From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()
Date: Mon, 13 Jul 2015 13:20:32 -0700 [thread overview]
Message-ID: <20150713202032.GZ3717@linux.vnet.ibm.com> (raw)
In-Reply-To: <20150713175029.GO2632@arm.com>
On Mon, Jul 13, 2015 at 06:50:29PM +0100, Will Deacon wrote:
> On Mon, Jul 13, 2015 at 04:54:47PM +0100, Peter Zijlstra wrote:
> > However I think we should look at the insides of the critical sections;
> > for example (from Documentation/memory-barriers.txt):
> >
> > " *A = a;
> > RELEASE M
> > ACQUIRE N
> > *B = b;
> >
> > could occur as:
> >
> > ACQUIRE N, STORE *B, STORE *A, RELEASE M"
> >
> > This could not in fact happen, even though we could flip M and N, A and
> > B will remain strongly ordered.
> >
> > That said, I don't think this could even happen on PPC because we have
> > load_acquire and store_release, this means that:
> >
> > *A = a
> > lwsync
> > store_release M
> > load_acquire N
> > lwsync
> > *B = b
> >
> > And since the store to M is wrapped inside two lwsync there must be
> > strong store order, and because the load from N is equally wrapped in
> > two lwsyncs there must also be strong load order.
> >
> > In fact, no store/load can cross from before the first lwsync to after
> > the latter and the other way around.
> >
> > So in that respect it does provide full load-store ordering. What it
> > does not provide is order for M and N, nor does it provide transitivity,
> > but looking at our documentation I'm not at all sure we guarantee that
> > in any case.
>
> So if I'm following along, smp_mb__after_unlock_lock *does* provide
> transitivity when used with UNLOCK + LOCK, which is stronger than your
> example here.
Yes, that is indeed the intent.
> I don't think we want to make the same guarantee for general RELEASE +
> ACQUIRE, because we'd end up forcing most architectures to implement the
> expensive macro for a case that currently has no users.
Agreed, smp_mb__after_unlock_lock() makes a limited guarantee.
> In which case, it boils down to the question of how expensive it would
> be to implement an SC UNLOCK operation on PowerPC and whether that justifies
> the existence of a complicated barrier macro that isn't used outside of
> RCU.
Given that it is either smp_mb() or nothing, I am not seeing the
"complicated" part...
Thanx, Paul
next prev parent reply other threads:[~2015-07-13 20:20 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-13 12:15 [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() Will Deacon
2015-07-13 13:09 ` Peter Hurley
2015-07-13 14:24 ` Will Deacon
2015-07-13 15:56 ` Peter Zijlstra
2015-07-13 13:11 ` Peter Zijlstra
2015-07-13 14:09 ` Will Deacon
2015-07-13 14:21 ` Will Deacon
2015-07-13 15:54 ` Peter Zijlstra
2015-07-13 17:50 ` Will Deacon
2015-07-13 20:20 ` Paul E. McKenney [this message]
2015-07-13 22:23 ` Peter Zijlstra
2015-07-13 23:04 ` Paul E. McKenney
2015-07-14 10:04 ` Will Deacon
2015-07-14 12:45 ` Paul E. McKenney
2015-07-14 12:51 ` Will Deacon
2015-07-14 14:00 ` Paul E. McKenney
2015-07-14 14:12 ` Will Deacon
2015-07-14 19:31 ` Paul E. McKenney
2015-07-15 1:38 ` Paul E. McKenney
2015-07-15 10:51 ` Will Deacon
2015-07-15 13:12 ` Paul E. McKenney
2015-07-24 11:31 ` Will Deacon
2015-07-24 15:30 ` Paul E. McKenney
2015-08-12 13:44 ` Will Deacon
2015-08-12 15:43 ` Paul E. McKenney
2015-08-12 17:59 ` Paul E. McKenney
2015-08-13 10:49 ` Will Deacon
2015-08-13 13:10 ` Paul E. McKenney
2015-08-17 4:06 ` Michael Ellerman
2015-08-17 6:15 ` Paul E. McKenney
2015-08-17 8:57 ` Will Deacon
2015-08-18 1:50 ` Michael Ellerman
2015-08-18 8:37 ` Will Deacon
2015-08-20 9:45 ` Michael Ellerman
2015-08-20 15:56 ` Will Deacon
2015-08-26 0:27 ` Paul E. McKenney
2015-08-26 4:06 ` Michael Ellerman
2015-07-13 18:23 ` Paul E. McKenney
2015-07-13 19:41 ` Peter Hurley
2015-07-13 20:16 ` Paul E. McKenney
2015-07-13 22:15 ` Peter Zijlstra
2015-07-13 22:43 ` Benjamin Herrenschmidt
2015-07-14 8:34 ` Peter Zijlstra
2015-07-13 22:53 ` Paul E. McKenney
2015-07-13 22:37 ` Benjamin Herrenschmidt
2015-07-13 22:31 ` Benjamin Herrenschmidt
2015-07-14 10:16 ` Will Deacon
2015-07-15 3:06 ` Michael Ellerman
2015-07-15 10:44 ` Will Deacon
2015-07-16 2:00 ` Michael Ellerman
2015-07-16 5:03 ` Benjamin Herrenschmidt
2015-07-16 5:14 ` Benjamin Herrenschmidt
2015-07-16 15:11 ` Paul E. McKenney
2015-07-16 22:54 ` Benjamin Herrenschmidt
2015-07-17 9:32 ` Will Deacon
2015-07-17 10:15 ` Peter Zijlstra
2015-07-17 12:40 ` Paul E. McKenney
2015-07-17 22:14 ` Benjamin Herrenschmidt
2015-07-20 13:39 ` Will Deacon
2015-07-20 13:48 ` Paul E. McKenney
2015-07-20 13:56 ` Will Deacon
2015-07-20 21:18 ` Benjamin Herrenschmidt
2015-07-22 16:49 ` Will Deacon
2015-09-01 2:57 ` Paul Mackerras
2015-07-15 14:18 ` Paul E. McKenney
2015-07-16 1:34 ` Michael Ellerman
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