batman-adv: Avoid recursive call_rcu for batadv_bla_claim

commit 63b39927 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: 23721387 ("batman-adv: add basic bridge loop avoidance code")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

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);
 }
 
 /**