Re: PROBLEM: Linux updates RTC secretly when clock synchronizes

[Pavel Machek <pavel@suse.cz>]

Hi!

> >> The offset needs to be sufficient to handle the case of the RTC
> >> being set to local time and the boot and first ntp sync occurring on
> >> opposite sides of a Daylight Savings Time change. A couple of
> >> timezones use a DST offset of 90 minutes, so if it's less than that,
> >> there will be problems.
> 
> > There is a related problem, where if you are running something which
> > can suspend, like a laptop, but not using integrated apm to do it
> > (for instance a laptop which has a broken BIOS), then suspending
> > 'freezes' the system time, which is wrong on resume (as the power
> > management event appears to be invisible to Linux). Then this goes
> > and blats over the (correct) RTC time. If you then get a network
> > connection up, ntp won't adjust the time as it's too far out.
> 
> That sounds like a configuration error to me, and here's why.
> 
>  1. One of the available reference clocks for xntpd is the local
>     RTC, type 1 in the list of reference clock types, and that
>     should ALWAYS be listed as one of the reference clocks to use,
>     but with a higher stratum than any other reference clock used.
> 
>  2. My experience with the xntpd driver suggests that if no better
>     reference is available and the RTC is one of the listed clocks,
>     then it ALWAYS adjusts the time to match the RTC, irrespective
>     of the time difference between them.

This sounds racy. What if kernel is faster and destroys RTC just after 
resume and before xntpd ca run?
								Pavel
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