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

[Peter Zijlstra]

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.

> +			__preempt_count_add(NMI_OFFSET);	\