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

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

On Fri, Sep 09, 2011 at 10:23:33AM -0700, Olof Johansson wrote:
> [+linuxppc-dev]
> 
> On Tue, Sep 6, 2011 at 11:00 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> 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.

As I understand it, although isync waits for the prior stwcx to execute,
it does not guarantee that the corresponding store is visible to all
processors before any following loads.

> > 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.

That does sound a bit more straightforward, now that you mention it.  ;-)

							Thanx, Paul