Re: [patch V4 part 1 06/36] compiler: Simple READ/WRITE_ONCE() implementations

[Will Deacon]

On Tue, May 05, 2020 at 03:16:08PM +0200, Thomas Gleixner wrote:
> READ/WRITE_ONCE_NOCHECK() is required for atomics in code which cannot be
> instrumented like the x86 int3 text poke code. As READ/WRITE_ONCE() is
> undergoing a rewrite, provide __{READ,WRITE}_ONCE_SCALAR().
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  include/linux/compiler.h |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -313,6 +313,14 @@ unsigned long read_word_at_a_time(const
>  	__u.__val;					\
>  })
>  
> +#define __READ_ONCE_SCALAR(x)				\
> +	(*(const volatile typeof(x) *)&(x))
> +
> +#define __WRITE_ONCE_SCALAR(x, val)			\
> +do {							\
> +	*(volatile typeof(x) *)&(x) = val;		\
> +} while (0)

FWIW, these end up being called __READ_ONCE() and __WRITE_ONCE() after
the rewrite; the *_SCALAR() variants will call into kcsan_check_atomic_*().

If you go with that naming now, then any later conflict should fall out in
the wash.

Will