Re: [PATCH] ext4: avoid unused variable warning

[Theodore Y. Ts'o]

On Wed, Oct 10, 2018 at 04:27:58PM +0200, Arnd Bergmann wrote:
> The two new variables are only used in an #ifdef, so they cause a
> warning without CONFIG_QUOTA:
> 
> fs/ext4/super.c: In function 'parse_options':
> fs/ext4/super.c:1977:26: error: unused variable 'grp_qf_name' [-Werror=unused-variable]
>   char *p, *usr_qf_name, *grp_qf_name;
>                           ^~~~~~~~~~~
> fs/ext4/super.c:1977:12: error: unused variable 'usr_qf_name' [-Werror=unused-variable]
>   char *p, *usr_qf_name, *grp_qf_name;
> 
> Fixes: 20cefcdc2040 ("ext4: fix use-after-free race in ext4_remount()'s error path")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Hmm, I wonder if we should do something like:

#define EXT4_UNUSED_VAR __attribute__ ((unused))

and then we could do:

	char *p, *usr_qf_name EXT4_UNUSED_VAR, *grp_qf_name EXT4_UNUSED_VAR;

More generally, I wonder if this is something we should have defined
for the whole kernel, as opposed to a one-off hack that ACPI and ext4
subsystems use.  It's a little ugly, but I think it's much nicer than
having extra #ifdefs such as:

	char *p;
#ifdef CONFIG_QUOTA
	char *usr_qf_name, *grp_qf_name;
#endif

After all, the compiler is perfectly capable of ignoring variables
which are unused.  And if it's only because of an #ifdef later in the
function, it would be nice to not have an extra #ifdef in the variable
declarations.

						- Ted