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

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Date:   Tue, 2 Apr 2019 09:26:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     paulmck@...ux.ibm.com, hpa@...or.com, linux-kernel@...r.kernel.org,
        dvyukov@...gle.com, jyknight@...gle.com, x86@...nel.org,
        mingo@...hat.com
Subject: Re: [PATCH] x86/asm: use memory clobber in bitops that touch
 arbitrary memory



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.