Re: [PATCH] x86/asm: use memory clobber in bitops that touch arbitrary memory

[Peter Zijlstra]

On Mon, Apr 01, 2019 at 06:24:08PM +0200, Alexander Potapenko wrote:
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index d153d570bb04..20e4950827d9 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -111,7 +111,7 @@ clear_bit(long nr, volatile unsigned long *addr)
>       } else {
>               asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
>                       : BITOP_ADDR(addr)
> -                     : "Ir" (nr));
> +                     : "Ir" (nr) : "memory");
>       }
>  }

clear_bit() doesn't have a return value, so why are we now still using
"+m" output ?

AFAICT the only reason we did that was to clobber the variable, which
you've (afaiu correctly) argued to be incorrect.

So whould we not write this as:

        asm volatile (LOCK_PREFIX __ASM_SIZE(btr) " %[nr], %[addr]"
                : : [addr] "m" (*addr), [nr] "Ir" (nr)
                : "memory");

?

And the very same for _all_ other sites touched in this patch.