Date:   Mon, 27 Feb 2023 14:21:20 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc:     Andrea Parri <parri.andrea@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Jonas Oberhauser <jonas.oberhauser@huawei.com>,
        will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com,
        npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk,
        luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com,
        joel@joelfernandes.org, urezki@gmail.com, quic_neeraju@quicinc.com,
        frederic@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] tools/memory-model: Make ppo a subrelation of po
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On Mon, Feb 27, 2023 at 09:13:01PM +0100, Jonas Oberhauser wrote:
> 
> 
> On 2/27/2023 8:40 PM, Andrea Parri wrote:
> > > The LKMM doesn't believe that a control or data dependency orders a
> > > plain write after a marked read.  Hence in this test it thinks that P1's
> > > store to u0 can happen before the load of x1.  I don't remember why we
> > > did it this way -- probably we just wanted to minimize the restrictions
> > > on when plain accesses can execute.  (I do remember the reason for
> > > making address dependencies induce order; it was so RCU would work.)
> > > 
> > > The patch below will change what the LKMM believes.  It eliminates the
> > > positive outcome of the litmus test and the data race.  Should it be
> > > adopted into the memory model?
> > (Unpopular opinion I know,) it should drop dependencies ordering, not
> > add/promote it.
> > 
> >    Andrea
> 
> Maybe not as unpopular as you think... :)
> But either way IMHO it should be consistent; either take all the
> dependencies that are true and add them, or drop them all.
> In the latter case, RCU should change to an acquire barrier. (also, one
> would have to deal with OOTA in some yet different way).
> 
> Generally my position is that unless there's a real-world benchmark with
> proven performance benefits of relying on dependency ordering, one should
> use an acquire barrier. I haven't yet met such a case, but maybe one of you
> has...

https://www.msully.net/thesis/thesis.pdf page 128 (PDF page 141).

Though this is admittedly for ARMv7 and PowerPC.

							Thanx, Paul