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

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Message-ID: <20080913012758.GA10459@gondor.apana.org.au>
Date:	Fri, 12 Sep 2008 18:27:58 -0700
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	jarkao2@...il.com, netdev@...r.kernel.org, kaber@...sh.net
Subject: Re: [PATCH take 2] pkt_sched: Fix qdisc_watchdog() vs.
	dev_deactivate() race

On Fri, Sep 12, 2008 at 06:22:59PM -0700, David Miller wrote:
> 
> This requires state, for the unlink.  And that unlink point could be
> several levels deep into the tree.
> 
> There is also currently no restriction on how a packet scheduler
> maintains it's queue of SKBs.
> 
> So, if the idea is to keep track of the skb_queue_head pointer to
> unlink from at the root, I don't think that's a good idea.

No non-leaf qdiscs would remember remember the child qdisc where
it peeked from.

> And then there are all of these complicated pieces of state the
> classful qdiscs modify when a SKB is removed from their visibility.
> So, the unlink isn't just a simple list delete operation.  There
> are queue lengths that need to be updated, watchdog timers to
> manage, etc.

The decision really comes down to whether it's harder to requeue
a seemingly arbitrary packet or whether it's harder to dequeue the
packet that was last peeked.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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