Re: [PATCH 10/24] preempt: Introduce HAS_SEPARATE_PREEMPT_RESCHED_BITS

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 31, 2026 at 01:30:11PM -0700, Boqun Feng wrote:

> diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
> index 1220656f3370..12353eeebc52 100644
> --- a/arch/x86/include/asm/preempt.h
> +++ b/arch/x86/include/asm/preempt.h

> @@ -24,18 +34,26 @@ DECLARE_PER_CPU_CACHE_HOT(int, __preempt_count);
>   */
>  static __always_inline int preempt_count(void)
>  {
> -	return raw_cpu_read_4(__preempt_count) & ~PREEMPT_NEED_RESCHED;
> +	return __pc_op(read, __preempt_count) & ~PREEMPT_NEED_RESCHED;
>  }
>  
> -static __always_inline void preempt_count_set(int pc)
> +/*
> + * unsigned long preempt count parameter works for both 32bit and 64bit cases:
> + *
> + * - For 32bit, "int" (the return of preempt_count()) and "unsigned long" have
> + *   the same size.
> + * - For 64bit, the effective bits of a preempt count sits in 32bit, and we
> + *   reserve the NEED_RESCHED bit from the old count.
> + */

The 64bit comment doesn't really make sense to me.

> +static __always_inline void preempt_count_set(unsigned long pc)
>  {
> -	int old, new;
> +	unsigned long old, new;
>  
> -	old = raw_cpu_read_4(__preempt_count);
> +	old = __pc_op(read, __preempt_count);
>  	do {
>  		new = (old & PREEMPT_NEED_RESCHED) |
>  			(pc & ~PREEMPT_NEED_RESCHED);
> -	} while (!raw_cpu_try_cmpxchg_4(__preempt_count, &old, new));
> +	} while (!__pc_op(try_cmpxchg, __preempt_count, &old, new));
>  }

> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index 33fc4c814a9f..87d5367f986c 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -30,18 +30,20 @@
>   * NMI nesting depth is tracked in a separate per-CPU variable
>   * (nmi_nesting) to save bits in preempt_count.
>   *
> - *         PREEMPT_MASK:	0x000000ff
> - *         SOFTIRQ_MASK:	0x0000ff00
> - * HARDIRQ_DISABLE_MASK:	0x00ff0000
> - *         HARDIRQ_MASK:	0x0f000000
> - *             NMI_MASK:	0x10000000
> - * PREEMPT_NEED_RESCHED:	0x80000000
> + *				32bit		HAS_SEPARATE_PREEMPT_RESCHED_BITS
> + *
> + *         PREEMPT_MASK:	0x000000ff	0x00000000000000ff
> + *         SOFTIRQ_MASK:	0x0000ff00	0x000000000000ff00
> + * HARDIRQ_DISABLE_MASK:	0x00ff0000	0x0000000000ff0000
> + *         HARDIRQ_MASK:	0x0f000000	0x000000000f000000
> + *             NMI_MASK:	0x10000000	0x00000000f0000000
> + * PREEMPT_NEED_RESCHED:	0x80000000	0x8000000000000000
>   */

Perhaps add a comment about how HAS_SEPARATE_PREEMPT_RESCHED_BITS really
is about having PREEMPT_NEED_RESCHED in its own word, rather than
preempt_count() being 64bit.

Because as presented it is very easy to confuse these two options.
Ideally it would explain the LOAD-STORE issue with NEED_RESCHED and
point to ARM64 or something.