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

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Date:   Mon, 24 Oct 2022 12:01:56 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Andy Shevchenko' <andriy.shevchenko@...ux.intel.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH 1/1] linux/container_of.h: Warn about loss of constness

On Mon, Oct 24, 2022 at 09:46:40AM +0000, David Laight wrote:
> 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'.

This is a pointless discussion, this macro will now be deleted, sorry.

greg k-h