linux-kernel - Re: Deadlocks due to per-process plugging

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Message-ID: <20120713123817.GC20361@quack.suse.cz>
Date:	Fri, 13 Jul 2012 14:38:17 +0200
From:	Jan Kara <jack@...e.cz>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Jan Kara <jack@...e.cz>, Jeff Moyer <jmoyer@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Jens Axboe <jaxboe@...ionio.com>, mgalbraith@...e.com
Subject: Re: Deadlocks due to per-process plugging

On Thu 12-07-12 00:12:44, 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 ?
  Yeah, that fix from Mike makes us unable to reproduce the problem. But
frankly it is a hack and I wouldn't bet a penny there isn't another
similar problem hiding elsewhere in the code. Just it would need a
different timing / load to trigger. So I think a better solution needs to
be found (an advice from JBD maintainer TM ;).

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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