From: Alan Stern <stern@rowland.harvard.edu>
To: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Will Deacon <will.deacon@arm.com>,
Andrea Parri <parri.andrea@gmail.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
mingo@kernel.org, peterz@infradead.org, boqun.feng@gmail.com,
npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk,
luc.maranget@inria.fr, akiyks@gmail.com
Subject: Re: [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
Date: Wed, 5 Sep 2018 21:25:40 -0400 (EDT) [thread overview]
Message-ID: <Pine.LNX.4.44L0.1809052115490.2261-100000@netrider.rowland.org> (raw)
Message-ID: <20180906012540.bUTtn6xP2YsdGJB7gk-REUF72UQfHTjPqXYMaG7sS-Y@z> (raw)
In-Reply-To: <20180903090153.GA4560@andrea>
On Mon, 3 Sep 2018, Andrea Parri wrote:
> I take this opportunity to summarize my viewpoint on these matters:
>
> Someone would have to write the commit message for the above diff ...
> that is, to describe -why- we should go RCtso (and update the documen-
> tation accordingly); by now, the only argument for this appears to be:
> "(most) people expect strong ordering" _and they will be "lazy enough"
> to not check their expectations by using the LKMM tool (paraphrasing
> from [1]); IAC, Linux "might work" better if we add this ordering to
> the LKMM. Agreeing on such an approach would mean agreeing that this
> argument "wins" over:
>
> "We want new architectures to implement acquire/release efficiently,
> and it's not unlikely that they will have acquire loads that are
> similar in semantics to LDAPR." [2]
>
> "RISC-V probably would have been RCpc [...] it takes extra fences
> to go from RCpc to either "RCtso" or RCsc." [3]
>
> (or similar instances) since, of course, there is no such thing as a
> "free strong ordering"; and I'm not only talking about "efficiency",
> I'm also thinking at the fact that someone will have to maintain that
> ordering across all the architectures and in the LKMM.
>
> If, OTOH, we agree that the above "win"/assumption is valid only for
> locks or, in other/better words, if we agree that we should maintain
> _two_ distinct release-acquire orderings (a first one for unlock-lock
> sequences and a second one for ordinary/atomic release-acquire, say,
> as proposed in the patch under RFC),
In fact, there have have been _two_ proposals along this line. One as
you describe here (which is what the 1/7 patch under discussion does),
and another in which unlock-lock sequences and atomic acquire-release
sequences both have "RCtso" semantics while ordinary acquire/release
sequences have RCpc semantics. You should consider the second
proposal. It could be put into the LKMM quite easily by building upon
this 1/7 patch.
> I ask that we audit and modify
> the generic code accordingly/as suggested in other posts _before_ we
> upstream the changes for the LKMM: we should identify those places
> where (the newly introduced) _gap_ between unlock-lock and the other
> release-acquire is not admissible and fix those places (notice that
> this entails, in part., agreeing on what/where the generic code is).
Have you noticed any part of the generic code that relies on ordinary
acquire-release (rather than atomic RMW acquire-release) in order to
implement locking constructs?
> Finally, if we don't agree with the above assumption at all (that is,
> no matter if we are considering unlock-lock or other release-acquire
> sequences), then we should go RCpc [4].
>
> I described three different approaches (which are NOT "independent",
> clearly; let us find an agreement...); even though some of them look
> insane to me, I'm currently open to all of them: thoughts?
How about this fourth approach?
Alan
> Andrea
>
> [1] http://lkml.kernel.org/r/20180712134821.GT2494@hirez.programming.kicks-ass.net
> http://lkml.kernel.org/r/CA+55aFwKpkU5C23OYt1HCiD3X5bJHVh1jz5G2dSnF1+kVrOCTA@mail.gmail.com
> [2] http://lkml.kernel.org/r/20180622183007.GD1802@arm.com
> [3] http://lkml.kernel.org/r/11b27d32-4a8a-3f84-0f25-723095ef1076@nvidia.com
> [4] http://lkml.kernel.org/r/20180711123421.GA9673@andrea
> http://lkml.kernel.org/r/Pine.LNX.4.44L0.1807132133330.26947-100000@netrider.rowland.org
next prev parent reply other threads:[~2018-09-06 5:58 UTC|newest]
Thread overview: 112+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-29 21:10 [PATCH RFC memory-model 0/7] Memory-model changes Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-30 12:50 ` Andrea Parri
2018-08-30 12:50 ` Andrea Parri
2018-08-30 21:31 ` Alan Stern
2018-08-30 21:31 ` Alan Stern
2018-08-31 9:17 ` Andrea Parri
2018-08-31 9:17 ` Andrea Parri
2018-08-31 14:52 ` Alan Stern
2018-08-31 14:52 ` Alan Stern
2018-08-31 16:06 ` Will Deacon
2018-08-31 16:06 ` Will Deacon
2018-08-31 18:28 ` Andrea Parri
2018-08-31 18:28 ` Andrea Parri
2018-09-03 9:01 ` Andrea Parri
2018-09-03 9:01 ` Andrea Parri
2018-09-03 17:04 ` Will Deacon
2018-09-03 17:04 ` Will Deacon
2018-09-04 8:11 ` Andrea Parri
2018-09-04 8:11 ` Andrea Parri
2018-09-04 19:09 ` Alan Stern
2018-09-04 19:09 ` Alan Stern
2018-09-05 7:21 ` Andrea Parri
2018-09-05 7:21 ` Andrea Parri
2018-09-05 14:33 ` Akira Yokosawa
2018-09-05 14:33 ` Akira Yokosawa
2018-09-05 14:53 ` Paul E. McKenney
2018-09-05 14:53 ` Paul E. McKenney
2018-09-05 15:00 ` Andrea Parri
2018-09-05 15:00 ` Andrea Parri
2018-09-05 15:04 ` Akira Yokosawa
2018-09-05 15:04 ` Akira Yokosawa
2018-09-05 15:24 ` Andrea Parri
2018-09-05 15:24 ` Andrea Parri
2018-09-03 17:52 ` Alan Stern
2018-09-03 17:52 ` Alan Stern
2018-09-03 18:28 ` Andrea Parri
2018-09-03 18:28 ` Andrea Parri
2018-09-06 1:25 ` Alan Stern [this message]
2018-09-06 1:25 ` Alan Stern
2018-09-06 9:36 ` Andrea Parri
2018-09-06 9:36 ` Andrea Parri
2018-09-07 16:00 ` Alan Stern
2018-09-07 16:00 ` Alan Stern
2018-09-07 16:09 ` Will Deacon
2018-09-07 16:09 ` Will Deacon
2018-09-07 16:39 ` Daniel Lustig
2018-09-07 16:39 ` Daniel Lustig
2018-09-07 17:38 ` Alan Stern
2018-09-07 17:38 ` Alan Stern
2018-09-08 0:04 ` Daniel Lustig
2018-09-08 0:04 ` Daniel Lustig
2018-09-08 9:58 ` Andrea Parri
2018-09-08 9:58 ` Andrea Parri
2018-09-11 19:31 ` Alan Stern
2018-09-11 19:31 ` Alan Stern
2018-09-11 20:03 ` Paul E. McKenney
2018-09-11 20:03 ` Paul E. McKenney
2018-09-12 14:24 ` Alan Stern
2018-09-12 14:24 ` Alan Stern
2018-09-13 17:07 ` Alan Stern
2018-09-13 17:07 ` Alan Stern
2018-09-14 14:37 ` Andrea Parri
2018-09-14 14:37 ` Andrea Parri
2018-09-14 16:29 ` Alan Stern
2018-09-14 16:29 ` Alan Stern
2018-09-14 19:44 ` Andrea Parri
2018-09-14 19:44 ` Andrea Parri
2018-09-14 21:08 ` [PATCH v5] " Alan Stern
2018-09-14 21:08 ` Alan Stern
2018-09-15 3:56 ` Paul E. McKenney
2018-09-15 3:56 ` Paul E. McKenney
2018-09-03 17:05 ` [PATCH RFC LKMM 1/7] " Will Deacon
2018-09-03 17:05 ` Will Deacon
2018-08-31 17:55 ` Andrea Parri
2018-08-31 17:55 ` Andrea Parri
2018-08-29 21:10 ` [PATCH RFC LKMM 2/7] doc: Replace smp_cond_acquire() with smp_cond_load_acquire() Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-09-14 16:59 ` Will Deacon
2018-09-14 16:59 ` Will Deacon
2018-09-14 18:20 ` Paul E. McKenney
2018-09-14 18:20 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 3/7] EXP tools/memory-model: Add more LKMM limitations Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-30 9:17 ` Andrea Parri
2018-08-30 9:17 ` Andrea Parri
2018-08-30 22:18 ` Paul E. McKenney
2018-08-30 22:18 ` Paul E. McKenney
2018-08-31 9:43 ` Andrea Parri
2018-08-31 9:43 ` Andrea Parri
2018-09-06 18:34 ` Paul E. McKenney
2018-09-06 18:34 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 4/7] tools/memory-model: Fix a README typo Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 5/7] EXP tools/memory-model: Add scripts to check github litmus tests Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 6/7] EXP tools/memory-model: Make scripts take "-j" abbreviation for "--jobs" Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 7/7] EXP tools/memory-model: Add .cfg and .cat files for s390 Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-31 16:06 ` Will Deacon
2018-08-31 16:06 ` Will Deacon
2018-09-01 17:08 ` Paul E. McKenney
2018-09-01 17:08 ` Paul E. McKenney
2018-09-14 16:36 ` [PATCH RFC memory-model 0/7] Memory-model changes Paul E. McKenney
2018-09-14 16:36 ` Paul E. McKenney
2018-09-14 17:19 ` Alan Stern
2018-09-14 17:19 ` Alan Stern
2018-09-14 18:29 ` Paul E. McKenney
2018-09-14 18:29 ` Paul E. McKenney
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