xen-netfront: Avoid deref'ing skbafter it is potentially freed.

xennet_tx_bug_gc can free the skb before we use it, so make sure we don't.
Signed-off-by: NKeir Fraser <keir@xensource.com>
Signed-off-by: NJeremy Fitzhardinge <jeremy@xensource.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: NJeff Garzik <jeff@garzik.org>

drivers/net/xen-netfront.c

@@ -566,6 +566,10 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (notify)
 		notify_remote_via_irq(np->netdev->irq);
 
+	np->stats.tx_bytes += skb->len;
+	np->stats.tx_packets++;
+
+	/* Note: It is not safe to access skb after xennet_tx_buf_gc()! */
 	xennet_tx_buf_gc(dev);
 
 	if (!netfront_tx_slot_available(np))
@@ -573,9 +577,6 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	spin_unlock_irq(&np->tx_lock);
 
-	np->stats.tx_bytes += skb->len;
-	np->stats.tx_packets++;
-
 	return 0;
 
  drop: