On Mon, Jan 26, 2026, at 20:54, Marco Elver wrote:

On Mon, Jan 26, 2026 at 08:56AM +0100, Arnd Bergmann wrote:

quoted

On Mon, Jan 26, 2026, at 01:25, Marco Elver wrote:

How does this work with CC_HAS_TYPEOF_UNQUAL=false?

As far as I can tell, TYPEOF_UNQUAL() falls back to __typeof__
on gcc-13, clang-18 and earlier, and not strip out qualifiers.

I think we only need to worry about Clang for LTO builds. But yeah, our
minimum supported Clang is 15, so between 15-18 it'd be broken.

Right, I missed the #ifdef CONFIG_LTO check, so indeed gcc is
fine here.

quoted

With fd69b2f7d5f4 ("compiler: Use __typeof_unqual__() for
__unqual_scalar_typeof()"), I would expect __unqual_scalar_typeof()
to do the right thing already.

It'd still be broken for Clang 15-18, so it won't help much. We need
this to work for more than "scalar", so even though it'll work for Clang
19+ given the redefinition to __typeof_unqual__, we should deprecate the
_Generic-based __unqual_scalar_typeof() sooner than later.

I was able to make this work for older compilers:

...

 #define __READ_ONCE(x)							\
 ({									\
 	auto __x = &(x);						\
-	auto __ret = (TYPEOF_UNQUAL(*__x) *)__x, *__retp = &__ret;	\
-	union { TYPEOF_UNQUAL(*__x) __val; char __c[1]; } __u;		\
+	auto __ret = (__read_once_typeof(*__x) *)__x, *__retp = &__ret;	\
+	union { __read_once_typeof(*__x) __val; char __c[1]; } __u;	\
 	*__retp = &__u.__val;						\

Thoughts?

Looks better than __unqual_scalar_typeof() to me. Would it make
sense to do the same __read_once_typeof() in the asm-generic
version of __READ_ONCE()? I don't remember if we discussed it
in the thread leading up to dee081bf8f82 ("READ_ONCE: Drop 
pointer qualifiers when reading from scalar types").
We probably didn't have __auto_type back then.

        Arnd