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Date:   Wed, 11 Jul 2018 11:10:41 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Will Deacon <will.deacon@arm.com>,
        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>,
        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
Reply-To: paulmck@linux.vnet.ibm.com
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On Wed, Jul 11, 2018 at 06:34:56PM +0200, Peter Zijlstra wrote:
> 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>

And I applies you ask as well, thank you!

							Thanx, Paul