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

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Message-ID: <alpine.LFD.2.02.1207120011070.32033@ionos>
Date:	Thu, 12 Jul 2012 00:12:44 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jan Kara <jack@...e.cz>
cc:	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 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 ?

Thanks,

	tglx
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