linux-kernel - Re: 2.6.39-rc4 BUG: unable to handle kernel NULL pointer dereference at 0000000c IP: cfq_insert

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Date:	Thu, 21 Apr 2011 17:29:14 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jens Axboe <axboe@...nel.dk>, Michal Hocko <mhocko@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.39-rc4 BUG: unable to handle kernel NULL pointer dereference
  at 0000000c IP: cfq_insert_request+0x1d/0x3f5

On 2011-04-21 16:38, Linus Torvalds wrote:
> On Thu, Apr 21, 2011 at 12:25 AM, Jens Axboe <axboe@...nel.dk> wrote:
>>>
>>> I am going to bisect, let's see if I can find anything.
>>
>> Thanks, that would be great!
> 
> I'd expect it to be very timing-dependent, and thus could easily be
> triggered (or hidden) by unrelated changes.
> 
> Just happening to have a request added to the elevator at _just_ the
> same moment that another CPU is changing it and getting rid of the
> data structures for the old one.

This particular bug does seem to trigger very reliably. The switching
works by ensuring that we have no requests with elevator data associated
with it before shutting down the old scheduler and attaching the new
one. I smells more like a bug on doing an insert sort on a non-priv
request. I bet this is it, Michal can you give it a spin?

diff --git a/block/elevator.c b/block/elevator.c
index 6f6abc0..45ca1e3 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -671,7 +671,8 @@ void __elv_add_request(struct request_queue *q, struct request *rq, int where)
 			q->boundary_rq = rq;
 		}
 	} else if (!(rq->cmd_flags & REQ_ELVPRIV) &&
-		    where == ELEVATOR_INSERT_SORT)
+		    (where == ELEVATOR_INSERT_SORT ||
+		     where == ELEVATOR_INSERT_SORT_MERGE))
 		where = ELEVATOR_INSERT_BACK;
 
 	switch (where) {

-- 
Jens Axboe

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