[PATCH] powerpc: Fix smp_wmb barrier definition use use lwsync consistently

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:

> asm/barrier.h is not always included after asm/synch.h, which meant
> it was missing __SUBARCH_HAS_LWSYNC, so in some files smp_wmb() would
> be eieio when it should be lwsync. kernel/time/hrtimer.c is one case.

Wow nice catch. Only broken since 2008 presumably.

Some days I think maybe we aren't very good at this writing software
thing, good to have some certainty :)

> __SUBARCH_HAS_LWSYNC is only used in one place, so just fold it in
> to where it's used. Previously with my small simulator config, 377
> instances of eieio in the tree. After this patch there are 55.

At least for Book3S this isn't actually a terrible bug AFAICS:

 - smp_wmb() is only defined to order accesses to cacheable memory.
 - smp_wmb() only orders prior stores vs later stores.
 - eieio orders all prior stores vs all later stores for cacheable
   memory.
 - lwsync orders everything except prior stores vs later loads for
   cacheable memory.

So eieio and lwsync are both valid to use as smp_wmb(), but it's still
terrible fishy that we were using both in different places depending on
include ordering.

I'm inclined to tag this for stable unless anyone can think of a reason
not to?

cheers

> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> index 10daa1d56e0a..c7c63959ba91 100644
> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -35,7 +35,8 @@
>  #define rmb()  __asm__ __volatile__ ("sync" : : : "memory")
>  #define wmb()  __asm__ __volatile__ ("sync" : : : "memory")
>  
> -#ifdef __SUBARCH_HAS_LWSYNC
> +/* The sub-arch has lwsync */
> +#if defined(__powerpc64__) || defined(CONFIG_PPC_E500MC)
>  #    define SMPWMB      LWSYNC
>  #else
>  #    define SMPWMB      eieio
> diff --git a/arch/powerpc/include/asm/synch.h b/arch/powerpc/include/asm/synch.h
> index 63e7f5a1f105..6ec546090ba1 100644
> --- a/arch/powerpc/include/asm/synch.h
> +++ b/arch/powerpc/include/asm/synch.h
> @@ -6,10 +6,6 @@
>  #include <linux/stringify.h>
>  #include <asm/feature-fixups.h>
>  
> -#if defined(__powerpc64__) || defined(CONFIG_PPC_E500MC)
> -#define __SUBARCH_HAS_LWSYNC
> -#endif
> -
>  #ifndef __ASSEMBLY__
>  extern unsigned int __start___lwsync_fixup, __stop___lwsync_fixup;
>  extern void do_lwsync_fixups(unsigned long value, void *fixup_start,
> -- 
> 2.16.1