linux-kernel - Re: [PATCH v2 1/4] container_of: add container_of_const() that preserves const-ness of the pointer

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Date:   Tue, 6 Dec 2022 16:54:45 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v2 1/4] container_of: add container_of_const() that
 preserves const-ness of the pointer

On Tue, Dec 06, 2022 at 03:13:29PM +0000, Matthew Wilcox wrote:
> On Mon, Dec 05, 2022 at 01:12:03PM +0100, Greg Kroah-Hartman wrote:
> > +/**
> > + * container_of_const - cast a member of a structure out to the containing
> > + *			structure and preserve the const-ness of the pointer
> > + * @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_const(ptr, type, member)				\
> > +	_Generic(ptr,							\
> > +		const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
> > +		default: ((type *)container_of(ptr, type, member))	\
> > +	)
> > +
> 
> Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> 
> I tried doing this:
> 
> +++ b/include/linux/container_of.h
> @@ -15,11 +15,17 @@
>   *
>   * WARNING: any const qualifier of @ptr is lost.
>   */
> -#define container_of(ptr, type, member) ({                             \
> +#define _c_of(ptr, type, member) ({                                    \
>         void *__mptr = (void *)(ptr);                                   \
>         static_assert(__same_type(*(ptr), ((type *)0)->member) ||       \
>                       __same_type(*(ptr), void),                        \
>                       "pointer type mismatch in container_of()");       \
>         ((type *)(__mptr - offsetof(type, member))); })
> 
> +#define container_of(ptr, type, m)                                     \
> +       _Generic(ptr,                                                   \
> +               const typeof(*(ptr)) *: (const type *)_c_of(ptr, type, m),\
> +               default: ((type *)_c_of(ptr, type, m))                  \
> +       )
> +
>  #endif /* _LINUX_CONTAINER_OF_H */
> 
> (whitespace damaged, yes the kernel-doc is now in the wrong place, etc)
> 
> It found a few problems; just building the mlx5 driver (I happened to be
> doing some work on it in that tree).  We're definitely not ready to do
> that yet, but I'll send a few patches to prepare for it.

Yeah, that's a good long-term goal to have here.  Once everything is
moved over, switching all container_of_const() to just container_of()
should be simple.

thanks,

greg k-h