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    SubjectRe: 2.6.39-rc4 BUG: unable to handle kernel NULL pointer dereference at 0000000c IP: cfq_insert_request+0x1d/0x3f5
    On Thu 21-04-11 17:29:14, Jens Axboe wrote:
    > On 2011-04-21 16:38, Linus Torvalds wrote:
    > > On Thu, Apr 21, 2011 at 12:25 AM, Jens Axboe <axboe@kernel.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?

    Will try tomorrow.

    >
    > 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
    >

    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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