linux-kernel - Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks

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Date:   Thu, 5 Jul 2018 16:15:53 +0100
From:   Will Deacon <will.deacon@....com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        dlustig@...dia.com
Subject: Re: [PATCH 2/2] tools/memory-model: Add write ordering by
 release-acquire and by locks

On Thu, Jul 05, 2018 at 10:57:28AM -0400, Alan Stern wrote:
> On Thu, 5 Jul 2018, Will Deacon wrote:
> > On Thu, Jul 05, 2018 at 10:21:36AM -0400, Alan Stern wrote:
> > > On Wed, 4 Jul 2018, Will Deacon wrote:
> > > > On Tue, Jul 03, 2018 at 01:28:17PM -0400, Alan Stern wrote:
> > > > > Would this be allowed if smp_load_acquire() was implemented with LDAPR?
> > > > > If the answer is yes then we will have to remove the rfi-rel-acq and
> > > > > rel-rf-acq-po relations from the memory model entirely.
> > > > 
> > > > I don't understand what you mean by "rfi-rel-acq-po", and I assume you mean
> > > > rel-rfi-acq-po for the other? Sounds like I'm confused here.
> > > 
> > > "rfi-rel-acq" is the relation which was removed by the first of my two
> > > patches (it is now back in business since Paul reverted the commits),
> > > and "rel-rf-acq-po" is the relation that was introduced to replace it.
> > 
> > Sorry, yes, I realised this after I'd replied. Curious: but why do you name
> > the relations this way around, as opposed to e.g. rel-rfi-acq? It's
> > obviously up to you, but I just couldn't figure out what inspired the
> > ordering.
> 
> I no longer remember the reason for naming "rfi-rel-acq" the way I did.  
> As you say, it doesn't make a lot of sense.

Fair enough!

> The reason for "rel-rf-acq-po" instead of "rel-rfi-acq-po" was because 
> the second of the two patches uses that relation in a context where the 
> release and the acquire might very well run on different CPUs.

Ok, that makes sense. I realised that I've only been thinking about RCpc
making a difference in the rfi case, because Armv8 is multi-copy atomic
so we don't allow early forwarding from a release to an RCpc acquire
if they are from different CPUs.

Again, I'd be interested in what other architectures have to say here (i.e.
whether RCpc acquire/release instructions are likely to exist in a non
multi-copy atomic architecture).

Will