On Tuesday April 24, [EMAIL PROTECTED] wrote: > [105449.653682] cfq: rbroot not empty, but ->next_rq == NULL! Fixing up, > report the issue to > [EMAIL PROTECTED] > [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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
