linux-kernel - Re: [PATCH 1/1] container_of: Document container_of

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Date: Tue, 18 Jun 2024 16:02:46 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Jason Gunthorpe <jgg@...dia.com>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Hans Verkuil <hverkuil@...all.nl>
Subject: Re: [PATCH 1/1] container_of: Document container_of_const() is
 preferred

On Tue, Jun 18, 2024 at 02:56:17PM +0100, Matthew Wilcox wrote:
> On Tue, Jun 18, 2024 at 02:54:53PM +0200, Greg Kroah-Hartman wrote:
> > > I didn't have any issues (apart from me misspelling function names ;)) with
> > > GCC 12, neither in using container_of_const() in a static inline function
> > > nor in using a static inline function as a _Generic() expression.
> > 
> > Really?  And how do you handle the pointer being either const or not,
> > and propagating that back out as the return type?  I'd like to see your
> > inline function please.
> 
> Here's how I did it for page_folio():
> 
> #define page_folio(p)           (_Generic((p),                          \
>         const struct page *:    (const struct folio *)_compound_head(p), \
>         struct page *:          (struct folio *)_compound_head(p)))
> 
> Is there something differently magic about container_of() that prevents
> this trick from working?

That's a #define, not an inline function, which is what I thought we
were talking about.

thanks,

greg k-h