From mboxrd@z Thu Jan  1 00:00:00 1970
From: paulmck@linux.vnet.ibm.com (Paul E. McKenney)
Date: Wed, 21 Sep 2011 16:43:25 -0700
Subject: [ltt-dev] [URCU PATCH] cmm: provide lightweight rmb/wmb on PPC
In-Reply-To: <4E799028.1010707@redhat.com>
References: <1316502769-576-1-git-send-email-pbonzini@redhat.com>
	<20110920163125.GA31802@Krystal> <4E78C1C5.2080001@redhat.com>
	<20110920165115.GA2955@Krystal> <4E799028.1010707@redhat.com>
Message-ID: <20110921234325.GH2394@linux.vnet.ibm.com>

On Wed, Sep 21, 2011 at 09:20:08AM +0200, Paolo Bonzini wrote:
> On 09/20/2011 06:51 PM, Mathieu Desnoyers wrote:
> >>>  I think you're right. "eieio;lwsync" is good for rmb, lwsync is good for
> >>>  wmb/smp_rmb/smp_wmb.
> >I'm not convinced that the "eieio; lwsync" combo would provide the
> >ordering we're looking for for cmm_rmb(). AFAIK, eieio orders,
> >separately, a) cacheable stores and b) loads and stores to non-cacheable
> >memory. AFAIK, lwsync orders cacheable memory ops, but not loads with
> >respect to previous stores. So basically, this combo lacks ordering of
> >non-cacheable memory accesses with respect to cachable memory accesses.
> 
> Yeah, better safe than sorry.
> 
> >Why would lwsync be good for cmm_wmb ? Does it order non-cacheable
> >writes ?
> 
> The manuals say non-cacheable writes are always ordered.

But at best only against other non-cacheable writes.  Again, they can
slip out of lock-based critical sections.

							Thanx, Paul