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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Will Deacon <will.deacon@arm.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.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>,
	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
Date: Wed, 11 Jul 2018 11:03:21 -0700
Message-ID: <20180711180321.GY3593@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180711161717.GA14635@andrea>

On Wed, Jul 11, 2018 at 06:17:17PM +0200, Andrea Parri wrote:
> On Wed, Jul 11, 2018 at 08:42:11AM -0700, Paul E. McKenney 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>
> > 
> > I have applied your Reviewed-by, and thank you both!
> > 
> > Given that this is a non-trivial change and given that I am posting
> > for -tip acceptance in a few days, I intend to send this one not
> > to the upcoming merge window, but to the one after that.
> > 
> > Please let me know if there is an urgent need for this to go into the
> > v4.19 merge window.
> 
> I raised some concerns in my review to v2; AFAICT, these concerns have
> not been resolved: so, until then, please feel free to add my NAK. ;-)

I will be keeping the prototype in my -rcu tree for the time being,
but I will not be submitting it into the v4.19 merge window.  I expect
that you all will be able to come to agreement in the couple of months
until the v4.20/v5.0 merge window.  ;-)

I will apply Peter's ack and at the same time mark it EXP so that its
state will be apparent.

							Thanx, Paul


  reply index

Thread overview: 85+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-09 20:01 [PATCH v2] " Alan Stern
2018-07-09 21:45 ` Paul E. McKenney
2018-07-10 13:57   ` Alan Stern
2018-07-10 16:25     ` Paul E. McKenney
     [not found]       ` <Pine.LNX.4.44L0.1807101416390.1449-100000@iolanthe.rowland.org>
2018-07-10 19:58         ` [PATCH v3] " Paul E. McKenney
2018-07-10 20:24           ` Alan Stern
2018-07-10 20:31             ` Paul E. McKenney
2018-07-11  9:43         ` Will Deacon
2018-07-11 15:42           ` Paul E. McKenney
2018-07-11 16:17             ` Andrea Parri
2018-07-11 18:03               ` Paul E. McKenney [this message]
2018-07-11 16:34           ` Peter Zijlstra
2018-07-11 18:10             ` Paul E. McKenney
2018-07-10  9:38 ` [PATCH v2] " Andrea Parri
2018-07-10 14:48   ` Alan Stern
2018-07-10 15:24     ` Andrea Parri
2018-07-10 15:34       ` Alan Stern
2018-07-10 23:14         ` Andrea Parri
2018-07-11  9:43   ` Will Deacon
2018-07-11 12:34     ` Andrea Parri
2018-07-11 12:54       ` Andrea Parri
2018-07-11 15:57       ` Will Deacon
2018-07-11 16:28         ` Andrea Parri
2018-07-11 17:00         ` Peter Zijlstra
2018-07-11 17:50           ` Daniel Lustig
2018-07-12  8:34             ` Andrea Parri
2018-07-12  9:29             ` Peter Zijlstra
2018-07-12  7:40       ` Peter Zijlstra
2018-07-12  9:34         ` Peter Zijlstra
2018-07-12  9:45           ` Will Deacon
2018-07-13  2:17             ` Daniel Lustig
2018-07-12 11:52         ` Andrea Parri
2018-07-12 12:01           ` Andrea Parri
2018-07-12 12:11             ` Peter Zijlstra
2018-07-12 13:48           ` Peter Zijlstra
2018-07-12 16:19             ` Paul E. McKenney
2018-07-12 17:04             ` Alan Stern
2018-07-12 17:14               ` Will Deacon
2018-07-12 17:28               ` Paul E. McKenney
2018-07-12 18:05                 ` Peter Zijlstra
2018-07-12 18:10                   ` Linus Torvalds
2018-07-12 19:52                     ` Andrea Parri
2018-07-12 20:24                       ` Andrea Parri
2018-07-13  2:05                     ` Daniel Lustig
2018-07-13  4:03                       ` Paul E. McKenney
2018-07-13  9:07                       ` Andrea Parri
2018-07-13  9:35                         ` Will Deacon
2018-07-13 17:16                           ` Linus Torvalds
2018-07-13 19:06                             ` Andrea Parri
2018-07-14  1:51                               ` Alan Stern
2018-07-14  2:58                                 ` Linus Torvalds
2018-07-16  2:31                                   ` Paul E. McKenney
2018-07-13 11:08                     ` Peter Zijlstra
2018-07-13 13:15                       ` Michael Ellerman
2018-07-13 16:42                         ` Peter Zijlstra
2018-07-13 19:56                           ` Andrea Parri
2018-07-16 14:40                           ` Michael Ellerman
2018-07-16 19:01                             ` Peter Zijlstra
2018-07-16 19:30                             ` Linus Torvalds
2018-07-17 14:45                               ` Michael Ellerman
2018-07-17 16:19                                 ` Linus Torvalds
2018-07-17 18:33                                   ` Paul E. McKenney
2018-07-17 18:42                                     ` Peter Zijlstra
2018-07-17 19:40                                       ` Paul E. McKenney
2018-07-17 19:47                                       ` Alan Stern
2018-07-17 18:44                                     ` Linus Torvalds
2018-07-17 18:49                                       ` Linus Torvalds
2018-07-17 19:42                                         ` Paul E. McKenney
2018-07-17 19:37                                       ` Alan Stern
2018-07-17 20:13                                         ` Linus Torvalds
2018-07-17 19:38                                       ` Paul E. McKenney
2018-07-17 19:40                                     ` Andrea Parri
2018-07-17 19:52                                       ` Paul E. McKenney
2018-07-18 12:31                                   ` Michael Ellerman
2018-07-18 13:16                             ` Michael Ellerman
2018-07-12 17:52               ` Andrea Parri
2018-07-12 20:43                 ` Alan Stern
2018-07-12 21:13                   ` Andrea Parri
2018-07-12 21:23                     ` Andrea Parri
2018-07-12 18:33               ` Peter Zijlstra
2018-07-12 17:45             ` Andrea Parri
2018-07-10 16:56 ` Daniel Lustig
     [not found]   ` <Pine.LNX.4.44L0.1807101315140.1449-100000@iolanthe.rowland.org>
2018-07-10 23:31     ` Andrea Parri
2018-07-11 14:19       ` Alan Stern
     [not found] <3344e7aeb09644758860ac343bd757a1@AcuMS.aculab.com>
2018-07-11 17:36 ` [PATCH v3] " Alan Stern

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