Re: [PATCH 1/1] container_of: Document container_of() is not to be used in new code

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

On Tue, May 20, 2025 at 05:16:45PM +0300, Andy Shevchenko wrote:
> On Tue, May 20, 2025 at 01:34:37PM +0300, Sakari Ailus wrote:
> > There is a warning in the kerneldoc documentation of container_of() that
> > constness of its ptr argument is lost. While this is a faible suggestion
> > container_of_const() should be used instead, the vast majority of new code
> > still uses container_of():
> > 
> > $ git diff v6.13 v6.14|grep container_of\(|wc -l
> > 646
> > $ git diff v6.13 v6.14|grep container_of_const|wc -l
> > 9
> > 
> > Make an explicit recommendation to use container_of_const().
> > 
> > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > ---
> > Hi Greg, Andy,
> > 
> > I guess we generally agree the additional constness check in
> > container_of_const() is useful, but adding the same check to
> > container_of() generates warnings -- there are some errors, too -- such as
> > this one currently:
> > 
> > In file included from /home/sailus/src/linux/include/linux/bcma/bcma.h:14,
> >                  from /home/sailus/src/linux/arch/x86/kernel/early-quirks.c:17:
> > /home/sailus/src/linux/include/linux/ssb/ssb.h: In function ‘dev_to_ssb_dev’:
> > /home/sailus/src/linux/include/linux/ssb/ssb.h:291:14: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
> >   291 |         wrap = container_of(dev, struct __ssb_dev_wrapper, dev);
> >       |              ^
> > 
> > As noted above, 646 new missing constness checks were introduced through
> > container_of() macro use during the 6.14 cycle alone. Most of these are
> > likely harmless, but with so many new users some are bound to be ignoring
> > constness.
> > 
> > Once the warnings from bad container_of() use are worked out in a way or
> > another, the constness check could be added to the container_of() macro
> > and the current container_of_const() be dropped altogether.
> > 
> > If this patch is accepted, I'll see how to add a warning on container_of()
> > to checkpatch.pl.
> 
> Hmm... Wouldn't be better to fix non-const cases and add the const check, etc
> to the container_of() instead of doing these comments?

Yes, fixing up the existing places where it is broken would be best, how
many of them are there now?

thanks,

greg k-h