netdev - Re: [PATCH v2 2/3] net_sched: Add accessor function for packet length for qdiscs

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Date:	Fri, 25 Jul 2008 11:53:55 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	jussi.kivilinna@...et.fi, kaber@...sh.net, netdev@...r.kernel.org
Subject: Re: [PATCH v2 2/3] net_sched: Add accessor function for packet
	length for qdiscs

On Fri, Jul 25, 2008 at 11:37:57AM +0000, Jarek Poplawski wrote:
> On Fri, Jul 25, 2008 at 03:57:12AM -0700, David Miller wrote:
...
> > It should be OK here, we have the root qdisc locked, so nobody
> > can remove it from the queue and if we got NET_XMIT_SUCCESS that
> > thing must not have been freed.
> > 
> > And anyways, we can't know the qdisc_pkt_len() until the enqueue
> > fucntion has been called.
> 
> OK, I see htb did this earlier with ".packets", so it looks like I'm
> wrong with this - sorry! (Anyway, it seems to unnecessarily restrict
> the way qdisc are working.)
> 
> > Even TCP depends upon that NET_XMIT_* return value having some real
> > meaning, remember the HTB bug we tracked down last week? :-)

As a matter of fact it's a good example: in your patch we have this:

+		ret = cl->un.leaf.q->enqueue(skb, cl->un.leaf.q);
+		if (ret == NET_XMIT_DROP) {
+			sch->qstats.drops++;
+			cl->qstats.drops++;
+		} else {
+			cl->bstats.packets +=
+				skb_is_gso(skb)?skb_shinfo(skb)->gso_segs:1;
+			cl->bstats.bytes += skb->len;

So, if something works like noop_enqueue() and returns NET_XMIT_CN
here, we're just using skb after kfree...

Jarek P.
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