Subject: Re: [patch 2/2] powerpc: optimise smp_wmb

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 21 May 2008 11:26:32 -0400

On Wed, 2008-05-21 at 16:12 +0200, Nick Piggin wrote:
> lwsync is the recommended method of store/store ordering on caching enabled
> memory. For those subarchs which have lwsync, use it rather than eieio for
> smp_wmb.

Yuck... existence of lwsync depends on the processor at boot time...

Ben.

> Signed-off-by: Nick Piggin <[email-masked]>
> ---
> Index: linux-2.6/include/asm-powerpc/system.h
> ===================================================================
> --- linux-2.6.orig/include/asm-powerpc/system.h
> +++ linux-2.6/include/asm-powerpc/system.h
> @@ -30,8 +30,8 @@
>   *
>   * For wmb(), we use sync since wmb is used in drivers to order
>   * stores to system memory with respect to writes to the device.
> - * However, smp_wmb() can be a lighter-weight eieio barrier on
> - * SMP since it is only used to order updates to system memory.
> + * However, smp_wmb() can be a lighter-weight lwsync or eieio barrier
> + * on SMP since it is only used to order updates to system memory.
>   */
>  #define mb()   __asm__ __volatile__ ("sync" : : : "memory")
>  #define rmb()  __asm__ __volatile__ ("sync" : : : "memory")
> @@ -43,9 +43,16 @@
>  #ifdef __KERNEL__
>  #define AT_VECTOR_SIZE_ARCH 6 /* entries in ARCH_DLINFO */
>  #ifdef CONFIG_SMP
> +
> +#ifdef __SUBARCH_HAS_LWSYNC
> +#    define SMPWMB      lwsync
> +#else
> +#    define SMPWMB      eieio
> +#endif
> +
>  #define smp_mb()	mb()
>  #define smp_rmb()	rmb()
> -#define smp_wmb()	eieio()
> +#define smp_wmb()	__asm__ __volatile__ (__stringify(SMPWMB) : : :"memory")
>  #define smp_read_barrier_depends()	read_barrier_depends()
>  #else
>  #define smp_mb()	barrier()
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