linux-kernel - Re: [tip:locking/urgent] locking/xchg/alpha: Clean up barrier usage by using smp_mb() in place of __ASM_

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Date:   Mon, 26 Feb 2018 18:05:52 +0000
From:   Will Deacon <will.deacon@....com>
To:     mattst88@...il.com, rth@...ddle.net, tglx@...utronix.de,
        hpa@...or.com, stern@...land.harvard.edu, parri.andrea@...il.com,
        ink@...assic.park.msu.ru, akpm@...ux-foundation.org,
        paulmck@...ux.vnet.ibm.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...nel.org
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:locking/urgent] locking/xchg/alpha: Clean up barrier usage
 by using smp_mb() in place of __ASM__MB

Hi Andrea,

I know this is in mainline now, but I think the way you've got the barriers
here:

On Fri, Feb 23, 2018 at 12:27:54AM -0800, tip-bot for Andrea Parri wrote:
> diff --git a/arch/alpha/include/asm/cmpxchg.h b/arch/alpha/include/asm/cmpxchg.h
> index 46ebf14aed4e..8a2b331e43fe 100644
> --- a/arch/alpha/include/asm/cmpxchg.h
> +++ b/arch/alpha/include/asm/cmpxchg.h
> @@ -6,7 +6,6 @@
>   * Atomic exchange routines.
>   */
>  
> -#define __ASM__MB
>  #define ____xchg(type, args...)		__xchg ## type ## _local(args)
>  #define ____cmpxchg(type, args...)	__cmpxchg ## type ## _local(args)
>  #include <asm/xchg.h>
> @@ -33,10 +32,6 @@
>  	cmpxchg_local((ptr), (o), (n));					\
>  })
>  
> -#ifdef CONFIG_SMP
> -#undef __ASM__MB
> -#define __ASM__MB	"\tmb\n"
> -#endif
>  #undef ____xchg
>  #undef ____cmpxchg
>  #define ____xchg(type, args...)		__xchg ##type(args)
> @@ -64,7 +59,6 @@
>  	cmpxchg((ptr), (o), (n));					\
>  })
>  
> -#undef __ASM__MB
>  #undef ____cmpxchg
>  
>  #endif /* _ALPHA_CMPXCHG_H */
> diff --git a/arch/alpha/include/asm/xchg.h b/arch/alpha/include/asm/xchg.h
> index e2660866ce97..e1facf6fc244 100644
> --- a/arch/alpha/include/asm/xchg.h
> +++ b/arch/alpha/include/asm/xchg.h
> @@ -28,12 +28,12 @@ ____xchg(_u8, volatile char *m, unsigned long val)
>  	"	or	%1,%2,%2\n"
>  	"	stq_c	%2,0(%3)\n"
>  	"	beq	%2,2f\n"
> -		__ASM__MB
>  	".subsection 2\n"
>  	"2:	br	1b\n"
>  	".previous"
>  	: "=&r" (ret), "=&r" (val), "=&r" (tmp), "=&r" (addr64)
>  	: "r" ((long)m), "1" (val) : "memory");
> +	smp_mb();
>  
>  	return ret;

ends up adding unnecessary barriers to the _local variants, which the
previous code took care to avoid. That's why I suggesting adding
the smp_mb() into the cmpxchg macro rather than the ____cmpxchg variants.

I think it's worth spinning another patch to fix this properly.

Will