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Date: Wed, 21 May 2008 17:47:02 +0200
From: Nick Piggin <npiggin@suse.de>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [patch 2/2] powerpc: optimise smp_wmb
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On Wed, May 21, 2008 at 11:43:00AM -0400, Benjamin Herrenschmidt wrote:
> 
> On Wed, 2008-05-21 at 17:34 +0200, Nick Piggin wrote:
> > On Wed, May 21, 2008 at 11:26:32AM -0400, Benjamin Herrenschmidt wrote:
> > > 
> > > 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...
> > 
> > Not according to the __stringify(LWSYNC) that I just removed. At least,
> > presumably it is always present on 64 bit processors, and 32 bit ones
> > will be no worse off as they'll continue just using eieio.
> 
> No, it doesn't exist on power3, but it degrades into a sync

OK, but I just don't understand what the problem is... your synch.h has

#ifdef __powerpc64__
#define __SUBARCH_HAS_LWSYNC
#endif

#ifdef __SUBARCH_HAS_LWSYNC
#    define LWSYNC      lwsync
#else
#    define LWSYNC      sync
#endif


And LWSYNC is then used for rmb()... how was that OK but this not?