netdev - Re: [PATCH take 2] pkt_sched: Fix qdisc_watchdog() vs. dev

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Message-Id: <20080911.044255.245121628.davem@davemloft.net>
Date:	Thu, 11 Sep 2008 04:42:55 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	jarkao2@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH take 2] pkt_sched: Fix qdisc_watchdog() vs.
 dev_deactivate() race

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Thu, 11 Sep 2008 21:00:35 +1000

> On Thu, Sep 11, 2008 at 03:49:55AM -0700, David Miller wrote:
> > 
> > Well some kind of check has to be there.
> > 
> > I _did_ remove it during my initial implementation, and that
> > turned into a reported performance regression.
> 
> I see.  How about looking at the queue that the head-of-qdisc
> packet maps to? That should be fairly cheap to compute.

This gets us back to the whole qdisc->ops->peek() discussion :)

And we don't have the qdisc lock here, taking it is undesirable,
and if we do take it we have to transfer that lock down into
__qdisc_run() which means adjusting all the other __qdisc_run()
callers.

It's very clumsy at best.

I therefore don't think it's wise peeking into the qdisc here.

But I do realize we have to do something about this, hmmm...
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