[3.16.y-ckt stable] Patch "batman-adv: Avoid recursive call_rcu for batadv_bla_claim" has been added to the 3.16.y-ckt tree

[Luis Henriques]

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

    batman-adv: Avoid recursive call_rcu for batadv_bla_claim

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt24.

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

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

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

>From 3816a728c9a27eb7db5fcc6ad3560ca8c06c0f96 Mon Sep 17 00:00:00 2001
From: Sven Eckelmann <sven at narfation.org>
Date: Thu, 14 Jan 2016 15:28:19 +0100
Subject: batman-adv: Avoid recursive call_rcu for batadv_bla_claim

commit 63b399272294e7a939cde41792dca38c549f0484 upstream.

The batadv_claim_free_ref function uses call_rcu to delay the free of the
batadv_bla_claim 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_bla_claim also contains a
reference to backbone_gw which must be removed.

The reference drop of backbone_gw was done in the call_rcu function
batadv_claim_free_rcu but should actually be done in the
batadv_claim_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: 23721387c409 ("batman-adv: add basic bridge loop avoidance code")
Signed-off-by: Sven Eckelmann <sven at narfation.org>
Acked-by: Simon Wunderlich <sw at simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner at neomailbox.ch>
Signed-off-by: Antonio Quartulli <a at unstable.cc>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/batman-adv/bridge_loop_avoidance.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/net/batman-adv/bridge_loop_avoidance.c b/net/batman-adv/bridge_loop_avoidance.c
index a957c8140721..90673ff8b87b 100644
--- a/net/batman-adv/bridge_loop_avoidance.c
+++ b/net/batman-adv/bridge_loop_avoidance.c
@@ -113,21 +113,17 @@ batadv_backbone_gw_free_ref(struct batadv_bla_backbone_gw *backbone_gw)
 }

 /* finally deinitialize the claim */
-static void batadv_claim_free_rcu(struct rcu_head *rcu)
+static void batadv_claim_release(struct batadv_bla_claim *claim)
 {
-	struct batadv_bla_claim *claim;
-
-	claim = container_of(rcu, struct batadv_bla_claim, rcu);
-
 	batadv_backbone_gw_free_ref(claim->backbone_gw);
-	kfree(claim);
+	kfree_rcu(claim, rcu);
 }

 /* free a claim, call claim_free_rcu if its the last reference */
 static void batadv_claim_free_ref(struct batadv_bla_claim *claim)
 {
 	if (atomic_dec_and_test(&claim->refcount))
-		call_rcu(&claim->rcu, batadv_claim_free_rcu);
+		batadv_claim_release(claim);
 }

 /**