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

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Message-Id: <20080911.033956.233317469.davem@davemloft.net>
Date:	Thu, 11 Sep 2008 03:39:56 -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: David Miller <davem@...emloft.net>
Date: Thu, 21 Aug 2008 00:52:50 -0700 (PDT)

> From: Herbert Xu <herbert@...dor.apana.org.au>
> Date: Thu, 21 Aug 2008 17:16:34 +1000
> 
> > Actually, why do we even keep a netdev_queue pointer in a qdisc?
> > A given qdisc can be used by multiple queues (which is why the
> > lock was moved into the qdisc in the first place).
> > 
> > How about keeping a pointer directly to the root qdisc plus a
> > pointer to the netdev (which seems to be the only other use for
> > qdisc->dev_queue)? That way there won't be any confusion as to
> > whether we want the sleeping or non-sleeping qdisc.
> 
> Not a bad idea at all.
> 
> The reason it's there is a left-over from earlier designs of my
> multiqueue stuff, I thought we'd always multiplex the qdiscs to be
> per-queue.  But once Patrick showed me we couldn't do that, we now
> have shared qdiscs.

I got to looking into this and we do need the qdisc->dev_queue member,
see qdisc_run().  So it's not like we can get rid of it if we replace
it with ->netdevdev and add a ->root_qdisc backpointer as well.

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