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

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Date:	Mon, 16 May 2011 15:38:17 -0400
From:	Arnaud Lacombe <lacombar@...il.com>
To:	Valdis.Kletnieks@...edu
Cc:	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	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

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

 - Arnaud
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