Messages in this thread | | | From | Neil Brown <> | Date | Wed, 25 Apr 2007 18:34:06 +1000 | Subject | Re: [OOPS] 2.6.21-rc6-git5 in cfq_dispatch_insert |
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On Tuesday April 24, brad@wasp.net.au wrote: > [105449.653682] cfq: rbroot not empty, but ->next_rq == NULL! Fixing up, report the issue to > lkml@vger.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.
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