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

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Message-ID: <20080725113757.GC10399@ff.dom.local>
Date:	Fri, 25 Jul 2008 11:37:57 +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 03:57:12AM -0700, David Miller wrote:
> From: Jarek Poplawski <jarkao2@...il.com>
> Date: Fri, 25 Jul 2008 10:57:48 +0000
> 
> > On 20-07-2008 01:35, Jussi Kivilinna wrote:
> > > Signed-off-by: Jussi Kivilinna <jussi.kivilinna@...et.fi>
> > ...
> > > diff --git a/net/sched/sch_cbq.c b/net/sched/sch_cbq.c
> > > index 1afe3ee..f1d2f8e 100644
> > > --- a/net/sched/sch_cbq.c
> > > +++ b/net/sched/sch_cbq.c
> > > @@ -370,7 +370,6 @@ static int
> > >  cbq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
> > >  {
> > >  	struct cbq_sched_data *q = qdisc_priv(sch);
> > > -	int len = skb->len;
> > >  	int uninitialized_var(ret);
> > >  	struct cbq_class *cl = cbq_classify(skb, sch, &ret);
> > >  
> > > @@ -391,7 +390,7 @@ cbq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
> > >  	if (ret == NET_XMIT_SUCCESS) {
> > >  		sch->q.qlen++;
> > >  		sch->bstats.packets++;
> > > -		sch->bstats.bytes+=len;
> > > +		sch->bstats.bytes += qdisc_pkt_len(skb);
> > 
> > Alas I didn't manage to read this earlier, but this type of changes
> > here and elsewhere in this patch looks wrong to me: we shouldn't
> > use skb pointer after ->enqueue().
> 
> 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? :-)

Do you mean this bug you've forgotten to fix yet?

Thanks,
Jarek P.
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