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Date:   Thu, 21 Jun 2018 20:34:40 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        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>,
        Will Deacon <will.deacon@arm.com>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] tools/memory-model: Add write ordering by
 release-acquire and by locks
Reply-To: paulmck@linux.vnet.ibm.com
References: <Pine.LNX.4.44L0.1806211322160.2381-100000@iolanthe.rowland.org>
 <20180621180422.GF3452@worktop.programming.kicks-ass.net>
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On Thu, Jun 21, 2018 at 08:04:22PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 21, 2018 at 01:27:12PM -0400, Alan Stern wrote:
> > More than one kernel developer has expressed the opinion that the LKMM
> > should enforce ordering of writes by release-acquire chains and by
> > locking.  In other words, given the following code:
> > 
> > 	WRITE_ONCE(x, 1);
> > 	spin_unlock(&s):
> > 	spin_lock(&s);
> > 	WRITE_ONCE(y, 1);
> > 
> > or the following:
> > 
> > 	smp_store_release(&x, 1);
> > 	r1 = smp_load_acquire(&x);	// r1 = 1
> > 	WRITE_ONCE(y, 1);
> > 
> > the stores to x and y should be propagated in order to all other CPUs,
> > even though those other CPUs might not access the lock s or be part of
> > the release-acquire chain.  In terms of the memory model, this means
> > that rel-rf-acq-po should be part of the cumul-fence relation.
> > 
> > All the architectures supported by the Linux kernel (including RISC-V)
> > do behave this way, albeit for varying reasons.  Therefore this patch
> > changes the model in accordance with the developers' wishes.
> > 
> > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> 
> Thanks for that Alan!
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

I queued both, with Peter's ack on the second, thank you both!

(Peter, if you meant to ack both, please let me know.)

							Thanx, Paul