linux-kernel - Re: [OOPS] 2.6.21-rc6-git5 in cfq_dispatch

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Date:	Wed, 25 Apr 2007 18:34:06 +1000
From:	Neil Brown <neilb@...e.de>
To:	Brad Campbell <brad@...p.net.au>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [OOPS] 2.6.21-rc6-git5 in cfq_dispatch_insert

On Tuesday April 24, brad@...p.net.au wrote:
> [105449.653682] cfq: rbroot not empty, but ->next_rq == NULL! Fixing up, report the issue to 
> lkml@...r.kernel.org
> [105449.683646] cfq: busy=1,drv=0,timer=0
> [105449.694871] cfq rr_list:
> [105449.702715]   3108: sort=0,next=00000000,q=0/1,a=1/0,d=0/0,f=69
> [105449.720693] cfq busy_list:
> [105449.729054] cfq idle_list:
> [105449.737418] cfq cur_rr:

Ok, I have a theory.

An ELEVATOR_FRONT_MERGE occurs which changes req->sector and calls
->elevator_merged_fn which is cfq_merged_request.

At this time there is already a request in cfq with the same sector
number, and that request is the only other request on the queue.

cfq_merged_request calls cfq_reposition_rq_rb which removes the
req from ->sortlist and then calls cfq_add_rq_rb to add it back (at
the new location because ->sector has changed).

cfq_add_rq_rb finds there is already a request with the same sector
number and so elv_rb_add returns an __alias which is passed to
cfq_dispatch_insert. 
This calls cfq_remove_request and as that is the only request present,
->next_rq gets set to NULL.
The old request with the new sector number is then added to the
->sortlist, but ->next_rq is never set - it remains NULL.

How likely it would be to get two requests with the same sector number
I don't know.  I wouldn't expect it to ever happen - I have seen it
before, but it was due to a bug in ext3.  Maybe XFS does it
intentionally some times?

You could test this theory by putting a
   WARN_ON(cfqq->next_rq == NULL);
at the end of cfq_reposition_rq_rb, just after the cfq_add_rq_rb call.

I will leave the development of a suitable fix up to Jens if he agrees
that this is possible.

NeilBrown
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