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

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Date:	Mon, 16 Jul 2012 12:13:05 +0200
From:	Mike Galbraith <mgalbraith@...ell.com>
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 Mon, 2012-07-16 at 11:59 +0200, Thomas Gleixner wrote: 
> On Mon, 16 Jul 2012, Mike Galbraith wrote:
> > On Mon, 2012-07-16 at 10:59 +0200, Thomas Gleixner wrote: 
> > > On Mon, 16 Jul 2012, Mike Galbraith wrote:
> > > > On Sun, 2012-07-15 at 11:14 +0200, Mike Galbraith wrote: 
> > > > > On Sun, 2012-07-15 at 10:59 +0200, Thomas Gleixner wrote: 
> > > > 
> > > > > > Can you figure out on which lock the stuck thread which did not unplug
> > > > > > due to tsk_is_pi_blocked was blocked?
> > > > > 
> > > > > I'll take a peek.
> > > > 
> > > > Sorry for late reply, took a half day away from box.  Jan had already
> > > > done the full ext3 IO deadlock analysis:
> > > > 
> > > > Again kjournald is waiting for buffer IO on block 4367635 (sector
> > > > 78364838) to finish.  Now it is dbench thread 0xffff88026f330e70 which
> > > > has submitted this buffer for IO and is still holding this buffer behind
> > > > its plug (request for sector 78364822..78364846). The dbench thread is
> > > > waiting on j_checkpoint mutex (apparently it has successfully got the
> > > > mutex in the past, checkpointed some buffers, released the mutex and
> > > > hung when trying to acquire it again in the next loop of
> > > > __log_wait_for_space()). 
> > > 
> > > And what's holding j_checkpoint mutex and not making progress?
> > 
> > Waiting for wakeup from kjournald.
> 
> So kicking the io in __log_wait_for_space()
> 
>                 spin_unlock(&journal->j_state_lock);
> 
> 		--> HERE
> 
>                 mutex_lock(&journal->j_checkpoint_mutex);
> 
> should fix the issue, right ?

Should for this scenario.  Jan said he could think of ways to likely
make the same happen in xfs.

-Mike

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