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    SubjectRe: Deadlocks due to per-process plugging
    On Thu, 2012-07-12 at 00:12 +0200, Thomas Gleixner wrote: 
    > On Wed, 11 Jul 2012, Jan Kara wrote:
    > > On Wed 11-07-12 12:05:51, Jeff Moyer wrote:
    > > > This eventually ends in a call to blk_run_queue_async(q) after
    > > > submitting the I/O from the plug list. Right? So is the question
    > > > really why doesn't the kblockd workqueue get scheduled?
    > > Ah, I didn't know this. Thanks for the hint. So in the kdump I have I can
    > > see requests queued in tsk->plug despite the process is sleeping in
    > > TASK_UNINTERRUPTIBLE state. So the only way how unplug could have been
    > > omitted is if tsk_is_pi_blocked() was true. Rummaging through the dump...
    > > indeed task has pi_blocked_on = 0xffff8802717d79c8. The dump is from an -rt
    > > kernel (I just didn't originally thought that makes any difference) so
    > > actually any mutex is rtmutex and thus tsk_is_pi_blocked() is true whenever
    > > we are sleeping on a mutex. So this seems like a bug in rtmutex code.
    > > Thomas, you seemed to have added that condition... Any idea how to avoid
    > > the deadlock?
    > Mike has sent out a fix related to the plug stuff, which I just posted
    > for the rt stable series. Can you verify against that ?

    btw, I called io_schedule() instead of a plain unplug thinking we're
    going to schedule anyway, but if we unplug and schedule, and we're not
    leftmost (non-rt task 'course), while we're away, likely contended mutex
    we're about to take may be released or at least become less contended.
    What a we won't be doing is accruing sleep time to help trigger yet more
    preemption. Anyone more deserving can move smartly rightward, and thus
    out of our way for a bit.

    If we're leftmost or rt, all was for naught, but it seemed worth a shot.


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