Subject: [PATCH] X.25: decrement netdev reference counts on unload

From: Apollon Oikonomopoulos <[email-masked]>
Date: Tue, 7 Dec 2010 21:43:30 +0200

Hello,

We came across this bug when we accidentally loaded and unloaded the x25 
kernel module on a host with KVM virtual machines. All subsequent 
attempts to stop a KVM instance resulted in the KVM process entering D 
state. Reproducing this behaviour can be done in the following way:

1. modprobe x25
2. tunctl && ifconfig tap0 up
3. rmmod x25
4. tunctl -d tap0 (hangs)

A patch follows - we don't use X.25 ourselves so I'm not sure it won't 
panic someone's system.

Regards,
Apollon

PS: I am not subscribed to the list, so please copy any replies to my 
address. Thanks!
----
x25 does not decrement the network device reference counts on module unload.
Thus unregistering any pre-existing interface after unloading the x25 module
hangs and results in

 unregister_netdevice: waiting for tap0 to become free. Usage count = 1

This patch decrements the reference counts of all interfaces in x25_link_free,
the way it is already done in x25_link_device_down for NETDEV_DOWN events.

Signed-off-by: Apollon Oikonomopoulos <[email-masked]>
---
 net/x25/x25_link.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/net/x25/x25_link.c b/net/x25/x25_link.c
index 73e7b95..b25c646 100644
--- a/net/x25/x25_link.c
+++ b/net/x25/x25_link.c
@@ -394,6 +394,7 @@ void __exit x25_link_free(void)
 	list_for_each_safe(entry, tmp, &x25_neigh_list) {
 		nb = list_entry(entry, struct x25_neigh, node);
 		__x25_remove_neigh(nb);
+		dev_put(nb->dev);
 	}
 	write_unlock_bh(&x25_neigh_list_lock);
 }
--
1.7.1