netfilter: nf_conncount: speculative garbage collection on empty lists

Instead of removing a empty list node that might be reintroduced soon
thereafter, tentatively place the empty list node on the list passed to
tree_nodes_free(), then re-check if the list is empty again before erasing
it from the tree.

[ Florian: rebase on top of pending nf_conncount fixes ]

Fixes: 5c789e131cbb9 ("netfilter: nf_conncount: Add list lock and gc worker, and RCU for init tree search")
Reviewed-by: Shawn Bohrer <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/net/netfilter/nf_conntrack_count.h

···
 	spinlock_t list_lock;
 	struct list_head head;	/* connections with the same filtering key */
 	unsigned int count;	/* length of list */
-	bool dead;
 };
 
 struct nf_conncount_data *nf_conncount_init(struct net *net, unsigned int family,

net/netfilter/nf_conncount.c

···
 	return memcmp(a, b, klen * sizeof(u32));
 }
 
-static bool conn_free(struct nf_conncount_list *list,
+static void conn_free(struct nf_conncount_list *list,
 		      struct nf_conncount_tuple *conn)
 {
-	bool free_entry = false;
-
 	lockdep_assert_held(&list->list_lock);
 
 	list->count--;
 	list_del(&conn->node);
-	if (list->count == 0) {
-		list->dead = true;
-		free_entry = true;
-	}
 
 	kmem_cache_free(conncount_conn_cachep, conn);
-	return free_entry;
 }
 
 static const struct nf_conntrack_tuple_hash *
 find_or_evict(struct net *net, struct nf_conncount_list *list,
-	      struct nf_conncount_tuple *conn, bool *free_entry)
+	      struct nf_conncount_tuple *conn)
 {
 	const struct nf_conntrack_tuple_hash *found;
 	unsigned long a, b;
···
 	 */
 	age = a - b;
 	if (conn->cpu == cpu || age >= 2) {
-		*free_entry = conn_free(list, conn);
+		conn_free(list, conn);
 		return ERR_PTR(-ENOENT);
 	}
 
···
 	struct nf_conncount_tuple *conn, *conn_n;
 	struct nf_conn *found_ct;
 	unsigned int collect = 0;
-	bool free_entry = false;
 
 	/* check the saved connections */
 	list_for_each_entry_safe(conn, conn_n, &list->head, node) {
 		if (collect > CONNCOUNT_GC_MAX_NODES)
 			break;
 
-		found = find_or_evict(net, list, conn, &free_entry);
+		found = find_or_evict(net, list, conn);
 		if (IS_ERR(found)) {
 			/* Not found, but might be about to be confirmed */
 			if (PTR_ERR(found) == -EAGAIN) {
···
 	spin_lock_init(&list->list_lock);
 	INIT_LIST_HEAD(&list->head);
 	list->count = 0;
-	list->dead = false;
 }
 EXPORT_SYMBOL_GPL(nf_conncount_list_init);
 
···
 	struct nf_conncount_tuple *conn, *conn_n;
 	struct nf_conn *found_ct;
 	unsigned int collected = 0;
-	bool free_entry = false;
 	bool ret = false;
 
 	/* don't bother if other cpu is already doing GC */
···
 		return false;
 
 	list_for_each_entry_safe(conn, conn_n, &list->head, node) {
-		found = find_or_evict(net, list, conn, &free_entry);
+		found = find_or_evict(net, list, conn);
 		if (IS_ERR(found)) {
-			if (PTR_ERR(found) == -ENOENT)  {
-				if (free_entry) {
-					spin_unlock(&list->list_lock);
-					return true;
-				}
+			if (PTR_ERR(found) == -ENOENT)
 				collected++;
-			}
 			continue;
 		}
 
···
 			 * closed already -> ditch it
 			 */
 			nf_ct_put(found_ct);
-			if (conn_free(list, conn)) {
-				spin_unlock(&list->list_lock);
-				return true;
-			}
+			conn_free(list, conn);
 			collected++;
 			continue;
 		}
···
 			break;
 	}
 
-	if (!list->count) {
-		list->dead = true;
+	if (!list->count)
 		ret = true;
-	}
 	spin_unlock(&list->list_lock);
 
 	return ret;
···
 	kmem_cache_free(conncount_rb_cachep, rbconn);
 }
 
+/* caller must hold tree nf_conncount_locks[] lock */
 static void tree_nodes_free(struct rb_root *root,
 			    struct nf_conncount_rb *gc_nodes[],
 			    unsigned int gc_count)
···
 	while (gc_count) {
 		rbconn = gc_nodes[--gc_count];
 		spin_lock(&rbconn->list.list_lock);
-		rb_erase(&rbconn->node, root);
-		call_rcu(&rbconn->rcu_head, __tree_nodes_free);
+		if (!rbconn->list.count) {
+			rb_erase(&rbconn->node, root);
+			call_rcu(&rbconn->rcu_head, __tree_nodes_free);
+		}
 		spin_unlock(&rbconn->list.list_lock);
 	}
 }
···
 	    struct rb_root *root,
 	    unsigned int hash,
 	    const u32 *key,
-	    u8 keylen,
 	    const struct nf_conntrack_tuple *tuple,
 	    const struct nf_conntrack_zone *zone)
 {
···
 	struct nf_conncount_rb *rbconn;
 	struct nf_conncount_tuple *conn;
 	unsigned int count = 0, gc_count = 0;
+	u8 keylen = data->keylen;
 	bool do_gc = true;
 
 	spin_lock_bh(&nf_conncount_locks[hash]);
···
 	if (!tuple)
 		return 0;
 
-	return insert_tree(net, data, root, hash, key, keylen, tuple, zone);
+	return insert_tree(net, data, root, hash, key, tuple, zone);
 }
 
 static void tree_gc_worker(struct work_struct *work)