Re: Crash in elevator_dispatch_fn() (e.g. deadline_dispatch()) whenchanging elevators.

From: Tejun Heo
Date: Thu Jan 23 2014 - 13:56:51 EST

On Thu, Jan 23, 2014 at 10:38:33AM -0800, Frank Mayhar wrote:
> On Wed, 2014-01-22 at 07:46 -0800, Frank Mayhar wrote:
> > On Tue, 2014-01-21 at 07:58 -0800, Frank Mayhar wrote:
> > > Replacing? Or adding to? Is BYPASS always set when DYING is set? (My
> > > guess is not but I haven't done an exhaustive analysis.) So the
> > > relevant code snippet in __elv_next_request() would be:
> > > if (unlikely(blk_queue_dying(q)) ||
> > > unlikely(blk_queue_bypass(q)) ||
> > > !q->elevator->type->ops.elevator_dispatch_fn(q, 0))
> > > return NULL;
> >
> > FYI, I've made this change and tested it. I can't say for certain that
> > it fixes the crash (since it's one of those races that's difficult to
> > reproduce), but it does seem to pass all the tests I've thrown at it so
> > far.
> Um, does anyone care about this? Tejun? Jens? Anyone?
> This is a real crash; it would be nice if someone would weigh in.

Yeah, we're gonna fix this and I *think* replacing dying with bypass
is the right thing to do as a queue is always bypassing when killed.
It's probably just that we're in the earlier part of the merge window
and I have some other things on my plate. Will post a patch in a
couple days.


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