On Tue, Aug 01, 2017 at 09:17:08AM -0400, J William Piggott wrote:
>
> Over the past decade a few commits for a corner case problem
> have left the RTC read branch in a bad place.
>
> The problem was: when a corrupted RTC could not be read, then
> it also could not be reset, because hwclock would error out
> due to the RTC read failure.
>
> 2.15-rc1 commit 3b96a7a Aug 9 2008
> 2.19-rc1 commit 5606df5 Dec 29 2010
> 2.23-rc1 commit ab8f402 Mar 21 2013
>
> The first fix was to ignore a synchronization timeout only for
> the busywait branch.
>
> Two and a half years later a commit describing the same problem
> took a little more heavy-handed approach by ignoring all
> synchronization failures and the RTC read after it, for both of
> the RTC set functions.
>
> Because the previous fix also ignored the select() branch timeout
> it caused a bogus warning. The chosen workaround for that was to
> only print the select() timeout message in debug mode (this is
> reverted by another patch).
>
> The problem with these fixes is that we cannot just ignore the
> synchronization timeout like that, because then the drift
> correction operations will be invalid. The original logic was
> correct; we must exit when synchronization fails.
... but will be possible to set RTC with invalid data?
> Another problem is that now there are statements between the
> timing-critical synchronize-read-timestamp trio (which were
> also in the wrong order, but that part of the problem goes
> back further in history).
>
> The solution is to skip the RTC read block completely for the
> RTC set functions when not also using the --update-drift
> option. If we are updating the drift correction factor during
> a set function then we must synchronize and read the RTC.
> Otherwise reading the RTC is not needed. Anyone trying to set
> a corrupt RTC should not be using --update-drift, because the
> resulting drift correction factor would be invalid.
OK, I see. The issue is --update-drift.
Maybe it would be nice to be more explicit in the man page and do
NOT recommend to use --systohc together with --update-drift.
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
http://karelzak.blogspot.com
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