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    SubjectRe: What am I doing wrong? submit_bio() suddenly stops working...
    On 2010-10-21 18:55, Ted Ts'o wrote:
    > On Thu, Oct 21, 2010 at 08:59:46AM +0200, Jens Axboe wrote:
    >> I don't see anything immediately wrong with your approach. I suspect
    >> we'll need to see sysrq-t traces of the relevant processes to make a
    >> more educated guess!
    > I've uploaded a trace output that includes the sysrq-t trace, but I
    > don't think it shows anything interesting. We're not hanging on any
    > kind of loack as near as I can tell. It looks like
    > __generic_make_request() is calling q->make_request_fn(), and this is
    > returning without actually doing anything.
    > In this trace, I added a patch to prove that __generic_make_request()
    > is calling __make_request (I wasn't sure what q->make_request_fn was
    > indirecting to, so I added a brute force lookup to make sure I
    > understood what was going on), but at one point, it just starts
    > queuing the request, and it enters cfq, but the request never gets
    > dispatched out. Maybe this is a failure of the plugging/unplugging
    > mechanisms?
    > I guess I can start putting in more brute-force printk's inside
    > __make_request and inside the cfq scheduler to try to understand what
    > is going on, but I'm really guessing at this point.
    > If you have any suggestions about more elegant ways of figuring what
    > is happening, please do let me know....

    I will take a look at the traces.

    By the sound of things, if I were you I'd turn on the mem and slab
    debugging to catch use-before-init and use-after-free. Mysterious hangs
    in the IO sub system are usually caused by such bugs. And the regular
    debugging aids, just to see if that produces anything of interest.

    Jens Axboe

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