Re: [PATCH 2/3] container_of: remove useless pair of parentheses

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Jul 15, 2026 at 06:27:31PM +0200, Vincent Mailhol wrote:
> On 15/07/2026 at 16:57, Greg Kroah-Hartman wrote:
> > On Wed, Jul 15, 2026 at 04:51:28PM +0200, Vincent Mailhol wrote:
> >> On 15/07/2026 at 16:30, Alexey Dobriyan wrote:
> >>> On Tue, Jul 14, 2026 at 08:18:02PM +0200, Vincent Mailhol wrote:
> >>>> The last expression in container_of() doesn't need an extra pair of
> >>>> parenthesis. Remove it.
> >>>>
> >>>> Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
> >>>> ---
> >>>>  include/linux/container_of.h | 2 +-
> >>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> >>>> index 28500a62ab7e..68153170db32 100644
> >>>> --- a/include/linux/container_of.h
> >>>> +++ b/include/linux/container_of.h
> >>>> @@ -21,7 +21,7 @@
> >>>>  	static_assert(__same_type(*(ptr), typeof_member(type, member)) || \
> >>>>  		      __same_type(*(ptr), void),			\
> >>>>  		      "pointer type mismatch in container_of()");	\
> >>>> -	((type *)(__mptr - offsetof(type, member))); })
> >>>> +	(type *)(__mptr - offsetof(type, member)); })
> >>>
> >>> container_of_const() has the same problem.
> >>
> >> Yes. But scoped I my series to the container_of() macro only.
> >>
> >> I don't plan to send a follow-up patch to remove the other useless
> >> parenthesis. The things which really bothered me was the __mptr
> >> variable which I removed in patch #3. And with that, I am done with
> >> what I wanted to contribute in this file.
> >>
> >> But you are welcome to do this clean-up if you feel the need for it.
> >>
> >>> And it should parenthesize first argument to _Generic.
> >>
> >> Do you mean like this:
> >>
> >> 	#define container_of_const(ptr, type, member)				\
> >> 		_Generic((ptr),							\
> >> 			const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
> >> 			default: ((type *)container_of(ptr, type, member))	\
> >> 		)
> >>
> >> ?
> >>
> >> Why would parenthesis be needed here? I don't see what kind of
> >> expression would make the current implementation unsafe.
> > 
> > It took me a while working with container_of_const() to get it right
> > with the (), so be careful removing any.
> 
> I just tested this patch:
> 
> ---8<---
> diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> index 1f6ebf27d962..03d5b91534a7 100644
> --- a/include/linux/container_of.h
> +++ b/include/linux/container_of.h
> @@ -34,8 +34,8 @@
>   */
>  #define container_of_const(ptr, type, member)				\
>  	_Generic(ptr,							\
> -		const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
> -		default: ((type *)container_of(ptr, type, member))	\
> +		const typeof(*(ptr)) *: (const type *)container_of(ptr, type, member),\
> +		default: (type *)container_of(ptr, type, member)	\
>  	)
>   #endif	/* _LINUX_CONTAINER_OF_H */
> ---8<---
> 
> I could compile and boot it, no problem (I am actually writing this
> message from a machine on which this patch is applied).
> 
> So, yes, I believe that these two pairs of parenthesis can be removed.
> Just not sure if it is worth it. For container_of(), the parenthesis
> removal was part of a series with more meaningful changes. I don't
> feel exited of sending another series to just remove the
> container_of_const() parenthesis.

Yeah, if it's not hurting anything, let's just leave it as-is for now.

thanks,

greg k-h