[PATCH 4.2.y-ckt 238/268] batman-adv: Avoid recursive call_rcu for batadv_bla_claim

From: Kamal Mostafa
Date: Wed Jan 27 2016 - 15:49:10 EST

4.2.8-ckt3 -stable review patch. If anyone has any objections, please let me know.


From: Sven Eckelmann <sven@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
Acked-by: Simon Wunderlich <sw@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Marek Lindner <mareklindner@xxxxxxxxxxxxx>
Signed-off-by: Antonio Quartulli <a@xxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
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 ba06092..7b521e7 100644
--- a/net/batman-adv/bridge_loop_avoidance.c
+++ b/net/batman-adv/bridge_loop_avoidance.c
@@ -126,21 +126,17 @@ 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);
- 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);