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    SubjectRe: [PATCH 05/10] block: remove per-queue plugging
    From
    2011/3/5 Jens Axboe <jaxboe@fusionio.com>:
    > On 2011-03-04 22:43, Mike Snitzer wrote:
    >> On Fri, Mar 04 2011 at  8:02am -0500,
    >> Shaohua Li <shli@kernel.org> wrote:
    >>
    >>> 2011/3/4 Mike Snitzer <snitzer@redhat.com>:
    >>>> I'm now hitting a lockdep issue, while running a 'for-2.6.39/stack-plug'
    >>>> kernel, when I try an fsync heavy workload to a request-based mpath
    >>>> device (the kernel ultimately goes down in flames, I've yet to look at
    >>>> the crashdump I took)
    >>>>
    >>>>
    >>>> =======================================================
    >>>> [ INFO: possible circular locking dependency detected ]
    >>>> 2.6.38-rc6-snitm+ #2
    >>>> -------------------------------------------------------
    >>>> ffsb/3110 is trying to acquire lock:
    >>>>  (&(&q->__queue_lock)->rlock){..-...}, at: [<ffffffff811b4c4d>] flush_plug_list+0xbc/0x135
    >>>>
    >>>> but task is already holding lock:
    >>>>  (&rq->lock){-.-.-.}, at: [<ffffffff8137132f>] schedule+0x16a/0x725
    >>>>
    >>>> which lock already depends on the new lock.
    >>> I hit this too. Can you check if attached debug patch fixes it?
    >>
    >> Fixes it for me.
    >
    > The preempt bit in block/ should not be needed. Can you check whether
    > it's the moving of the flush in sched.c that does the trick?
    yes, it's not related to the lockdep issue. but I think we still need
    it. if there is a preempt between attempt_plub_merge(), we do queue
    flush, then we might hit an incomplete list of request->biotail. Am I
    missing anything?

    Thanks,
    Shaohua
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