linux-kernel - Re: BUG: soft lockup

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Date:	Mon, 5 Jan 2009 05:19:59 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Peter Klotz <peter.klotz@....at>,
	Roman Kononov <kernel@...onov.ftml.net>,
	linux-kernel@...r.kernel.org, xfs@....sgi.com
Subject: Re: BUG: soft lockup - is this XFS problem?

On Mon, Jan 05, 2009 at 02:48:21AM +0100, Nick Piggin wrote:
> On Sat, Jan 03, 2009 at 04:44:43PM -0500, Christoph Hellwig wrote:
> > On Tue, Dec 30, 2008 at 05:23:33AM +0100, Nick Piggin wrote:
> > > On Tue, Dec 23, 2008 at 12:12:59PM -0500, Christoph Hellwig wrote:
> > > > 
> > > > Nick, I've seen various reports like this by Roman.  It seems to be
> > > > caused by an interaction of the lockless pagecache with the xfs
> > > > I/O code.  Any idea what might be wrong here:
> > > 
> > > Hmm, it could get into a loop here if there is a page in the pagecache
> > > with a zero refcount, which might be a problem with XFS... other looping
> > > conditions might indicate a problem iwth lockless pagecache or radix
> > > tree. It would be very helpful to know what condition it is looping on...
> > 
> > See http://oss.sgi.com/bugzilla/show_bug.cgi?id=805
> 
> OK.. Hmm, well here is a modification to your patch which might help further.
> I'll see if I can reproduce it here meanwhile.

I have reproduced it. It seems like it might be a livelock condition
because the system ended up recovering after I terminated the dd (and
did so before I collected any real info, oops, hopefully I can
reproduce it again).

This would fit with the problem going away when the debugging patch
was applied. Timing changes...
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