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SubjectRe: [PATCH] tcpdump may trace some outbound packets twice.
On Mon, 15 May 2006, David S. Miller wrote:

> From: Ranjit Manomohan <ranjitm@google.com>
> Date: Mon, 15 May 2006 14:19:06 -0700 (PDT)
>
> > Heres a new version which does a copy instead of the clone to avoid
> > the double cloning issue.
>
> I still very much dislike this patch because it is creating
> 1 more clone per packet than is actually necessary and that
> is very expensive.
>
> dev_queue_xmit_nit() is going to clone whatever SKB you send into
> there, so better to just bump the reference count (with skb_get())
> instead of cloning or copying.
>

I was a bit apprehensive about just incrementing the refcnt but that works
too. Attached is the modified version.

-Thanks,
Ranjit

--- linux-2.6/net/sched/sch_generic.c 2006-05-10 12:34:52.000000000 -0700
+++ linux/net/sched/sch_generic.c 2006-05-15 15:48:03.000000000 -0700
@@ -136,8 +136,12 @@

if (!netif_queue_stopped(dev)) {
int ret;
+ struct sk_buff *skbc = NULL;
+ /* Increment the reference count on the skb so
+ * that we can use it after a successful xmit.
+ */
if (netdev_nit)
- dev_queue_xmit_nit(skb, dev);
+ skbc = skb_get(skb);

ret = dev->hard_start_xmit(skb, dev);
if (ret == NETDEV_TX_OK) {
@@ -145,9 +149,20 @@
dev->xmit_lock_owner = -1;
spin_unlock(&dev->xmit_lock);
}
+ if (skbc) {
+ /* transmit succeeded,
+ * trace the buffer. */
+ dev_queue_xmit_nit(skbc,dev);
+ kfree_skb(skbc);
+ }
spin_lock(&dev->queue_lock);
return -1;
}
+
+ /* Call free in case we incremented refcnt */
+ if (skbc)
+ kfree_skb(skbc);
+
if (ret == NETDEV_TX_LOCKED && nolock) {
spin_lock(&dev->queue_lock);
goto collision;
-
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