Re: [PATCH v17 02/16] preempt: Track NMI nesting to separate per-CPU counter

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jan 21, 2026 at 05:39:05PM -0500, 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);			\
> +		__preempt_count_add(HARDIRQ_OFFSET);		\
> +		preempt_count_set(preempt_count() | NMI_MASK);	\
>  	} while (0)
>  
>  #define nmi_enter()						\
> @@ -124,8 +128,12 @@ void irq_exit_rcu(void);
>  
>  #define __nmi_exit()						\
>  	do {							\
> +		unsigned int nesting;				\
>  		BUG_ON(!in_nmi());				\
> -		__preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET);	\
> +		__preempt_count_sub(HARDIRQ_OFFSET);		\
> +		nesting = __this_cpu_dec_return(nmi_nesting);	\
> +		if (!nesting)					\
> +			__preempt_count_sub(NMI_OFFSET);	\
>  		arch_nmi_exit();				\
>  		lockdep_on();					\
>  	} while (0)

While not wrong like last time; it is pretty awful. 

preempt_count_set() is a cmpxchg() loop.

Would not something like so be better?

#define __nmi_enter()                                           \
        do {                                                    \
+               unsigned int _o = NMI_MASK + HARDIRQ_OFFSET;    \
                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) == ~0U);    \
+               __this_cpu_inc(nmi_nesting);                    \
+               _o -= (preempt_count() & NMI_MASK);             \
+               __preempt_count_add(_o);                        \
        } while (0)


 #define __nmi_exit()                                           \
        do {                                                    \
+               unsigned int _o = HARDIRQ_OFFSET;               \
                BUG_ON(!in_nmi());                              \
-               __preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET);       \
+               if (!__this_cpu_dec_return(nmi_nesting))        \
+                       _o += NMI_MASK;                         \
+               __preempt_count_sub(_o);                        \
                arch_nmi_exit();                                \
                lockdep_on();                                   \
        } while (0)


But I'm really somewhat sad that 64bit can't do better than this.