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Date:   Wed, 11 Jul 2018 18:34:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Will Deacon <will.deacon@arm.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        "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>,
        Daniel Lustig <dlustig@nvidia.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nicholas Piggin <npiggin@gmail.com>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3] tools/memory-model: Add extra ordering for locks and
 remove it for ordinary release/acquire
Message-ID: <20180711163456.GL2476@hirez.programming.kicks-ass.net>
References: <20180710162555.GV3593@linux.vnet.ibm.com>
 <Pine.LNX.4.44L0.1807101416390.1449-100000@iolanthe.rowland.org>
 <20180711094344.GE13963@arm.com>
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On Wed, Jul 11, 2018 at 10:43:45AM +0100, Will Deacon wrote:
> Hi Alan,
> 
> On Tue, Jul 10, 2018 at 02:18:13PM -0400, Alan Stern wrote:
> > More than one kernel developer has expressed the opinion that the LKMM
> > should enforce ordering of writes by locking.  In other words, given
> > the following code:
> > 
> > 	WRITE_ONCE(x, 1);
> > 	spin_unlock(&s):
> > 	spin_lock(&s);
> > 	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.  In terms of
> > the memory model, this means expanding the cumul-fence relation.
> > 
> > Locks should also provide read-read (and read-write) ordering in a
> > similar way.  Given:
> > 
> > 	READ_ONCE(x);
> > 	spin_unlock(&s);
> > 	spin_lock(&s);
> > 	READ_ONCE(y);		// or WRITE_ONCE(y, 1);
> > 
> > the load of x should be executed before the load of (or store to) y.
> > The LKMM already provides this ordering, but it provides it even in
> > the case where the two accesses are separated by a release/acquire
> > pair of fences rather than unlock/lock.  This would prevent
> > architectures from using weakly ordered implementations of release and
> > acquire, which seems like an unnecessary restriction.  The patch
> > therefore removes the ordering requirement from the LKMM for that
> > case.
> > 
> > All the architectures supported by the Linux kernel (including RISC-V)
> > do provide this ordering for locks, 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, I'm happy with this version of the patch:
> 
> Reviewed-by: Will Deacon <will.deacon@arm.com>

Me too! Thanks Alan.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>