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

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Message-ID: <aCyOzUIIvMk6Gp8o@smile.fi.intel.com>
Date: Tue, 20 May 2025 17:16:45 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] container_of: Document container_of() is not to be
 used in new code

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@...ux.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?

-- 
With Best Regards,
Andy Shevchenko