linux-kernel - Re: boot panic regression introduced in 3.5-rc7

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Message-ID: <alpine.LFD.2.02.1208010847590.32033@ionos>
Date:	Wed, 1 Aug 2012 08:50:12 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <johnstul@...ibm.com>
cc:	CAI Qian <caiqian@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Prarit Bhargava <prarit@...hat.com>,
	Zhouping Liu <zliu@...hat.com>
Subject: Re: boot panic regression introduced in 3.5-rc7

On Mon, 30 Jul 2012, John Stultz wrote:
> On 07/29/2012 08:51 PM, CAI Qian wrote:
> Now, one of the reasons Thomas and I changed the logic was that using the
> precalculated realtime_offset was slightly more efficient then re-adding xtime
> and wall_to_monotonic's components separately. But how valuable this
> unmeasured slight efficiency is vs extra robustness for crazy time values is
> questionable.

Well, I guess it unearthed a weakness which has been there forever:

  Trusting random values which are supplied by cmos or whatever.

So the right fix is:
 
> 2) Validate that time values we accept are smaller the ktime_t before using
> them.

I really don't like the magic workaround by nulling out the effect of
crap data with a more expensive calculation.

Thanks,

	tglx
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