Re: [PATCH 2/5] kernel/jump_label: implement generic support for relative references

[Ard Biesheuvel]

On 28 June 2018 at 11:25, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Jun 28, 2018 at 11:04:45AM +0200, Ard Biesheuvel wrote:
>> On 28 June 2018 at 11:02, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>> >>> @@ -56,7 +72,9 @@ jump_label_sort_entries(struct jump_entry *start, struct jump_entry *stop)
>> >>>
>> >>>       size = (((unsigned long)stop - (unsigned long)start)
>> >>>                                       / sizeof(struct jump_entry));
>> >>> -     sort(start, size, sizeof(struct jump_entry), jump_label_cmp, NULL);
>> >>> +     sort(start, size, sizeof(struct jump_entry), jump_label_cmp,
>> >>> +          IS_ENABLED(CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE) ? jump_label_swap
>> >>> +                                                           : NULL);
>> >>>  }
>> >>
>> >> That will result in jump_label_swap being an unused symbol for some
>> >> compile options.
>> >
>> > No, and isn't that the point of IS_ENABLED()? The compiler sees a
>> > reference to jump_label_swap(), so it won't complain about it being
>> > unused.
>
> Ah, ok. I hadn't figured it was quite that smart about it.
>

Yeah. I could use a temp variable to make the indentation less
obnoxious, but since this is an opt-in feature, I'd like to preserve
the NULL (*swap)() argument for the existing users.

>> > Meh. I thought IS_ENABLED() was preferred over #ifdef, no?
>
> Dunno, I just reacted to the proposed code's uglyness :-)
>

I will try to come up with something that rhymes, ok? :-)

>> ... and it means the sort() routine will unconditionally perform an
>> indirect function call even if the arch does not require it.
>
> Yeah, not sure I care about that here, this is a one time affair, very
> far away from any fast paths.
>

Fair enough.