linux-kernel - Re: [PATCH] x86: cmpxchg_double: Add missing memory clobber

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Date:	Tue, 6 Oct 2015 13:16:46 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Pranith Kumar <bobby.prani@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: cmpxchg_double: Add missing memory clobber

On 10/06/2015 11:54 AM, Pranith Kumar wrote:
> We are reading from memory locations pointed to by p1 and p2 in the asm
> block. Add a memory clobber flag to make gcc aware of this.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> ---
>  arch/x86/include/asm/cmpxchg.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
> index 4a2e5bc..3e83949 100644
> --- a/arch/x86/include/asm/cmpxchg.h
> +++ b/arch/x86/include/asm/cmpxchg.h
> @@ -214,7 +214,8 @@ extern void __add_wrong_size(void)
>  		     : "=a" (__ret), "+d" (__old2),			\
>  		       "+m" (*(p1)), "+m" (*(p2))			\
>  		     : "i" (2 * sizeof(long)), "a" (__old1),		\
> -		       "b" (__new1), "c" (__new2));			\
> +		       "b" (__new1), "c" (__new2)			\
> +		     : "memory");					\
>  	__ret;								\
>  })

NAK.  We already have the "+m" for exactly this reason; adding an
explicit memory clobber should only be used to prevent movement of
*other* memory operations around this one (i.e. a barrier).

	-hpa


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