linux-kernel - Re: [PATCH v3 4/9] cpumask: Introduce for_each_cpu

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Date:   Mon, 05 Sep 2022 17:44:05 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Tony Luck <tony.luck@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Gal Pressman <gal@...dia.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>
Subject: Re: [PATCH v3 4/9] cpumask: Introduce for_each_cpu_andnot()

On 25/08/22 14:14, Yury Norov wrote:
> On Thu, Aug 25, 2022 at 07:12:05PM +0100, Valentin Schneider wrote:
>> +#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)))
>

IIUC the order of execution *is* guaranteed as this is a comma operator,
not argument passing:

  6.5.17 Comma operator

  The left operand of a comma operator is evaluated as a void expression;
  there is a sequence point after its evaluation. Then the right operand is
  evaluated; the result has its type and value.

for_each_cpu{_and}() uses the same pattern (which I simply copied here).

Still, I'd be up for making this a bit more readable. I did a bit of
digging to figure out how we ended up with that pattern, and found

  7baac8b91f98 ("cpumask: make for_each_cpu_mask a bit smaller")

so this appears to have been done to save up on generated instructions.
*if* it is actually OK standard-wise, I'd vote to leave it as-is.

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