Re: [RFC][PATCH 11/12] slub: Replace cmpxchg_double()

[H. Peter Anvin]

On 1/9/23 14:22, H. Peter Anvin wrote:
> > Another alternative is to try to avoid casting to "u64" as long as
> > humanly possible, and use only "typeof((*ptr))" everywhere. Then when
> > the type actually *is* 128-bit, it all works out fine, because it
> > won't be a pointer. That's the approach the uaccess macros tend to
> > take, and then they hit the reverse issue on clang, where using the
> > "byte register" constraints would cause warnings for non-byte
> > accesses, and we had to do
> >
> >                 unsigned char x_u8__;
> >                 __get_user_asm(x_u8__, ptr, "b", "=q", label);
> >                 (x) = x_u8__;
> >
> > because using '(x)' directly would then warn when 'x' wasn't a
> > char-sized thing - even if that asm case never actually was _used_ for
> > that case, since it was all inside a "switch (sizeof) case 1:"
> > statement.
> >
> >             Linus

> I wrote a crazy macro for dealing with exactly this at one point,
> basically producing the "right type" to cast to. It would need to
> have 128-bit support added to it, but that should be trivial. It is
> called something like int_type() ... not in front of a computer right
> now so can't double check.
Right, it is called __inttype and is defined in 
arch/x86/include/asm/uaccess.h (and, apparently, a few other 
architectures; probably should be centralized.)

It has been rewritten since my first version using a nice little macro 
called __typefits, also would we worth centralizing.

	-hpa