batman-adv: Avoid recursive call_rcu for batadv_nc_node

The batadv_nc_node_free_ref function uses call_rcu to delay the free of the
batadv_nc_node object until no (already started) rcu_read_lock is enabled
anymore. This makes sure that no context is still trying to access the
object which should be removed. But batadv_nc_node also contains a
reference to orig_node which must be removed.

The reference drop of orig_node was done in the call_rcu function
batadv_nc_node_free_rcu but should actually be done in the
batadv_nc_node_release function to avoid nested call_rcus. This is
important because rcu_barrier (e.g. batadv_softif_free or batadv_exit) will
not detect the inner call_rcu as relevant for its execution. Otherwise this
barrier will most likely be inserted in the queue before the callback of
the first call_rcu was executed. The caller of rcu_barrier will therefore
continue to run before the inner call_rcu callback finished.

Fixes: d56b1705e28c ("batman-adv: network coding - detect coding nodes and remove these after timeout")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>

net/batman-adv/network-coding.c

@@ -203,28 +203,25 @@ void batadv_nc_init_orig(struct batadv_orig_node *orig_node)
 }
 
 /**
- * batadv_nc_node_free_rcu - rcu callback to free an nc node and remove
- *  its refcount on the orig_node
- * @rcu: rcu pointer of the nc node
+ * batadv_nc_node_release - release nc_node from lists and queue for free after
+ *  rcu grace period
+ * @nc_node: the nc node to free
  */
-static void batadv_nc_node_free_rcu(struct rcu_head *rcu)
+static void batadv_nc_node_release(struct batadv_nc_node *nc_node)
 {
-	struct batadv_nc_node *nc_node;
-
-	nc_node = container_of(rcu, struct batadv_nc_node, rcu);
 	batadv_orig_node_free_ref(nc_node->orig_node);
-	kfree(nc_node);
+	kfree_rcu(nc_node, rcu);
 }
 
 /**
- * batadv_nc_node_free_ref - decrements the nc node refcounter and possibly
- * frees it
+ * batadv_nc_node_free_ref - decrement the nc node refcounter and possibly
+ *  release it
  * @nc_node: the nc node to free
  */
 static void batadv_nc_node_free_ref(struct batadv_nc_node *nc_node)
 {
 	if (atomic_dec_and_test(&nc_node->refcount))
-		call_rcu(&nc_node->rcu, batadv_nc_node_free_rcu);
+		batadv_nc_node_release(nc_node);
 }
 
 /**