Re: [PATCH tip/core/rcu 48/55] powerpc: strengthenvalue-returning-atomics memory barriers

[Olof Johansson]

[+linuxppc-dev]

On Tue, Sep 6, 2011 at 11:00 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> The trailing isync/lwsync in PowerPC value-returning atomics needs
> to be a sync in order to provide the required ordering properties.
> The leading lwsync/eieio can remain, as the remainder of the required
> ordering guarantees are provided by the atomic instructions: Any
> reordering will cause the stwcx to fail, which will result in a retry.

Admittedly, my powerpc barrier memory is starting to fade, but isn't
isync sufficient here? It will make sure all instructions before it
have retired, and will restart any speculative/issued instructions
beyond it.

lwsync not being sufficient makes sense since a load can overtake it.

> diff --git a/arch/powerpc/include/asm/synch.h b/arch/powerpc/include/asm/synch.h
> index d7cab44..4d97fbe 100644
> --- a/arch/powerpc/include/asm/synch.h
> +++ b/arch/powerpc/include/asm/synch.h
> @@ -37,11 +37,7 @@ static inline void isync(void)
>  #endif
>
>  #ifdef CONFIG_SMP
> -#define __PPC_ACQUIRE_BARRIER                          \
> -       START_LWSYNC_SECTION(97);                       \
> -       isync;                                          \
> -       MAKE_LWSYNC_SECTION_ENTRY(97, __lwsync_fixup);
> -#define PPC_ACQUIRE_BARRIER    "\n" stringify_in_c(__PPC_ACQUIRE_BARRIER)
> +#define PPC_ACQUIRE_BARRIER    "\n" stringify_in_c(sync;)

This can just be done as "\n\tsync\n" instead of the stringify stuff.


-Olof
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