linux-kernel - Re: [BUG] fatal hang untarring 90GB file, possibly writeback related.

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Date:	Fri, 29 Apr 2011 11:31:12 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	sedat.dilek@...il.com
Cc:	Chris Mason <chris.mason@...cle.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG] fatal hang untarring 90GB file, possibly writeback
 related.

On Fri, 2011-04-29 at 10:37 -0500, James Bottomley wrote:
> On Fri, 2011-04-29 at 12:23 +0200, Sedat Dilek wrote:
> > But as I see these RCU (CPU) stalls, the patch from [1] might be worth a try.
> > First, I have seen negative effects on my UP-system was when playing
> > with linux-next [2].
> > It was not clear what the origin was and the the side-effects were
> > somehow "bizarre".
> > The issue could be easily reproduced by tar-ing the kernel build-dir
> > to an external USB-hdd.
> > The issue kept RCU and TIP folks really busy.
> > Before stepping 4 weeks in the dark, give it a try and let me know in
> > case of success.
> 
> Well, it's highly unlikely because that's a 2.6.39 artifact and the bug
> showed up in 2.6.38 ... I tried it just in case with no effect, so we
> know it isn't the cause.

Actually, I tell a lie: it does't stop kswapd spinning on PREEMPT, but
it does seem to prevent non-PREEMPT from locking up totally (at least it
survives three back to back untar runs).

It's probable it alters the memory pin conditions that cause the spin,
so it's masking the problem rather than fixing it.

The question now is, what causes it in 2.6.38 stable?

James


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