Re: [PATCH] sysctl: add register_sysctl() dummy helper

[Arnd Bergmann]

On Mon, Nov 6, 2017 at 6:41 PM, Dave Martin <Dave.Martin@xxxxxxx> wrote:
> On Mon, Nov 06, 2017 at 05:12:03PM +0000, Arnd Bergmann wrote:
>> On Mon, Nov 6, 2017 at 5:50 PM, Dave Martin <Dave.Martin@xxxxxxx> wrote:
>> > On Mon, Nov 06, 2017 at 01:36:40PM +0000, Arnd Bergmann wrote:
>> >> The register_sysctl() function has been around for five years with commit
>> >> fea478d4101a ("sysctl: Add register_sysctl for normal sysctl users")
>> >> but now that arm64 started using it, I ran into a compile error:
>> >>
>> >> arch/arm64/kernel/armv8_deprecated.c: In function 'register_insn_emulation_sysctl':
>> >> arch/arm64/kernel/armv8_deprecated.c:257:2: error: implicit declaration of function 'register_sysctl'
>> >
>> > Hmmm, looks like I missed this combination in testing.
>> >
>> > I wonder whether ARMV8_DEPRECATED without SYSCTL is really a good idea
>> > though: in that config, we build a lot of dead code and leak some
>> > memory today.  The default emulation is still potentially useful, but
>> > all the support for runtime twiddling of the emulation modes becomes
>> > useless.
>> >
>> > For parallel reasons, the SVE sysctl stuff is protected by #ifdef
>> > CONFIG_SYSCTL, which is why I didn't get a similar splat there.
>> >
>> >
>> > So, although this patch is superficially sensible, it may tend to hide
>> > bugs: code that calls register_sysctl() when CONFIG_SYSCTL=n is
>> > suspicious and probably needs review... no?
>>
>> I think your analysis for this code is correct, we waste a lot of memory
>> if we do it like this. However in the general case where we register a
>> statically allocated 'struct ctl_table', it wouldn't be an issue, because
>> gcc could then eliminate all the dead code.
>>
>> Adding a CONFIG_SYSCTL #ifdef or Kconfig dependency would
>> probably be reasonable for armv8_deprecated.c, for the rest of
>> the kernel, having that wrapper is probably better.
>>
>> I don't really care how this gets fixed, as long as some solution gets
>> merged.
>
> Well, since your patch makes things more consistent, I'm happy to
>
> Reviewed-by: Dave Martin <Dave.Martin@xxxxxxx>
>
>
> The armv8_deprecated code looks suspicious in any case, so I'll propose
> a fix for that separately -- adding a Kconfig dependency seems simplest.

Thanks, your other patch looks good too (replied with an Ack there).

Andrew, can you pick up my patch into -mm then? It seems we still
want it anyway, but there is no longer a dependency with the arm64
tree, so no reason to pick merge it there.

       Arnd