Re: [OOPS] 2.6.21-rc6-git5 in cfq_dispatch_insert

[Jens Axboe]

On Wed, Apr 25 2007, Jens Axboe wrote:
> On Wed, Apr 25 2007, Neil Brown wrote:
> > On Tuesday April 24, brad@xxxxxxxxxxx wrote:
> > > [105449.653682] cfq: rbroot not empty, but ->next_rq == NULL! Fixing up, report the issue to 
> > > lkml@xxxxxxxxxxxxxxx
> > > [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.
> 
> That's pretty close to where I think the problem is (the front merging
> and cfq_reposition_rq_rb()). The issue with that is that you'd only get
> aliases for O_DIRECT and/or raw IO, and that doesn't seem to be the case
> here. Given that front merges are equally not very likely, I'd be
> surprised is something like that has ever happened.
> 
> BUT... That may explain while we are only seeing it on md. Would md
> ever be issuing such requests that trigger this condition?
> 
> I'll try and concoct a test case.

I made a test case and reproduced it, this is indeed what is happening.
md must be issuing direct requests in such a manner, that you do get a
front merge and then an alias to an existing request. Will test a fix!

-- 
Jens Axboe

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