[PATCH tip/core/rcu 48/55] powerpc: strengthen value-returning-atomics memory barriers
Paul E. McKenney
paulmck at linux.vnet.ibm.com
Sat Sep 10 03:34:55 EST 2011
On Fri, Sep 09, 2011 at 10:23:33AM -0700, Olof Johansson wrote:
> On Tue, Sep 6, 2011 at 11:00 AM, Paul E. McKenney
> <paulmck at 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. ;-)
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