From: Mathieu Desnoyers Date: Mon, 19 Sep 2011 16:45:26 +0000 (-0400) Subject: rculfhash: remove helper scheme X-Git-Tag: v0.7.0~43^2~148^2 X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=f9c8034108dccf1434c99da6b5768aad0b8c10f1;p=urcu.git rculfhash: remove helper scheme 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 --- diff --git a/rculfhash.c b/rculfhash.c index 824a940..08d024c 100644 --- a/rculfhash.c +++ b/rculfhash.c @@ -169,7 +169,6 @@ /* * Define the minimum table size. */ -//#define MIN_TABLE_SIZE 128 #define MIN_TABLE_SIZE 1 #if (CAA_BITS_PER_LONG == 32) @@ -196,7 +195,7 @@ #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))) {