From: Peter Zijlstra <peterz@infradead.org>
To: stern@rowland.harvard.edu, alexander.shishkin@linux.intel.com,
hpa@zytor.com, parri.andrea@gmail.com, mingo@kernel.org,
paulmck@kernel.org, vincent.weaver@maine.edu, tglx@linutronix.de,
jolsa@redhat.com, acme@redhat.com, torvalds@linux-foundation.org,
linux-kernel@vger.kernel.org, eranian@google.com,
will@kernel.org
Cc: linux-tip-commits@vger.kernel.org
Subject: Re: [tip:locking/core] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
Date: Wed, 8 Sep 2021 13:44:11 +0200 [thread overview]
Message-ID: <YTiiC1mxzHyUJ47F@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YTiXyiA92dM9726M@hirez.programming.kicks-ass.net>
On Wed, Sep 08, 2021 at 01:00:26PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 02, 2018 at 03:11:10AM -0700, tip-bot for Alan Stern wrote:
> > Commit-ID: 6e89e831a90172bc3d34ecbba52af5b9c4a447d1
> > Gitweb: https://git.kernel.org/tip/6e89e831a90172bc3d34ecbba52af5b9c4a447d1
> > Author: Alan Stern <stern@rowland.harvard.edu>
> > AuthorDate: Wed, 26 Sep 2018 11:29:17 -0700
> > Committer: Ingo Molnar <mingo@kernel.org>
> > CommitDate: Tue, 2 Oct 2018 10:28:01 +0200
> >
> > tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
> >
> > 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.
>
> Let me revive this old thread... recently we ran into the case:
>
> WRITE_ONCE(x, 1);
> spin_unlock(&s):
> spin_lock(&r);
> WRITE_ONCE(y, 1);
>
> which is distinct from the original in that UNLOCK and LOCK are not on
> the same variable.
>
> I'm arguing this should still be RCtso by reason of:
>
> spin_lock() requires an atomic-acquire which:
>
> TSO-arch) implies smp_mb()
> ARM64) is RCsc for any stlr/ldar
> Power) LWSYNC
> Risc-V) fence r , rw
> *) explicitly has smp_mb()
>
>
> However, Boqun points out that the memory model disagrees, per:
>
> https://lkml.kernel.org/r/YTI2UjKy+C7LeIf+@boqun-archlinux
>
> Is this an error/oversight of the memory model, or did I miss a subtlety
> somewhere?
Hmm.. that argument isn't strong enough for Risc-V if I read that FENCE
thing right. That's just R->RW ordering, which doesn't constrain the
first WRITE_ONCE().
However, that spin_unlock has "fence rw, w" with a subsequent write. So
the whole thing then becomes something like:
WRITE_ONCE(x, 1);
FENCE RW, W
WRITE_ONCE(s.lock, 0);
AMOSWAP %0, 1, r.lock
FENCE R, WR
WRITE_ONCE(y, 1);
Which I think is still sufficient, irrespective of the whole s!=r thing.
next prev parent reply other threads:[~2021-09-08 11:44 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-26 18:28 [PATCH memory-model 0/5] Updates to the formal memory model Paul E. McKenney
2018-09-26 18:29 ` [PATCH memory-model 1/5] tools/memory-model: Add litmus-test naming scheme Paul E. McKenney
2018-10-02 10:10 ` [tip:locking/core] " tip-bot for Paul E. McKenney
2018-09-26 18:29 ` [PATCH memory-model 2/5] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Paul E. McKenney
2018-10-02 10:11 ` [tip:locking/core] " tip-bot for Alan Stern
2021-09-08 11:00 ` Peter Zijlstra
2021-09-08 11:44 ` Peter Zijlstra [this message]
2021-09-08 14:42 ` Alan Stern
2021-09-08 15:12 ` Peter Zijlstra
2021-09-08 16:08 ` Linus Torvalds
2021-09-09 7:25 ` Peter Zijlstra
2021-09-09 13:35 ` Will Deacon
2021-09-09 17:02 ` Linus Torvalds
2021-09-09 18:59 ` Alan Stern
2021-09-09 17:03 ` Dan Lustig
2021-09-09 18:00 ` Paul E. McKenney
2021-09-10 14:20 ` Boqun Feng
2021-09-10 15:33 ` Palmer Dabbelt
2021-09-10 16:36 ` Alan Stern
2021-09-10 17:12 ` Peter Zijlstra
2021-09-10 17:56 ` Alan Stern
2021-09-10 17:17 ` Peter Zijlstra
2021-09-12 0:26 ` Boqun Feng
2021-09-10 0:01 ` Boqun Feng
2021-09-10 5:37 ` Boqun Feng
2021-09-10 9:33 ` Peter Zijlstra
2021-09-10 10:04 ` Boqun Feng
2021-09-10 13:48 ` Dan Lustig
2021-09-10 14:15 ` Boqun Feng
2021-09-09 17:46 ` Paul E. McKenney
2021-09-10 11:08 ` Will Deacon
2021-09-17 3:21 ` Nicholas Piggin
2021-09-17 5:31 ` Nicholas Piggin
2021-09-17 14:36 ` Michael Ellerman
2018-09-26 18:29 ` [PATCH memory-model 3/5] tools/memory-model: Fix a README typo Paul E. McKenney
2018-10-02 10:11 ` [tip:locking/core] " tip-bot for SeongJae Park
2018-09-26 18:29 ` [PATCH memory-model 4/5] tools/memory-model: Add more LKMM limitations Paul E. McKenney
2018-10-02 10:12 ` [tip:locking/core] " tip-bot for Paul E. McKenney
2018-09-26 18:29 ` [PATCH memory-model 5/5] doc: Replace smp_cond_acquire() with smp_cond_load_acquire() Paul E. McKenney
2018-10-02 10:12 ` [tip:locking/core] locking/memory-barriers: " tip-bot for Andrea Parri
2018-10-02 8:28 ` [PATCH memory-model 0/5] Updates to the formal memory model Ingo Molnar
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