhash: set minimum table size, add todo about helping resize

[urcu.git] / rculfhash.c

diff --git a/rculfhash.c b/rculfhash.c
index 193857647439ef9cc3cfbc222184cab4c32c3360..96351be6a53af6306e30957f37f3e7fa0aace907 100644 (file)
--- a/rculfhash.c
+++ b/rculfhash.c
@@ -166,6 +166,16 @@
 #define CHAIN_LEN_TARGET               1
 #define CHAIN_LEN_RESIZE_THRESHOLD     3
 
+/*
+ * Define the minimum table size. Protects against hash table resize overload
+ * when too many entries are added quickly before the resize can complete.
+ * This is especially the case if the table could be shrinked to a size of 1.
+ * TODO: we might want to make the add/remove operations help the resize to
+ * add or remove dummy nodes when a resize is ongoing to ensure upper-bound on
+ * chain length.
+ */
+#define MIN_TABLE_SIZE                 128
+
 #ifndef max
 #define max(a, b)      ((a) > (b) ? (a) : (b))
 #endif
@@ -699,7 +709,7 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht, struct rcu_table *t,
                 */
                index = hash & (t->size - 1);
                order = get_count_order_ulong(index + 1);
-               lookup = &t->tbl[order]->nodes[index & ((1UL << (order - 1)) - 1)];
+               lookup = &t->tbl[order]->nodes[index & ((!order ? 0 : (1UL << (order - 1))) - 1)];
                iter_prev = (struct cds_lfht_node *) lookup;
                /* We can always skip the dummy node initially */
                iter = rcu_dereference(iter_prev->p.next);
@@ -755,7 +765,7 @@ gc_end:
        /* Garbage collect logically removed nodes in the bucket */
        index = hash & (t->size - 1);
        order = get_count_order_ulong(index + 1);
-       lookup = &t->tbl[order]->nodes[index & ((1UL << (order - 1)) - 1)];
+       lookup = &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);
        return node;
@@ -799,7 +809,7 @@ int _cds_lfht_remove(struct cds_lfht *ht, struct rcu_table *t,
        assert(t->size > 0);
        index = hash & (t->size - 1);
        order = get_count_order_ulong(index + 1);
-       lookup = &t->tbl[order]->nodes[index & ((1UL << (order - 1)) - 1)];
+       lookup = &t->tbl[order]->nodes[index & (!order ? 0 : ((1UL << (order - 1)) - 1))];
        dummy = (struct cds_lfht_node *) lookup;
        _cds_lfht_gc_bucket(dummy, node);
 end:
@@ -917,7 +927,7 @@ struct cds_lfht *cds_lfht_new(cds_lfht_hash_fct hash_fct,
        ht->percpu_count = alloc_per_cpu_items_count();
        /* this mutex should not nest in read-side C.S. */
        pthread_mutex_init(&ht->resize_mutex, NULL);
-       order = get_count_order_ulong(max(init_size, 1)) + 1;
+       order = get_count_order_ulong(max(init_size, MIN_TABLE_SIZE)) + 1;
        ht->t = calloc(1, sizeof(struct cds_lfht)
                       + (order * sizeof(struct rcu_level *)));
        ht->t->size = 0;
@@ -940,9 +950,9 @@ struct cds_lfht_node *cds_lfht_lookup(struct cds_lfht *ht, void *key, size_t key
        t = rcu_dereference(ht->t);
        index = hash & (t->size - 1);
        order = get_count_order_ulong(index + 1);
-       lookup = &t->tbl[order]->nodes[index & ((1UL << (order - 1)) - 1)];
+       lookup = &t->tbl[order]->nodes[index & (!order ? 0 : ((1UL << (order - 1))) - 1)];
        dbg_printf("lookup hash %lu index %lu order %lu aridx %lu\n",
-                  hash, index, order, index & ((1UL << (order - 1)) - 1));
+                  hash, index, order, index & (!order ? 0 : ((1UL << (order - 1)) - 1)));
        node = (struct cds_lfht_node *) lookup;
        for (;;) {
                if (unlikely(!node))
@@ -1154,7 +1164,7 @@ void _do_cds_lfht_shrink(struct cds_lfht *ht, struct rcu_table *old_t,
        unsigned long old_order, new_order;
        struct rcu_table *new_t;
 
-       new_size = max(new_size, 1);
+       new_size = max(new_size, MIN_TABLE_SIZE);
        old_order = get_count_order_ulong(old_size) + 1;
        new_order = get_count_order_ulong(new_size) + 1;
        printf("resize from %lu (order %lu) to %lu (order %lu) buckets\n",
@@ -1214,7 +1224,7 @@ static
 unsigned long resize_target_update_count(struct rcu_table *t,
                                   unsigned long count)
 {
-       count = max(count, 1);
+       count = max(count, MIN_TABLE_SIZE);
        return uatomic_set(&t->resize_target, count);
 }