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

[Joel Fernandes]

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?

Thanks!

 - Joel