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

From: David Laight <David.Laight@ACULAB.COM>
To: 'Andy Shevchenko' <andriy.shevchenko@linux.intel.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 09:46:40 +0000	[thread overview]
Message-ID: <57360b976d5944babe1e85fc51b3f6f6@AcuMS.aculab.com> (raw)
In-Reply-To: <Y1ZcyBd2Yjir/dNO@smile.fi.intel.com>

From: 'Andy Shevchenko'
> Sent: 24 October 2022 10:37
> ...
> 
> > > > > 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?

I meant in the function itself.

It might be interesting to check how many of the function
can actually have a NULL pointer?
Especially in staging code might be being 'defensive'.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)