Re: [PATCH 2/2] kbuild: clang: Disable the 'duplicate-decl-specifier' warning

Masahiro Yamada
2017-05-08T08:29:32Z

Hi Matthias,

2017-05-05 4:50 GMT+09:00 Matthias Kaehlcke <mka@chromium.org>:
> Hi Masahiro,
>
> El Sun, Apr 30, 2017 at 11:01:11PM +0900 Masahiro Yamada ha dit:
>
>> 2017-04-22 6:39 GMT+09:00 Matthias Kaehlcke <mka@chromium.org>:
>> > clang generates plenty of these warnings in different parts of the code.
>> > They are mostly caused by container_of() and other macros which declare
>> > a "const <type> *" variable for their internal use which triggers a
>> > "duplicate 'const' specifier" warning if the <type> is already const
>> > qualified.
>> >
>> > Wording-mostly-from: Michael Davidson <md@google.com>
>> > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
>>
>>
>>
>> I think container_of() can be more simple,
>> dropping the 'const'.
>>
>> The following patch worked for me.
>>
>>
>> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>> index 4c26dc3..d53672b 100644
>> --- a/include/linux/kernel.h
>> +++ b/include/linux/kernel.h
>> @@ -846,11 +846,9 @@ static inline void ftrace_dump(enum
>> ftrace_dump_mode oops_dump_mode) { }
>> * @ptr: the pointer to the member.
>> * @type: the type of the container struct this is embedded in.
>> * @member: the name of the member within the struct.
>> - *
>> */
>> #define container_of(ptr, type, member) ({ \
>> - const typeof( ((type *)0)->member ) *__mptr = (ptr); \
>> - (type *)( (char *)__mptr - offsetof(type,member) );})
>> + (type *)((void *)(ptr) - offsetof(type, member));})
>>
>> /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
>> #ifdef CONFIG_FTRACE_MCOUNT_RECORD
>
> Thanks, this eliminates indeed a huge amount of these warnings.

I noticed this solution will drop precious type checking
from container_of(). So, my workaround is not nice.

> The other big source of warnings is MODULE_DEVICE_TABLE which declares
> a const alias of a type that in most cases is already const. One possible
> solution would be to remove the 'additional' const qualifier, which
> might leave some tables non-const. Another option could be some
> hackery to suppress the warning just for the macro. Not sure if any of
> this would be acceptable.

Me neither.

>> For also this one,
>> I'd like to try to fix the code rather than hiding warnings.
>
> I totally agree with the general approach. My clang kernel builds
> started with plenty of warnings disabled. I fixed the code for most of
> them until I was left with these two extremely noisy ones, for which I
> didn't see a clear path.

If we do not come up with a good solution,
I will pick up this patch.

--
Best Regards
Masahiro Yamada