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:

quoted

On Wed, Nov 23, 2022 at 01:29:56PM -0400, Jason Gunthorpe wrote:

quoted

#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