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From: Daniel Lustig <dlustig@nvidia.com>
To: Alan Stern <stern@rowland.harvard.edu>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: 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>,
	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
Date: Tue, 10 Jul 2018 09:56:14 -0700
Message-ID: <e8e62634-0b5b-faa9-ac43-557a59b16100@nvidia.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1807091519180.2462-100000@iolanthe.rowland.org>

On 7/9/2018 1:01 PM, 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>
> 
> ---
> 
> v.2: Restrict the ordering to lock operations, not general release
> and acquire fences.
> 
> [as1871b]
> 
> 
>  tools/memory-model/Documentation/explanation.txt                           |  186 +++++++---
>  tools/memory-model/linux-kernel.cat                                        |    8 
>  tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus |    5 
>  3 files changed, 149 insertions(+), 50 deletions(-)
> 
> Index: usb-4.x/tools/memory-model/linux-kernel.cat
> ===================================================================
> --- usb-4.x.orig/tools/memory-model/linux-kernel.cat
> +++ usb-4.x/tools/memory-model/linux-kernel.cat
> @@ -38,7 +38,7 @@ let strong-fence = mb | gp
>  (* Release Acquire *)
>  let acq-po = [Acquire] ; po ; [M]
>  let po-rel = [M] ; po ; [Release]
> -let rfi-rel-acq = [Release] ; rfi ; [Acquire]
> +let unlock-rf-lock-po = [UL] ; rf ; [LKR] ; po

It feels slightly weird that unlock-rf-lock-po is asymmetrical.  And in
fact, I think the current RISC-V solution we've been discussing (namely,
putting a fence.tso instead of a fence rw,w in front of the release)
may not even technically respect that particular sequence.  The
fence.tso solution really enforces "po; [UL]; rf; [LKR]", right?

Does something like "po; [UL]; rf; [LKR]; po" fit in with the rest
of the model?  If so, maybe that solves the asymmetry and also
legalizes the approach of putting fence.tso in front?

Or, other suggestions?

Dan

>  (**********************************)
>  (* Fundamental coherence ordering *)
> @@ -60,13 +60,13 @@ let dep = addr | data
>  let rwdep = (dep | ctrl) ; [W]
>  let overwrite = co | fr
>  let to-w = rwdep | (overwrite & int)
> -let to-r = addr | (dep ; rfi) | rfi-rel-acq
> +let to-r = addr | (dep ; rfi)
>  let fence = strong-fence | wmb | po-rel | rmb | acq-po
> -let ppo = to-r | to-w | fence
> +let ppo = to-r | to-w | fence | (unlock-rf-lock-po & int)
>  
>  (* Propagation: Ordering from release operations and strong fences. *)
>  let A-cumul(r) = rfe? ; r
> -let cumul-fence = A-cumul(strong-fence | po-rel) | wmb
> +let cumul-fence = A-cumul(strong-fence | po-rel) | wmb | unlock-rf-lock-po
>  let prop = (overwrite & ext)? ; cumul-fence* ; rfe?

  parent reply index

Thread overview: 84+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-09 20:01 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
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 [this message]
     [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

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