linux-kernel - Re: [PATCH] clocksource, prevent overflow in clocksource

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Message-ID: <alpine.LFD.2.02.1204191451350.2542@ionos>
Date:	Thu, 19 Apr 2012 14:52:24 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <john.stultz@...aro.org>
cc:	Prarit Bhargava <prarit@...hat.com>, linux-kernel@...r.kernel.org,
	Salman Qazi <sqazi@...gle.com>, stable@...nel.org
Subject: Re: [PATCH] clocksource, prevent overflow in clocksource_cyc2ns

On Thu, 19 Apr 2012, Thomas Gleixner wrote:

> On Wed, 18 Apr 2012, John Stultz wrote:
> > On 04/18/2012 04:59 PM, Prarit Bhargava wrote:
> > > 
> > > Hey John,
> > > 
> > > Thanks for continuing to work on this.  Coincidentally that exact patch was
> > > my
> > > first attempt at resolving the problem as well.  The problem is that even
> > > after
> > > touching the clocksource watchdog and restoring irqs the printk buffer can
> > > take
> > > a LONG time to flush -- and that still will cause an overflow comparison.
> > > So
> > > fixing it with just a touch_clocksource_watchdog() isn't the right thing to
> > > do
> > > IMO.  Maybe a combination of the printk() patch you suggested earlier and
> > > the
> > > touch_clocksource_watchdog() is the right way to go but I'll leave that up
> > > to
> > > tglx and yourself to decide on a correct fix.
> > :( That's a bummer. Something similar may be useful on the printk side as
> > well.
> 
> No. The show_state() part prints into the buffer. But it's not
> guaranteed that the buffer is flushed right away. It could be flushed
> later as well in a different context. And of course the flush code
> runs with interrupts disabled and dumping out a gazillion of lines
> over serial will cause the same hickup. Just planting random
> touch_watchdog() calls into the code is not the right approach,
> really.
> 
> We should think about the reasons why we have interrupts disabled for
> so much time. Is that really, really necessary ?

I'm not against making the clocksource code more robust, but I don't
want to add crap there just to cope with complete madness elsewhere.

Thanks,

	tglx
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