linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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Date:	Mon, 13 Jul 2015 17:54:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Mon, Jul 13, 2015 at 03:21:10PM +0100, Will Deacon wrote:
> On Mon, Jul 13, 2015 at 03:09:15PM +0100, Will Deacon wrote:
> > On Mon, Jul 13, 2015 at 02:11:43PM +0100, Peter Zijlstra wrote:
> > > On Mon, Jul 13, 2015 at 01:15:04PM +0100, Will Deacon wrote:
> > > > smp_mb__after_unlock_lock is used to promote an UNLOCK + LOCK sequence
> > > > into a full memory barrier.
> > > > 
> > > > However:
> > > 
> > > >   - The barrier only applies to UNLOCK + LOCK, not general
> > > >     RELEASE + ACQUIRE operations
> > > 
> > > No it does too; note that on ppc both acquire and release use lwsync and
> > > two lwsyncs do not make a sync.
> > 
> > Really? IIUC, that means smp_mb__after_unlock_lock needs to be a full
> > barrier on all architectures implementing smp_store_release as smp_mb() +
> > STORE, otherwise the following isn't ordered:
> > 
> >   RELEASE X
> >   smp_mb__after_unlock_lock()
> >   ACQUIRE Y
> > 
> > On 32-bit ARM (at least), the ACQUIRE can be observed before the RELEASE.
> 
> I knew we'd had this conversation before ;)
> 
>   http://lkml.kernel.org/r/20150120093443.GA11596@twins.programming.kicks-ass.net

Ha! yes. And I had indeed forgotten about this argument.

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.

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