Patch "batman-adv: Avoid recursive call_rcu for batadv_nc_node" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

batman-adv: Avoid recursive call_rcu for batadv_nc_node

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
batman-adv-avoid-recursive-call_rcu-for-batadv_nc_node.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Sven Eckelmann <[email protected]>
Date: Tue, 5 Jan 2016 12:06:19 +0100
Subject: batman-adv: Avoid recursive call_rcu for batadv_nc_node

From: Sven Eckelmann <[email protected]>

[ Upstream commit 44e8e7e91d6c7c7ab19688750f7257292640d1a0 ]

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.

---
 net/batman-adv/network-coding.c |   19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

@@ -203,28 +203,25 @@ void batadv_nc_init_orig(struct batadv_o
 }
 
 /**
- * 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);
 }
 
 /**

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/batman-adv-drop-immediate-batadv_neigh_node-free-function.patch queue-4.4/batman-adv-drop-immediate-neigh_ifinfo-free-function.patch queue-4.4/batman-adv-drop-immediate-orig_node-free-function.patch queue-4.4/batman-adv-drop-immediate-batadv_orig_ifinfo-free-function.patch queue-4.4/batman-adv-avoid-recursive-call_rcu-for-batadv_nc_node.patch queue-4.4/batman-adv-drop-immediate-batadv_hard_iface-free-function.patch queue-4.4/batman-adv-avoid-recursive-call_rcu-for-batadv_bla_claim.patch