[4.2.y-ckt stable] Patch "batman-adv: Avoid recursive call_rcu for batadv_nc_node" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    batman-adv: Avoid recursive call_rcu for batadv_nc_node

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt3.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 2b909afa71d376fcfddca89cced3e574d4991047 Mon Sep 17 00:00:00 2001
From: Sven Eckelmann <sven at narfation.org>
Date: Tue, 5 Jan 2016 12:06:19 +0100
Subject: batman-adv: Avoid recursive call_rcu for batadv_nc_node

commit 44e8e7e91d6c7c7ab19688750f7257292640d1a0 upstream.

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 at narfation.org>
Signed-off-by: Marek Lindner <mareklindner at neomailbox.ch>
Signed-off-by: Antonio Quartulli <a at unstable.cc>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 net/batman-adv/network-coding.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/net/batman-adv/network-coding.c b/net/batman-adv/network-coding.c
index 4660401..3e750c5 100644
--- a/net/batman-adv/network-coding.c
+++ b/net/batman-adv/network-coding.c
@@ -204,28 +204,25 @@ 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);
 }

 /**
--
1.9.1