Fix: take RCU read-side lock within hash table functions

After review, a great deal of caller sites miss the RCU read-side lock
when using the hash table modification functions. This is a case where
having a slight performance degradation might be worthwhile if we can be
a bit more stability. So instead of playing whack-a-mole, add the RCU
read-side lock in the hash table modification functions to ensure
protection from ABA.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>

diff --git a/src/common/hashtable/hashtable.c b/src/common/hashtable/hashtable.c
index b08a57e5cc0a07f53582a3181443d395c8b97765..8f2f2bf03221944d9297cc89fb4e146e672b58cb 100644 (file)
--- a/src/common/hashtable/hashtable.c
+++ b/src/common/hashtable/hashtable.c
@@ -32,6 +32,13 @@ unsigned long lttng_ht_seed;
 static unsigned long min_hash_alloc_size = 1;
 static unsigned long max_hash_buckets_size = 0;
 
+/*
+ * Getter/lookup functions need to be called with RCU read-side lock
+ * held. However, modification functions (add, add_unique, replace, del)
+ * take the RCU lock internally, so it does not matter whether the
+ * caller hold the RCU lock or not.
+ */
+
 /*
  * Match function for string node.
  */
@@ -252,8 +259,11 @@ void lttng_ht_add_unique_str(struct lttng_ht *ht,
        assert(ht->ht);
        assert(node);
 
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
        node_ptr = cds_lfht_add_unique(ht->ht, ht->hash_fct(node->key, lttng_ht_seed),
                        ht->match_fct, node->key, &node->node);
+       rcu_read_unlock();
        assert(node_ptr == &node->node);
 }
 
@@ -267,8 +277,11 @@ void lttng_ht_add_str(struct lttng_ht *ht,
        assert(ht->ht);
        assert(node);
 
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
        cds_lfht_add(ht->ht, ht->hash_fct(node->key, lttng_ht_seed),
                        &node->node);
+       rcu_read_unlock();
 }
 
 /*
@@ -280,8 +293,11 @@ void lttng_ht_add_ulong(struct lttng_ht *ht, struct lttng_ht_node_ulong *node)
        assert(ht->ht);
        assert(node);
 
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
        cds_lfht_add(ht->ht, ht->hash_fct((void *) node->key, lttng_ht_seed),
                        &node->node);
+       rcu_read_unlock();
 }
 
 /*
@@ -294,8 +310,11 @@ void lttng_ht_add_u64(struct lttng_ht *ht, struct lttng_ht_node_u64 *node)
        assert(ht->ht);
        assert(node);
 
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
        cds_lfht_add(ht->ht, ht->hash_fct(&node->key, lttng_ht_seed),
                        &node->node);
+       rcu_read_unlock();
 }
 
 /*
@@ -309,9 +328,12 @@ void lttng_ht_add_unique_ulong(struct lttng_ht *ht,
        assert(ht->ht);
        assert(node);
 
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
        node_ptr = cds_lfht_add_unique(ht->ht,
                        ht->hash_fct((void *) node->key, lttng_ht_seed), ht->match_fct,
                        (void *) node->key, &node->node);
+       rcu_read_unlock();
        assert(node_ptr == &node->node);
 }
 
@@ -326,9 +348,12 @@ void lttng_ht_add_unique_u64(struct lttng_ht *ht,
        assert(ht->ht);
        assert(node);
 
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
        node_ptr = cds_lfht_add_unique(ht->ht,
                        ht->hash_fct(&node->key, lttng_ht_seed), ht->match_fct,
                        &node->key, &node->node);
+       rcu_read_unlock();
        assert(node_ptr == &node->node);
 }
 
@@ -343,9 +368,12 @@ void lttng_ht_add_unique_two_u64(struct lttng_ht *ht,
        assert(ht->ht);
        assert(node);
 
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
        node_ptr = cds_lfht_add_unique(ht->ht,
                        ht->hash_fct((void *) &node->key, lttng_ht_seed), ht->match_fct,
                        (void *) &node->key, &node->node);
+       rcu_read_unlock();
        assert(node_ptr == &node->node);
 }
 
@@ -360,9 +388,12 @@ struct lttng_ht_node_ulong *lttng_ht_add_replace_ulong(struct lttng_ht *ht,
        assert(ht->ht);
        assert(node);
 
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
        node_ptr = cds_lfht_add_replace(ht->ht,
                        ht->hash_fct((void *) node->key, lttng_ht_seed), ht->match_fct,
                        (void *) node->key, &node->node);
+       rcu_read_unlock();
        if (!node_ptr) {
                return NULL;
        } else {
@@ -382,9 +413,12 @@ struct lttng_ht_node_u64 *lttng_ht_add_replace_u64(struct lttng_ht *ht,
        assert(ht->ht);
        assert(node);
 
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
        node_ptr = cds_lfht_add_replace(ht->ht,
                        ht->hash_fct(&node->key, lttng_ht_seed), ht->match_fct,
                        &node->key, &node->node);
+       rcu_read_unlock();
        if (!node_ptr) {
                return NULL;
        } else {
@@ -398,11 +432,17 @@ struct lttng_ht_node_u64 *lttng_ht_add_replace_u64(struct lttng_ht *ht,
  */
 int lttng_ht_del(struct lttng_ht *ht, struct lttng_ht_iter *iter)
 {
+       int ret;
+
        assert(ht);
        assert(ht->ht);
        assert(iter);
 
-       return cds_lfht_del(ht->ht, iter->iter.node);
+       /* RCU read lock protects from ABA. */
+       rcu_read_lock();
+       ret = cds_lfht_del(ht->ht, iter->iter.node);
+       rcu_read_unlock();
+       return ret;
 }
 
 /*
@@ -440,7 +480,10 @@ unsigned long lttng_ht_get_count(struct lttng_ht *ht)
        assert(ht);
        assert(ht->ht);
 
+       /* RCU read lock protects from ABA and allows RCU traversal. */
+       rcu_read_lock();
        cds_lfht_count_nodes(ht->ht, &scb, &count, &sca);
+       rcu_read_unlock();
 
        return count;
 }