Re: [PATCH v2 00/12] s390: More this_cpu_*() changes

On Fri, Sep 18, 2026 at 11:31:25AM +0100, Mark Rutland wrote:
> > v2:
> > - Yet another brown paper bug: Re-add CPU migration check to last
> >   patch. Unconditionally recalculating and updating the percpu variable
> >   address and percpu offset register can corrupt previous context
> >   register state in several cases (not only the single case reported by
> >   Sashiko).
> 
> Do all of those case boil down to:
> 
> * During exception entry from context A to context B, a nested exception
>   is taken from context B to context C before percpu_entry() is called,
>   and the nested exception handler updates B's live regs using A's
>   percpu register indices.
> 
> * During exception return from context B to context A, a nested
>   exception is taken from context B to context C after percpu_entry() is
                                                         ^^^^^^^^^^^^
That should have been percpu_exit(), I guess.

>   called, and the nested exception handler updates B's live regs using
>   A's percpu register indices.
> 
> ... or was there another case that you spotted (e.g. some case where
> percpu_{entry,exit}() aren't called)?

Yes, all scenarios boil down to exactly the above.

> On arm64, I think we avoid those two cases by virtue of not taking
> nested (architectural) exceptions during those windows, and by not
> applying fixups for SDEI events (SW NMIs that can be taken during those
> windows).
> 
> If there's another case to consider, it'd be handy to know.

I'm not aware of more cases.