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Date:   Thu, 25 Aug 2022 14:14:08 -0700
From:   Yury Norov <yury.norov@gmail.com>
To:     Valentin Schneider <vschneid@redhat.com>
Cc:     netdev@vger.kernel.org, linux-rdma@vger.kernel.org,
        linux-kernel@vger.kernel.org, Saeed Mahameed <saeedm@nvidia.com>,
        Leon Romanovsky <leon@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Heiko Carstens <hca@linux.ibm.com>,
        Tony Luck <tony.luck@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Gal Pressman <gal@nvidia.com>,
        Tariq Toukan <tariqt@nvidia.com>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>
Subject: Re: [PATCH v3 4/9] cpumask: Introduce for_each_cpu_andnot()
Message-ID: <YwfmIDEbRT4JfsZp@yury-laptop>
References: <20220825181210.284283-1-vschneid@redhat.com>
 <20220825181210.284283-5-vschneid@redhat.com>
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On Thu, Aug 25, 2022 at 07:12:05PM +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 1414ce8cd003..372a642bf9ba 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);
> +	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;)

The standard doesn't guarantee the order of execution of last 2 lines,
so you might end up with unreliable code. Can you do it in a more
conventional style:
   #define for_each_cpu_andnot(cpu, mask1, mask2)			\
   	for ((cpu) = cpumask_next_andnot(-1, (mask1), (mask2));	        \
   		(cpu) < nr_cpu_ids;                                     \
   		(cpu) = cpumask_next_andnot((cpu), (mask1), (mask2)))	

> +
>  /**
>   * cpumask_any_but - return a "random" in a cpumask, but not this one.
>   * @mask: the cpumask to search
> -- 
> 2.31.1