From: Patrick Palka <patrick@parcs.ath.cx>
Subject: Re: [PATCH] ext4: address a benign compiler warning
Date: Wed, 12 Feb 2014 23:03:03 -0500
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Cc: linux-kernel@vger.kernel.org, "Theodore Ts'o" <tytso@mit.edu>,
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To: Joe Perches <joe@perches.com>
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On Wed, Feb 12, 2014 at 9:59 PM, Joe Perches <joe@perches.com> wrote:
> On Wed, 2014-02-12 at 21:13 -0500, Patrick Palka wrote:
>> When !defined(CONFIG_EXT4_DEBUG), mb_debug() should be defined as an
>> empty do-while statement so as to suppress the following compiler
>> warning:
>
> Hello Patrick.
>
>> fs/ext4/mballoc.c: In function 'ext4_mb_cleanup_pa':
>> fs/ext4/mballoc.c:2659:47: warning: suggest braces around empty body in an 'if' statement [-Wempty-body]
>>    mb_debug(1, "mballoc: %u PAs left\n", count);
>> ---
>> diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
> []
>> @@ -48,7 +48,7 @@ extern ushort ext4_mballoc_debug;
>>               }                                                       \
>>       } while (0)
>>  #else
>> -#define mb_debug(n, fmt, a...)
>> +#define mb_debug(n, fmt, a...)               do { } while (0)
>
> Ideally, this section should be something like below
> so the !CONFIG_EXT4_DEBUG case still verifies that
> the format and argument types match.
>
> This can help avoid people adding debug statements but
> not compiling with the proper CONFIG_<foo> variable set.
>
> ---
>
> #ifdef CONFIG_EXT4_DEBUG
> extern ushort ext4_mballoc_debug;
>
> #define mb_debug(n, fmt, ...)                                   \
> do {                                                            \
>         if ((n) <= ext4_mballoc_debug)                          \
>                 pr_debug(fmt, ##__VA_ARGS__);                   \
>         }                                                       \
> } while (0)
> #else
> #define mb_debug(n, fmt, ...)                                   \
>         no_printk(KERN_DEBUG fmt, ##__VA_ARGS__)
> #endif
>
>

Hi Joe,

Thanks!  I was not aware of that idiom.  It makes a good amount of
sense.  I'm going to send a revised version of the patch shortly.

Patrick