Re: [PATCH 1/2] locking/xchg/alpha: Use smp_mb() in place of __ASM__MB

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Feb 22, 2018 at 10:24:29AM +0100, Andrea Parri wrote:
> Replace each occurrence of __ASM__MB with a (trailing) smp_mb() in
> xchg(), cmpxchg(), and remove the now unused __ASM__MB definitions;
> this improves readability, with no additional synchronization cost.
> 
> Suggested-by: Will Deacon <will.deacon@arm.com>
> Signed-off-by: Andrea Parri <parri.andrea@gmail.com>

I am a bit confused by the use of out-of-line branches to do a backwards
branch, but those were in place to start with.  Maybe the point is to
defeat backwards-branch prediction or some such.

Regardless...

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
> Cc: Matt Turner <mattst88@gmail.com>
> Cc: Richard Henderson <rth@twiddle.net>
> Cc: linux-alpha@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  arch/alpha/include/asm/cmpxchg.h |  6 ------
>  arch/alpha/include/asm/xchg.h    | 16 ++++++++--------
>  2 files changed, 8 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/alpha/include/asm/cmpxchg.h b/arch/alpha/include/asm/cmpxchg.h
> index 46ebf14aed4e5..8a2b331e43feb 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 e2660866ce972..e1facf6fc2446 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;
>  }
> @@ -52,12 +52,12 @@ ____xchg(_u16, volatile short *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;
>  }
> @@ -72,12 +72,12 @@ ____xchg(_u32, volatile int *m, unsigned long val)
>  	"	bis $31,%3,%1\n"
>  	"	stl_c %1,%2\n"
>  	"	beq %1,2f\n"
> -		__ASM__MB
>  	".subsection 2\n"
>  	"2:	br 1b\n"
>  	".previous"
>  	: "=&r" (val), "=&r" (dummy), "=m" (*m)
>  	: "rI" (val), "m" (*m) : "memory");
> +	smp_mb();
> 
>  	return val;
>  }
> @@ -92,12 +92,12 @@ ____xchg(_u64, volatile long *m, unsigned long val)
>  	"	bis $31,%3,%1\n"
>  	"	stq_c %1,%2\n"
>  	"	beq %1,2f\n"
> -		__ASM__MB
>  	".subsection 2\n"
>  	"2:	br 1b\n"
>  	".previous"
>  	: "=&r" (val), "=&r" (dummy), "=m" (*m)
>  	: "rI" (val), "m" (*m) : "memory");
> +	smp_mb();
> 
>  	return val;
>  }
> @@ -150,12 +150,12 @@ ____cmpxchg(_u8, volatile char *m, unsigned char old, unsigned char new)
>  	"	stq_c	%2,0(%4)\n"
>  	"	beq	%2,3f\n"
>  	"2:\n"
> -		__ASM__MB
>  	".subsection 2\n"
>  	"3:	br	1b\n"
>  	".previous"
>  	: "=&r" (prev), "=&r" (new), "=&r" (tmp), "=&r" (cmp), "=&r" (addr64)
>  	: "r" ((long)m), "Ir" (old), "1" (new) : "memory");
> +	smp_mb();
> 
>  	return prev;
>  }
> @@ -177,12 +177,12 @@ ____cmpxchg(_u16, volatile short *m, unsigned short old, unsigned short new)
>  	"	stq_c	%2,0(%4)\n"
>  	"	beq	%2,3f\n"
>  	"2:\n"
> -		__ASM__MB
>  	".subsection 2\n"
>  	"3:	br	1b\n"
>  	".previous"
>  	: "=&r" (prev), "=&r" (new), "=&r" (tmp), "=&r" (cmp), "=&r" (addr64)
>  	: "r" ((long)m), "Ir" (old), "1" (new) : "memory");
> +	smp_mb();
> 
>  	return prev;
>  }
> @@ -200,12 +200,12 @@ ____cmpxchg(_u32, volatile int *m, int old, int new)
>  	"	stl_c %1,%2\n"
>  	"	beq %1,3f\n"
>  	"2:\n"
> -		__ASM__MB
>  	".subsection 2\n"
>  	"3:	br 1b\n"
>  	".previous"
>  	: "=&r"(prev), "=&r"(cmp), "=m"(*m)
>  	: "r"((long) old), "r"(new), "m"(*m) : "memory");
> +	smp_mb();
> 
>  	return prev;
>  }
> @@ -223,12 +223,12 @@ ____cmpxchg(_u64, volatile long *m, unsigned long old, unsigned long new)
>  	"	stq_c %1,%2\n"
>  	"	beq %1,3f\n"
>  	"2:\n"
> -		__ASM__MB
>  	".subsection 2\n"
>  	"3:	br 1b\n"
>  	".previous"
>  	: "=&r"(prev), "=&r"(cmp), "=m"(*m)
>  	: "r"((long) old), "r"(new), "m"(*m) : "memory");
> +	smp_mb();
> 
>  	return prev;
>  }
> -- 
> 2.7.4
>