Re: [PATCH v5] x86/mce: Avoid infinite loop for copy from user recovery

[Luck, Tony]

On Thu, Jan 28, 2021 at 06:57:35PM +0100, Borislav Petkov wrote:
> Crazy idea: if you still can reproduce on -rc3, you could bisect: i.e.,
> if you apply the patch on -rc3 and it explodes and if you apply the same
> patch on -rc5 and it works, then that could be a start... Yeah, don't
> have a better idea here. :-\

I tried reporoducing (applied the original patch I posted back to -rc3) and
the same issue stubbornly refused to show up again.

But I did hit something with the same signature (overflow bit set in
bank 1) while running my futex test (which has two processes mapping
the poison page).  This time I *do* understand what happened.  The test
failed when the two processes were running on the two hyperhtreads of
the same core. Seeing overflow in this case is understandable because
bank 1 MSRs on my test machine are shared between the HT threads. When
I run the test again using taskset(1) to only allowing running on
thread 0 of each core, it keeps going for hunderds of iterations.

I'm not sure I can stitch together how this overflow also happened for
my single process test. Maybe a migration from one HT thread to the
other at an awkward moment?

-Tony