Re: [PATCH v3 4/9] cpumask: Introduce for_each_cpu_andnot()

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

On Mon, Sep 5, 2022 at 9:44 AM Valentin Schneider <vschneid@redhat.com> wrote:
>
> 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.

Indeed. I probably messed with ANSI C.

Sorry for the noise.