Re: [PATCH 5/5] cpumask: fix comment of cpumask_xxx

[Linus Torvalds]

On Mon, Mar 6, 2023 at 9:47 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> The drivers/char/random.c code is very wrong, and does
>
>              if (cpu == nr_cpumask_bits)
>                              cpu = cpumask_first(&timer_cpus);
>
> which fails miserably exactly because it doesn't use ">=".

Turns out this "cpu == nr_cpumask_bits" pattern exists in a couple of
other places too.

It was always wrong, but it always just happened to work. The lpfc
SCSI driver in particular seems to *love* this pattern:

        start_cpu = cpumask_next(new_cpu, cpu_present_mask);
        if (start_cpu == nr_cpumask_bits)
                start_cpu = first_cpu;

and has repeated it multiple times, all incorrect.

We do have "cpumask_next_wrap()", and that *seems* to be what the lpcf
driver actually wants to do.

.. and then we have kernel/sched/fair.c, which is actually not buggy,
just odd. It uses nr_cpumask_bits too, but it uses it purely for its
own internal nefarious reasons - it's not actually related to the
cpumask functions at all, its just used as a "not valid CPU number".

I think that scheduler use is still very *wrong*, but it doesn't look
actively buggy.

The other cases all look very buggy indeed, but yes, they happened to
work, and now they don't. So commit 596ff4a09b89 ("cpumask:
re-introduce constant-sized cpumask optimizations") did break them.

I'd rather fix these bad users than revert, but there does seem to be
an alarming number of these things, which worries me:

     git grep '== nr_cpumask_bits'

and that's just checking for this *exact* thing.

                Linus