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

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Date:	Mon, 15 Sep 2008 16:44:08 -0700
From:	"Duyck, Alexander H" <alexander.h.duyck@...el.com>
To:	Jarek Poplawski <jarkao2@...il.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
CC:	Alexander Duyck <alexander.duyck@...il.com>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"kaber@...sh.net" <kaber@...sh.net>
Subject: RE: [PATCH take 2] pkt_sched: Fix qdisc_watchdog() vs.
	dev_deactivate() race

Jarek Poplawski wrote:
> On Mon, Sep 15, 2008 at 07:20:08AM +0000, Jarek Poplawski wrote:
> ...
>> Of course preserving a flow consistency is must-be here, but I think
>> there are rehashing algorithms used in similar cases (sch_sfq) which
>> take care for this. As a matter of fact, I've thought of requeuing as
>> a best place to detect possible problems, but now I see that
>> Alexander's proposal let's to do this simply by observing this
>> TCQ_F_STOPPED flag [...]
>
> Hmm... or maybe it doesn't? Since this is qdisc flag we don't know at
> the top which tx_queue is a problem at the bottom...
>
The problem is with the nature of a qdisc.  For example let's say we have a
prio qdisc with a packet in lowest priority that fails to be transmitted due
to a stopped subqueue.  If we add an skb for a non-stopped queue to a higher
prio then the qdisc should be no longer stopped since we can dequeue from the
ring and transmit.  Thus, keeping a memory of which queue is stopped may not be
useful in a situation such as this.

The only thing I really prefer about my solution as opposed to the solution
Dave implemented was that it would mean only one dequeue instead of a peek
followed by a dequeue.  I figure the important thing is to push the
discovery of us being stopped to as soon as possible in the process.

It will probably be a few days before I have a patch with my approach ready.
I didn't realize how complex it would be to resolve this issue for CBQ, HTB,
HFSC, etc.  Also it is starting to look like I will probably need to implement
another function to support this since it seems like the dequeue operations
would need to be split into a multiqueue safe version, and a standard version
to support some workarounds like those found in qdisc_peek_len() for HFSC.

Thanks,

Alex

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