[patch 09/11] x86/vdso: Simplify the invalid vclock case

Peter Zijlstra
Tue, 18 Sep 2018 08:30:55 -0000

On Tue, Sep 18, 2018 at 09:52:26AM +0200, Thomas Gleixner
wrote:
> On Mon, 17 Sep 2018, John Stultz wrote:
> > On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto at
kernel.org> wrote:
> > > Also, I'm not entirely convinced that this "last"
thing is needed at
> > > all.  John, what's the scenario under which we need it?
> >
> > So my memory is probably a bit foggy, but I recall that as we
> > accelerated gettimeofday, we found that even on systems that claimed
> > to have synced TSCs, they were actually just slightly out of sync.
> > Enough that right after cycles_last had been updated, a read on
> > another cpu could come in just behind cycles_last, resulting in a
> > negative interval causing lots of havoc.
> >
> > So the sanity check is needed to avoid that case.
>
> Your memory serves you right. That's indeed observable on CPUs which
> lack TSC_ADJUST.

But, if the gtod code can observe this, then why doesn't the code that
checks the sync?