Re: include/net/sock.h:2100:16: sparse: sparse: cast to non-scalar

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Jan 10, 2026 at 10:35:48PM +0000, Al Viro wrote:

> #define __READ_ONCE(x)                                                  \
> ({                                                                      \
>         __unqual_scalar_typeof(x) __x =                                 \
> 		(*(volatile typeof(__x) *)(&(x)));                      \
> 	mb();                                                           \
> 	(typeof(x))__x;                                                 \
> })
> combined with
> typedef struct {
>         uid_t val;
> } kuid_t;
> 
> IOW, it complains about a cast from structure to itself, which is fair
> enough - C is pretty clear about not allowing any typecasts to or from
> non-scalar types, tautological or not.
> 
> Why do we even need that cast?  Seeing that generic __READ_ONCE() is
> #define __READ_ONCE(x)  (*(const volatile __unqual_scalar_typeof(x) *)&(x))
> the cast added on alpha seems to be pointless.

The problem was things like test_bit() that take a volatile argument,
doing READ_ONCE() on them would instantiate a volatile temporary and GCC
would end up generating shit code.

The __unqual_scalar_typeof() was the result of trying to remove CV
qualifiers from a type.