Re: [PATCH v13 01/17] preempt: Track NMI nesting to separate per-CPU counter

Peter Zijlstra
2025-10-14T19:44:03Z

On Tue, Oct 14, 2025 at 01:55:47PM -0400, Joel Fernandes wrote:
>
>
> On 10/14/2025 6:48 AM, Peter Zijlstra wrote:
> > On Mon, Oct 13, 2025 at 11:48:03AM -0400, Lyude Paul wrote:
> >
> >> #define __nmi_enter() \
> >> do { \
> >> lockdep_off(); \
> >> arch_nmi_enter(); \
> >> - BUG_ON(in_nmi() == NMI_MASK); \
> >> - __preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET); \
> >> + BUG_ON(__this_cpu_read(nmi_nesting) == UINT_MAX); \
> >> + __this_cpu_inc(nmi_nesting); \
> >
> > An NMI that nests from here..
> >
> >> + __preempt_count_add(HARDIRQ_OFFSET); \
> >> + if (__this_cpu_read(nmi_nesting) == 1) \
> >
> > .. until here, will see nmi_nesting > 1 and not set NMI_OFFSET.
>
> This is true, I can cure it by setting NMI_OFFSET unconditionally when
> nmi_nesting >= 1. Then the outer most NMI will then reset it. I think that will
> work. Do you see any other issue with doing so?

unconditionally set NMI_FFSET, regardless of nmi_nesting
and only clear on exit when nmi_nesting == 0.

Notably, when you use u64 __preempt_count, you can limit this to 32bit
only. The NMI nesting can happen in the single instruction window
between ADD and ADC. But on 64bit you don't have that gap and so don't
need to fix it.