Messages in this thread |  | | | Date | Mon, 16 Aug 2010 11:44:43 -0400 | | From | Mike Snitzer <> | | Subject | Re: Switching block device elevators |
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On Mon, Aug 16 2010 at 10:52am -0400, Peter Oberparleiter <oberpar@linux.vnet.ibm.com> wrote:
> After commit 1abec4fdbb142e3ccb6ce99832fae42129134a96, "block: make > blk_init_free_list and elevator_init idempotent", we're seeing > kernel panics in our s390 tape block device driver. The panic is > triggered because our driver tries to replace the default elevator > with a noop elevator by calling elevator_exit() directly followed by > elevator_init().
Maybe we should look to export elevator_switch() -- rather than confining its use to the sysfs interface.
> Since the commit, elevator_init() returns 0 if > request_queue->elevator is non-null, even though it does not install > a new elevator. As a result, the next access to the elevator finds a > pointer to the old one which was already freed and a panic is > triggered. Our current fix consists of setting the elevator pointer > to NULL after elevator_exit().
elevator_exit() triggers a call, via kobj, to elevator_release() which doesn't have access to the request_queue to reset it.
Unfortunately, commit 1abec4fdbb imposes that the elevator_exit() caller must take care to reset q->elevator to NULL -- like dasd_alloc_queue() does.
Though I suppose we _could_ pass request_queue to elevator_exit.
> There is at least one other driver where the problem currently > exists (drivers/block/mg_disk.c, author on cc) and another s390 > driver where the problem was only accidentally fixed before 2.6.35. > I'm wondering if there's a better solution (apart from not forcing > an elevator) and would like to hear everyone's opinion on this > matter. How about declaring elevator_switch() non-static, for > example?
Right, updating drivers/block/mg_disk.c and drivers/s390/block/dasd.c to use elevator_switch would work.
Mike
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