[PATCH 4.1 089/127] batman-adv: Avoid recursive call_rcu for batadv_nc_node

From: Greg Kroah-Hartman
Date: Wed Jan 27 2016 - 14:45:51 EST

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


From: Sven Eckelmann <sven@xxxxxxxxxxxxx>

[ 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.

Fixes: d56b1705e28c ("batman-adv: network coding - detect coding nodes and remove these after timeout")
Signed-off-by: Sven Eckelmann <sven@xxxxxxxxxxxxx>
Signed-off-by: Marek Lindner <mareklindner@xxxxxxxxxxxxx>
Signed-off-by: Antonio Quartulli <a@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
net/batman-adv/network-coding.c | 19 ++++++++-----------
1 file changed, 8 insertions(+), 11 deletions(-)

--- a/net/batman-adv/network-coding.c
+++ b/net/batman-adv/network-coding.c
@@ -175,28 +175,25 @@ 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);
- 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);