From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>,
Alan Stern <stern@rowland.harvard.edu>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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>,
Kernel development list <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
Date: Thu, 12 Jul 2018 19:45:27 +0200 [thread overview]
Message-ID: <20180712174527.GA3533@andrea> (raw)
In-Reply-To: <20180712134821.GT2494@hirez.programming.kicks-ass.net>
> Anyway, back to the problem of being able to use the memory model to
> describe locks. This is I think a useful property.
>
> My earlier reasoning was that:
>
> - smp_store_release() + smp_load_acquire() := RCpc
>
> - we use smp_store_release() as unlock()
>
> Therefore, if we want unlock+lock to imply at least TSO (ideally
> smp_mb()) we need lock to make up for whatever unlock lacks.
>
> Hence my proposal to strenghten rmw-acquire, because that is the basic
> primitive used to implement lock.
>
> But as you (and Will) point out, we don't so much care about rmw-acquire
> semantics as much as that we care about unlock+lock behaviour. Another
> way to look at this is to define:
>
> smp-store-release + rmw-acquire := TSO (ideally smp_mb)
>
> But then we also have to look at:
>
> rmw-release + smp-load-acquire
> rmw-release + rmw-acquire
>
> for completeness sake, and I would suggest they result in (at least) the
> same (TSO) ordering as the one we really care about.
Indeed (unless I'm not seeing something... ;-).
>
> One alternative is to no longer use smp_store_release() for unlock(),
> and say define atomic_set_release() to be in the rmw-release class
> instead of being a simple smp_store_release().
>
> Another, and I like this proposal least, is to introduce a new barrier
> to make this all work.
An smp_tso__after_unlock_lock()? (In a certain sense, the solution
adopted by RCU aligns to this approach: live with powerpc's RCpc and
introduce smp_mb__after_unlock_lock().) Or did you have something
else in mind?
But I wouldn't hasten to introduce such a barrier, given that: (1)
this would be a "do { } while (0)" for all the supported arch. _if_
we sticked to the current implementations, and (2) even if these
implementations changed or some new arch. required a non-trivial
definition, we still would have to find a "pure/TSO" case ;-).
Andrea
next prev parent reply other threads:[~2018-07-12 17:45 UTC|newest]
Thread overview: 84+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-09 20:01 [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire 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 [this message]
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
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