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Date:   Wed, 23 Nov 2022 14:25:59 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
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Subject: Re: [PATCH 3/5] driver core: make struct device_type.uevent() take a
 const *
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On Wed, Nov 23, 2022 at 07:10:49PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Nov 23, 2022 at 05:49:36PM +0000, Matthew Wilcox wrote:
> > On Wed, Nov 23, 2022 at 01:29:56PM -0400, Jason Gunthorpe wrote:
> > > #define generic_container_of(in_type, in, out_type, out_member) \
> > > 	_Generic(in,                                        \
> > >                   const in_type *: ((const out_type *)container_of(in, out_type, out_member)),   \
> > >                   in_type *: ((out_type *)container_of(in, out_type, out_member)) \
> > > 		  )
> > 
> > There's a neat trick I found in seqlock.h:
> > 
> > #define generic_container_of(in_t, in, out_t, m)			\
> > 	_Generic(*(in),							\
> > 		const in_t: ((const out_t *)container_of(in, out_t, m)), \
> > 		in_t: ((out_t *)container_of(in, out_type, m))	\
> > 	)
> > 
> > and now it fits in 80 columns ;-)
> 
> Nice trick!  Dropping the inline functions is a bit different, let me
> see if that still gives a sane error if we pass an incorrect type or
> mess with the const * the wrong way.  I'll run some tests tomorrow
> afternoon...

The errors in some cases are very verbose, but it is somewhat
understandable - the worst is when _Generic fails to match anything,
but also at least clang partially expanded container_of and it throws
other assertions too.

I also wonder if this could just be rolled into the normal
container_of.

in_type would have to be derived like:

  in_type = typeof((out_type *)NULL)->out_member)

But I don't know if you can use typeof in a generic type matching expression..

Jason