linux-kernel - Re: [PATCH v2] kconfig: autogenerated config_is

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Date:	Tue, 17 May 2011 03:03:29 +0200
From:	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>
To:	Arnaud Lacombe <lacombar@...il.com>
Cc:	Valdis.Kletnieks@...edu, Michal Marek <mmarek@...e.cz>,
	linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
	x86@...nel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2] kconfig: autogenerated config_is_xxx macro

On 15:38 Mon 16 May     , Arnaud Lacombe wrote:
> Hi,
> 
> On Mon, May 16, 2011 at 3:03 PM,  <Valdis.Kletnieks@...edu> wrote:
> > On Fri, 13 May 2011 10:09:09 +0200, Jean-Christophe PLAGNIOL-VILLARD said:
> >> On 10:52 Mon 09 May     , Michal Marek wrote:
> >> > Do you have proof of concept patches that make use of the
> >> > config_is_xxx macros? Acked by the respective subsystem maintainers?
> >> > It would be a good idea to send them along to show that this feature
> >> > is going to be actually used.
> >> I've seen thousands of place in the kernel we can use
> >> so I'll just take one example on x86
> >>
> >> the patch attached is just an example
> >
> > Out of curiosity, will this Do The Right Thing for cases where things simply won't
> > build for some configs?  For example, consider this code snippet from kernel/timer.c,
> > in __mod_timer() (near line 682):
> >
> >        debug_activate(timer, expires);
> >
> >        cpu = smp_processor_id();
> >
> > #if defined(CONFIG_NO_HZ) && defined(CONFIG_SMP)
> >        if (!pinned && get_sysctl_timer_migration() && idle_cpu(cpu))
> >                cpu = get_nohz_timer_target();
> > #endif
> >        new_base = per_cpu(tvec_bases, cpu);
> >
> > If you convert this to an if statement, will it still compile?  Which will
> > happen first, dead code elimination, or the warning that get_nohz_timer_target()
> > is an implicit declaration because the definition in the .h file is also
> > guarded by #ifdef CONFIG_NO_HZ?
> >
> I already exposed this case, but let's prove it:
> 
> % grep CONFIG_SMP .config
> # CONFIG_SMP is not set
> 
> % git diff
> diff --git a/kernel/timer.c b/kernel/timer.c
> index fd61986..ea4a5ba 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -681,10 +681,8 @@ __mod_timer(struct timer_list *timer, unsigned
> long expires,
> 
>         cpu = smp_processor_id();
> 
> -#if defined(CONFIG_NO_HZ) && defined(CONFIG_SMP)
> -       if (!pinned && get_sysctl_timer_migration() && idle_cpu(cpu))
> +       if (0 && 0 && !pinned && get_sysctl_timer_migration() && idle_cpu(cpu))
>                 cpu = get_nohz_timer_target();
> -#endif
>         new_base = per_cpu(tvec_bases, cpu);
> 
> % gmake kernel/timer.o
>   CHK     include/linux/version.h
>   CHK     include/generated/utsrelease.h
>   CALL    scripts/checksyscalls.sh
>   CC      kernel/timer.o
> kernel/timer.c: In function '__mod_timer':
> kernel/timer.c:685:3: error: implicit declaration of function
> 'get_nohz_timer_target'
> gmake[1]: *** [kernel/timer.o] Error 1
> gmake: *** [kernel/timer.o] Error 2
because we do not define the inline function if the CONFIG_ is not define

as we are supposed to do if we want to compile without ifdef everywhere

Best Regards,
J.
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