Re: [PATCH 1/1] linux/container_of.h: Warn about loss of constness

From: 'Andy Shevchenko' <andriy.shevchenko@linux.intel.com>
To: David Laight <David.Laight@aculab.com>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>
Subject: Re: [PATCH 1/1] linux/container_of.h: Warn about loss of constness
Date: Mon, 24 Oct 2022 12:37:12 +0300	[thread overview]
Message-ID: <Y1ZcyBd2Yjir/dNO@smile.fi.intel.com> (raw)
In-Reply-To: <fbe735d131a44c2a95248cc4ad51485b@AcuMS.aculab.com>

On Mon, Oct 24, 2022 at 09:34:42AM +0000, David Laight wrote:
> From: Andy Shevchenko
> > Sent: 24 October 2022 10:23

...

> > > > Wait, no one uses this macro, so why not just remove it entirely?
> > >
> > > Good question. It appears to be a (relatively) common pattern to look up
> > > something and the return its containing object if the lookup was
> > > successful. Doing a quick
> > >
> > > 	$ git grep 'container_of.*:' drivers include
> > >
> > > reveals more than 20 instances of the pattern. There are probably more
> > > those that use if for testing for NULL. I guess people don't know about
> > > this macro, apart from the developers of the staging driver it was added
> > > for (commit 05e6557b8ed833546ee2b66ce6b58fecf09f439e).
> > 
> > Maybe we can provide an example to keep this macro in the kernel, meaning
> > convert one of the drivers / subsystem to actually use it?
> 
> Adding _safe() to a function name doesn't actually tell you anything.
> You still need to look up what it is 'safe' against.
> 
> In this case the full code pattern is actually much clearer.
> 
> It is also quite likely that it is followed by an:
> 	if (!ptr)
> 		return xxx;
> You that can/should really be put before the container_of() call.

return statements in macros are no go. Or you meant something else?

-- 
With Best Regards,
Andy Shevchenko