On Friday 04 March 2016 16:21:32 Andrew Lunn wrote:
Hi
I'm sometimes getting a crash after removing a hard interface when the batadv_send_outstanding_bat_org_packet() is called in a work queue. It calls
static void batadv_iv_ogm_aggregate_new(const unsigned char *packet_buff, int packet_len, unsigned long send_time, bool direct_link, struct batadv_hard_iface *if_incoming, struct batadv_hard_iface *if_outgoing, int own_packet) { struct batadv_priv *bat_priv = netdev_priv(if_incoming->soft_iface); struct batadv_forw_packet *forw_packet_aggr; unsigned char *skb_buff; unsigned int skb_size;
if (!kref_get_unless_zero(&if_incoming->refcount)) return; if (!kref_get_unless_zero(&if_outgoing->refcount)) goto out_free_incoming;Given that we have:
static inline void batadv_hardif_put(struct batadv_hard_iface *hard_iface) { kref_put(&hard_iface->refcount, batadv_hardif_release); }
does using kref_get_unless_zero() make sense? If it is zero, hasn't it been freed by the kref_put that set it to zero?
At least it makes sense for the outgoing interface because it is only in a rcu_read_lock in batadv_iv_ogm_schedule (batadv_iv_ogm_queue_add -> batadv_iv_ogm_aggregate_new). The batadv_hardif_list is traversed with list_for_each_entry_rcu and it is expected that one entry (maybe) gets dropped from the list. The batadv_hardif_release will only queue the actual free of the memory (kfree_rcu) and every function which wants to get a reference has to increase the counter with kref_get_unless_zero to check that it is not actually in the waiting-to-be-freed-phase.
But you have something which needs to be fixed (you see a crash). Question is what is causing the crash and what can be done against it. I am currently wondering how the if_incoming interface is being protected. It is not fetched from a list via a rcu list access primitive and it is not protected via rcu_read_lock. I can also not see where the reference for the forw_packet-
if_incoming is increased. It is just accessed in
batadv_send_outstanding_bat_ogm_packet (and later send to the mentioned function via batadv_schedule_bat_ogm). Also batadv_add_bcast_packet_to_list doesn't increase the reference counter for if_incoming before adding to the forward packet. So I would just say that the reference counting for batadv_hard_iface is broken.
Kind regards, Sven