[PATCH] tcpdump may trace some outbound packets twice.

[Ranjit Manomohan <ranjitm@google.com>]

This patch fixes the problem where tcpdump shows duplicate packets
while tracing outbound packets on drivers which support lockless
transmit. The patch changes the current behaviour to tracing the
packets only on a successful transmit.

Signed-off-by: Ranjit Manomohan <ranjitm@google.com>

--- linux-2.6/net/sched/sch_generic.c	2006-05-10 12:34:52.000000000 -0700
+++ linux/net/sched/sch_generic.c	2006-05-10 12:39:38.000000000 -0700
@@ -136,8 +136,12 @@
 
 			if (!netif_queue_stopped(dev)) {
 				int ret;
+				struct sk_buff *skbc = NULL;
+				/* Clone the skb so that we hold a reference
+				 * to its data and we can trace it after a
+				 * successful transmit. */
 				if (netdev_nit)
-					dev_queue_xmit_nit(skb, dev);
+					skbc = skb_clone(skb, GFP_ATOMIC);
 
 				ret = dev->hard_start_xmit(skb, dev);
 				if (ret == NETDEV_TX_OK) { 
@@ -145,6 +149,15 @@
 						dev->xmit_lock_owner = -1;
 						spin_unlock(&dev->xmit_lock);
 					}
+					if(skbc) {
+						/* transmit succeeded, 
+						 * trace the clone. */
+						dev_queue_xmit_nit(skbc,dev);
+						kfree_skb(skbc);
+					}
+					/* Free clone if it exists */
+					if(skbc)
+						kfree_skb(skbc);
 					spin_lock(&dev->queue_lock);
 					return -1;
 				}