Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 10 Jul 2018 09:25:55 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     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>,
        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
Reply-To: paulmck@linux.vnet.ibm.com
References: <20180709214514.GJ3593@linux.vnet.ibm.com>
 <Pine.LNX.4.44L0.1807100954490.1449-100000@iolanthe.rowland.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Pine.LNX.4.44L0.1807100954490.1449-100000@iolanthe.rowland.org>
User-Agent: Mutt/1.5.21 (2010-09-15)
Message-Id: <20180710162555.GV3593@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Jul 10, 2018 at 09:57:17AM -0400, Alan Stern wrote:
> On Mon, 9 Jul 2018, Paul E. McKenney wrote:
> 
> > On Mon, Jul 09, 2018 at 04:01:57PM -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>
> > 
> > Nice!
> > 
> > However, it doesn't apply against current -rcu.  Am I missing a patch?
> > Or is this supposed to apply against origin/lkmm?
> 
> I wrote it based on 4.18-rc.  However, I can rebase it against your
> current dev branch.

Could you please?  Against either the dev or lkmm branch should well.

If you don't have time for this, my approach would be to apply against
4.18-rc, then cherry-pick onto my branch, resolving the conflicts and
emailing you both the "<<<<"-marked file and my proposed resolution.
(Or git might just resolve everything automatically -- that does
sometimes happen.  But it would still be good to double-check its work,
as it sometimes does "interesting" resolutions.)

							Thanx, Paul