Re: [PATCH v2] m68k: Define NR_CPUS

[Geert Uytterhoeven]

Hi Günter,

Thanks for your patch!

On Tue, Sep 24, 2024 at 1:56 AM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> SPLIT_PTE_PTLOCKS depends on "NR_CPUS >= 4". Unfortunately, that evaluates
> to true if there is no NR_CPUS configuration option. This results in
> CONFIG_SPLIT_PTE_PTLOCKS=y for mac_defconfig. This in turn causes the m68k
> "q800" and "virt" machines to crash in qemu if debugging options are
> enabled.
>
> Making CONFIG_SPLIT_PTE_PTLOCKS dependent on the existence of NR_CPUS
> does not work since a dependency on the existence of a numeric Kconfig
> entry always evaluates to false. Example:
>
> config HAVE_NO_NR_CPUS
>        def_bool y
>        depends on !NR_CPUS
>
> After adding this to a Kconfig file, "make defconfig" includes:
> $ grep NR_CPUS .config
> CONFIG_NR_CPUS=64
> CONFIG_HAVE_NO_NR_CPUS=y
>
> Define NR_CPUS for m68k instead to solve the problem.
>
> Fixes: 394290cba966 ("mm: turn USE_SPLIT_PTE_PTLOCKS / USE_SPLIT_PTE_PTLOCKS into Kconfig options")
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

I think it's a bit premature to add David's tag, as v2 is completely
different from v1.

> Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> ---
> v2: Instead of trying to make SPLIT_PTE_PTLOCKS depend on the
>     existence of NR_CPUS, define NR_CPUS for m68k.
>
> v1: https://lore.kernel.org/lkml/202409240546.SJwj9tUj-lkp@xxxxxxxxx/T/#t

I replied on v1 why I think this is not the right solution.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds