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 3.16: 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

@@ -1440,6 +1440,8 @@ static void xennet_disconnect_backend(struct netfront_info *info)
 	for (i = 0; i < num_queues; ++i) {
 		struct netfront_queue *queue = &info->queues[i];
 
+		del_timer_sync(&queue->rx_refill_timer);
+
 		if (queue->tx_irq && (queue->tx_irq == queue->rx_irq))
 			unbind_from_irqhandler(queue->tx_irq, queue);
 		if (queue->tx_irq && (queue->tx_irq != queue->rx_irq)) {
@@ -2360,8 +2362,6 @@ static int xennet_remove(struct xenbus_device *dev)
 {
 	struct netfront_info *info = dev_get_drvdata(&dev->dev);
 	unsigned int num_queues = info->netdev->real_num_tx_queues;
-	struct netfront_queue *queue = NULL;
-	unsigned int i = 0;
 
 	dev_dbg(&dev->dev, "%s\n", dev->nodename);
 
@@ -2371,11 +2371,6 @@ static int xennet_remove(struct xenbus_device *dev)
 
 	unregister_netdev(info->netdev);
 
-	for (i = 0; i < num_queues; ++i) {
-		queue = &info->queues[i];
-		del_timer_sync(&queue->rx_refill_timer);
-	}
-
 	if (num_queues) {
 		kfree(info->queues);
 		info->queues = NULL;