Re: [PATCH v10 07/14] unwind_user/deferred: Make unwind deferral requests NMI-safe

[Peter Zijlstra]

On Thu, Jun 19, 2025 at 05:42:31AM -0400, Steven Rostedt wrote:
> 
> 
> On June 19, 2025 5:32:26 AM EDT, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >On Thu, Jun 19, 2025 at 05:07:10AM -0400, Steven Rostedt wrote:
> >
> >> Does #DB make in_nmi() true? If that's the case then we do need to handle that.
> >
> >Yes: #DF, #MC, #BP (int3), #DB and NMI all have in_nmi() true.
> >
> >Ignoring #DF because that's mostly game over, you can get them all
> >nested for up to 4 (you're well aware of the normal NMI recursion
> >crap).
> 
> We probably can implement this with stacked counters.

I would seriously consider dropping support for anything that can't do
cmpxchg at the width you need.