linux-kernel - Re: [PATCH] printk: fix pr_debug and pr

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Date:	Fri, 04 Dec 2015 11:38:59 -0500
From:	Aaron Conole <aconole@...hat.com>
To:	Jason Baron <jbaron@...mai.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Joe Perches <joe@...ches.com>
Subject: Re: [PATCH] printk: fix pr_debug and pr_devel to elide function calls

Jason Baron <jbaron@...mai.com> writes:

> On 12/03/2015 05:45 PM, Aaron Conole wrote:
>> Currently, pr_debug and pr_devel will not elide function call arguments
>> appearing in calls to no_printk for these macros. This is because all
>> side effects must be honored before proceeding to the 0-value assignment
>> in no_printk.
>> 
>> The behavior is contrary to documentation found in the CodingStyle and
>> header file where these functions are declared. 
>> 
>> This patch corrects that behavior by shunting out the call to no_printk
>> completely. The format string is still checked by gcc for correctness, but
>> no code seems to be emitted in common cases.
>> 
>> fixes commit 5264f2f75d86 ("include/linux/printk.h: use and neaten
>> no_printk")
>> 
>> Signed-off-by: Aaron Conole <aconole@...hat.com>
>> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
>> Cc: Joe Perches <joe@...ches.com>
>
> I think we should just convert no_printk() to not emit anything. This
> will avoid us adding unwrapped calls to 'no_printk()' in the future, and
> I think makes the code more readable. Based on Joe's previous
> 'eliminated_printk()' thing. IE:
>
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 9729565..58632bf 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -108,11 +108,11 @@ struct va_format {
>   * Dummy printk for disabled debugging statements to use whilst maintaining
>   * gcc's format and side-effect checking.
>   */
> -static inline __printf(1, 2)
> -int no_printk(const char *fmt, ...)
> -{
> -       return 0;
> -}
> +#define no_printk(fmt, ...)                    \
> +do {                                           \
> +       if (0)                                  \
> +               printk(fmt, ##__VA_ARGS__);     \
> +} while (0)
>
>  #ifdef CONFIG_EARLY_PRINTK
>  extern asmlinkage __printf(1, 2)
>
> Thanks,
>
> -Jason

I like this fix the best, but reading some other upstream mails it seems
like that approach isn't likely to be accepted? I'll happily respin to
have your proposed code because it makes the most sense, if no one else
has any objections.
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