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Message-ID: <46F0117A.4060807@trash.net>
Date: Tue, 18 Sep 2007 19:57:14 +0200
From: Patrick McHardy <kaber@...sh.net>
To: Chuck Ebbert <cebbert@...hat.com>
CC: Netdev <netdev@...r.kernel.org>
Subject: Re: SFQ qdisc crashes with limit of 2 packets
Patrick McHardy wrote:
> Chuck Ebbert wrote:
>
>>Limit of 1 is forbidden, crashes with 2, works with 3:
>>
>>>>From disassembling sch_sfq.ko it seems that it is on line 360 of sch_sfq.c:
>> sch->qstats.backlog -= skb->len;
>>where "skb" is an invalid pointer:
>
>
>
> Is it a NULL pointer or something random?
Never mind, I found the reason. When enqueuing the packet, sfq_enqueue
contains an off-by-one in the limit check (which IIRC is there for a
reason, but I can't remember right now) and drops the packet again.
dev_queue_xmit() calls qdisc_run() anyway and the empty qdisc is
dequeued, which is not handled by SFQ.
I see three possibilities to fix this (in my preferred order):
1) figure out why the off-by-one is there, if not needed remove
2) don't dequeue qdiscs even once if empty
3) check for NULL in sfq_dequeue
So I'll try to remeber why the off-by-one is there ..
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