| Message ID | 20181010142813.1918180-1-arnd@arndb.de |
|---|---|
| State | New |
| Headers | show |
| Series | ext4: avoid unused variable warning | expand |
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@arndb.de> 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
Hi Ted, On Wed, Oct 10, 2018 at 9:27 PM Theodore Y. Ts'o <tytso@mit.edu> wrote: > > 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@arndb.de> > > Hmm, I wonder if we should do something like: > > #define EXT4_UNUSED_VAR __attribute__ ((unused)) We have __maybe_unused already, so you can go ahead! :-) (Also __always_unused, same definition as well, but here it does not may sense). > > 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. Indeed, it looks clean --- I like it. Although I am not sure how people will feel about that :-) Someone may argue that, for consistency, we shouldn't, because inside structs we have to use #ifdefs still. Cheers, Miguel
On 10/10/18, Theodore Y. Ts'o <tytso@mit.edu> wrote: > 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@arndb.de> > > 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. I think the global __maybe_unused macro should work fine here. I though about using that instead, but picked the #ifdef for consistency with the other ifdef in the same function. > 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. Another alternative that often results in more readable code is to use a check like if (IS_ENABLED(CONFIG_QUOTA)) { .... } around the conditional code instead of the #ifdef. This should usually work unless the code accesses some struct members that are also hidden in that #ifdef. I have not checked if that is the case here. Arnd
diff --git a/fs/ext4/super.c b/fs/ext4/super.c index e9ca8312457b..d60199510c94 100644 --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -1974,7 +1974,10 @@ static int parse_options(char *options, struct super_block *sb, int is_remount) { struct ext4_sb_info *sbi = EXT4_SB(sb); - char *p, *usr_qf_name, *grp_qf_name; + char *p; +#ifdef CONFIG_QUOTA + char *usr_qf_name, *grp_qf_name; +#endif substring_t args[MAX_OPT_ARGS]; int token;
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@arndb.de> --- fs/ext4/super.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) -- 2.18.0