Re: [PATCH v4 2/7] cpumask: Introduce for_each_cpu_andnot()

[Yury Norov <yury.norov@gmail.com>]

On Fri, Sep 23, 2022 at 04:55:37PM +0100, Valentin Schneider wrote:
> for_each_cpu_and() is very convenient as it saves having to allocate a
> temporary cpumask to store the result of cpumask_and(). The same issue
> applies to cpumask_andnot() which doesn't actually need temporary storage
> for iteration purposes.
> 
> Following what has been done for for_each_cpu_and(), introduce
> for_each_cpu_andnot().
> 
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>  include/linux/cpumask.h | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 1b442fb2001f..4c69e338bb8c 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -238,6 +238,25 @@ unsigned int cpumask_next_and(int n, const struct cpumask *src1p,
>  		nr_cpumask_bits, n + 1);
>  }
>  
> +/**
> + * cpumask_next_andnot - get the next cpu in *src1p & ~*src2p
> + * @n: the cpu prior to the place to search (ie. return will be > @n)
> + * @src1p: the first cpumask pointer
> + * @src2p: the second cpumask pointer
> + *
> + * Returns >= nr_cpu_ids if no further cpus set in *src1p & ~*src2p
> + */
> +static inline
> +unsigned int cpumask_next_andnot(int n, const struct cpumask *src1p,
> +				 const struct cpumask *src2p)
> +{
> +	/* -1 is a legal arg here. */
> +	if (n != -1)
> +		cpumask_check(n);

This is wrong. n-1 should be illegal here. The correct check is:
cpumask_check(n+1);

> +	return find_next_andnot_bit(cpumask_bits(src1p), cpumask_bits(src2p),
> +		nr_cpumask_bits, n + 1);
> +}
> +
>  /**
>   * for_each_cpu - iterate over every cpu in a mask
>   * @cpu: the (optionally unsigned) integer iterator
> @@ -317,6 +336,26 @@ unsigned int __pure cpumask_next_wrap(int n, const struct cpumask *mask, int sta
>  		(cpu) = cpumask_next_and((cpu), (mask1), (mask2)),	\
>  		(cpu) < nr_cpu_ids;)
>  
> +/**
> + * for_each_cpu_andnot - iterate over every cpu present in one mask, excluding
> + *			 those present in another.
> + * @cpu: the (optionally unsigned) integer iterator
> + * @mask1: the first cpumask pointer
> + * @mask2: the second cpumask pointer
> + *
> + * This saves a temporary CPU mask in many places.  It is equivalent to:
> + *	struct cpumask tmp;
> + *	cpumask_andnot(&tmp, &mask1, &mask2);
> + *	for_each_cpu(cpu, &tmp)
> + *		...
> + *
> + * After the loop, cpu is >= nr_cpu_ids.
> + */
> +#define for_each_cpu_andnot(cpu, mask1, mask2)				\
> +	for ((cpu) = -1;						\
> +		(cpu) = cpumask_next_andnot((cpu), (mask1), (mask2)),	\
> +		(cpu) < nr_cpu_ids;)

This would raise cpumaks_check() warning at the very last iteration.
Because cpu is initialized insize the loop, you don't need to check it
at all. You can do it like this:

 #define for_each_cpu_andnot(cpu, mask1, mask2)				\
         for_each_andnot_bit(...)

Check this series for details (and please review).
https://lore.kernel.org/all/20220919210559.1509179-8-yury.norov@gmail.com/T/

Thanks,
Yury