xen-netfront: Delete rx_refill_timer in xennet_disconnect_backend()

commit 74470954 upstream.

rx_refill_timer should be deleted as soon as we disconnect from the
backend since otherwise it is possible for the timer to go off before
we get to xennet_destroy_queues(). If this happens we may dereference
queue->rx.sring which is set to NULL in xennet_disconnect_backend().
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: there's only one RX queue here, and del_timer_sync()
 was called from xennet_remove() but that's also too late]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/xen-netfront.c

@@ -1404,6 +1404,8 @@ static void xennet_disconnect_backend(struct netfront_info *info)
 	spin_unlock_irq(&info->tx_lock);
 	spin_unlock_bh(&info->rx_lock);
 
+	del_timer_sync(&info->rx_refill_timer);
+
 	if (info->netdev->irq)
 		unbind_from_irqhandler(info->netdev->irq, info->netdev);
 	info->evtchn = info->netdev->irq = 0;
@@ -1940,8 +1942,6 @@ static int __devexit xennet_remove(struct xenbus_device *dev)
 
 	unregister_netdev(info->netdev);
 
-	del_timer_sync(&info->rx_refill_timer);
-
 	free_percpu(info->stats);
 
 	free_netdev(info->netdev);