Re: [PATCH] smp.h: Use local_irq_{save,restore}() in !SMP version of on_each_cpu().

[David Daney <ddaney@caviumnetworks.com>]

On 06/13/2013 10:46 PM, Linus Torvalds wrote:
> On Thu, Jun 13, 2013 at 6:07 PM, David Daney <ddaney.cavm@gmail.com> wrote:
>>
>> Suggested fix: Do what we already do in the SMP version of
>> on_each_cpu(), and use local_irq_save/local_irq_restore.
>
> I was going to apply this, but started looking a bit more.
>
> Using "flags" as a variable name inside a macro like this is a
> *really* bad idea.
>
> Lookie here:
>
>      [torvalds@pixel linux]$ git grep on_each_cpu.*flags
>      arch/s390/kernel/perf_cpum_cf.c:        on_each_cpu(setup_pmc_cpu,
> &flags, 1);
>      arch/s390/kernel/perf_cpum_cf.c:        on_each_cpu(setup_pmc_cpu,
> &flags, 1);
>
> and ask yourself what happens when the "info" argument expands to
> "&flags", and it all compiles perfectly fine, but the "&flags" takes
> the address of the new _inner_ variable called "flags" from the macro
> expansion. Not the one that the caller actually intends..
>
> Oops.
>
> Not a good idea.
>

Yeah,  I think making it a static inline function may be the best approach.

I am going to test doing that and send a new patch very soon.

David Daney


> So I would suggest trivially renaming "flags" as "__flags" or
> something, or perhaps even just making it a real function and avoiding
> the whole namespace issue.
>
> And rather than doing that blindly by editing the patch at after -rc5,
> I'm just going to ask you to re-send a tested patch. Ok?
>
>                      Linus
>