Re: [PATCH v4 08/11] READ_ONCE: Drop pointer qualifiers when reading from scalar types

[Rasmus Villemoes]

On 21/04/2020 17.15, Will Deacon wrote:
> Passing a volatile-qualified pointer to READ_ONCE() is an absolute
> trainwreck for code generation: the use of 'typeof()' to define a
> temporary variable inside the macro means that the final evaluation in
> macro scope ends up forcing a read back from the stack. When stack
> protector is enabled (the default for arm64, at least), this causes
> the compiler to vomit up all sorts of junk.
> 
> Unfortunately, dropping pointer qualifiers inside the macro poses quite
> a challenge, especially since the pointed-to type is permitted to be an
> aggregate, and this is relied upon by mm/ code accessing things like
> 'pmd_t'. Based on numerous hacks and discussions on the mailing list,
> this is the best I've managed to come up with.

Hm, maybe this can be brought to work, only very lightly tested. It
basically abuses what -Wignored-qualifiers points out:

  warning: type qualifiers ignored on function return type

Example showing the idea:

const int c(void);
volatile int v(void);

int hack(int x, int y)
{
	typeof(c()) a = x;
	typeof(v()) b = y;

	a += b;
	b += a;
	a += b;
	return a;
}

Since that compiles, a cannot be const-qualified, and the generated code
certainly suggests that b is not volatile-qualified. So something like

#define unqual_type(x) _unqual_type(x, unique_id_dance)
#define _unqual_type(x, id) typeof( ({
  typeof(x) id(void);
  id();
}) )

and perhaps some _Pragma("GCC diagnostic push")/_Pragma("GCC diagnostic
ignored -Wignored-qualifiers")/_Pragma("GCC diagnostic pop") could
prevent the warning (which is in -Wextra, so I don't think it would
appear in a normal build anyway).

No idea how well any of this would work across gcc versions or with clang.

Rasmus