linux-kernel - Re: gettimeofday() jumping into the future

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Message-ID: <alpine.LFD.1.10.0804030827180.3219@apollo.tec.linutronix.de>
Date:	Thu, 3 Apr 2008 08:28:25 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Karsten Wiese <fzu@...gehoertderstaat.de>
cc:	John Stultz <johnstul@...ibm.com>, Tim Ricketts <tr@...th.li>,
	Michael Smith <msmith@...h.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andy Wingo <wingo@...endo.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: gettimeofday() jumping into the future

On Thu, 3 Apr 2008, Karsten Wiese wrote:
> > To prevent this TSC specific wreckage we need to compare the TSC value
> > against the reference value and return the latter when it is larger
> > than the actual TSC value.
> 
> Last paragraph IMO should be:
> 	To prevent this TSC specific wreckage we need to substract the
> 	reference value from the TSC value, interpret the result as
> 	signed. If the interpreted result is negative, return the reference
> 	value, else the TSC Value.
> Your patch misses the corner case where TSC value is < reference _and_ valid
> at TSC wrap around.

In about 200 years on a 3Ghz box.
 
> The access to the reference value needs a (the xtime ?) lock on 32bit, no?

The readout is under xtime lock anyway.

Thanks,

	tglx
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