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Date:   Tue, 10 Jul 2018 13:17:50 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Daniel Lustig <dlustig@nvidia.com>
cc:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Andrea Parri <andrea.parri@amarulasolutions.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nicholas Piggin <npiggin@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and
 remove it for ordinary release/acquire
In-Reply-To: <e8e62634-0b5b-faa9-ac43-557a59b16100@nvidia.com>
Message-ID: <Pine.LNX.4.44L0.1807101315140.1449-100000@iolanthe.rowland.org>
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On Tue, 10 Jul 2018, Daniel Lustig wrote:

> > --- usb-4.x.orig/tools/memory-model/linux-kernel.cat
> > +++ usb-4.x/tools/memory-model/linux-kernel.cat
> > @@ -38,7 +38,7 @@ let strong-fence = mb | gp
> >  (* Release Acquire *)
> >  let acq-po = [Acquire] ; po ; [M]
> >  let po-rel = [M] ; po ; [Release]
> > -let rfi-rel-acq = [Release] ; rfi ; [Acquire]
> > +let unlock-rf-lock-po = [UL] ; rf ; [LKR] ; po
> 
> It feels slightly weird that unlock-rf-lock-po is asymmetrical.  And in
> fact, I think the current RISC-V solution we've been discussing (namely,
> putting a fence.tso instead of a fence rw,w in front of the release)
> may not even technically respect that particular sequence.  The
> fence.tso solution really enforces "po; [UL]; rf; [LKR]", right?
> 
> Does something like "po; [UL]; rf; [LKR]; po" fit in with the rest
> of the model?  If so, maybe that solves the asymmetry and also
> legalizes the approach of putting fence.tso in front?

That would work just as well.  For this version of the patch it 
doesn't make any difference, because nothing that comes po-after the 
LKR is able to directly read the value stored by the UL.

Alan