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

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Message-ID: <20150714083415.GH19282@twins.programming.kicks-ass.net>
Date:	Tue, 14 Jul 2015 10:34:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Will Deacon <will.deacon@....com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Tue, Jul 14, 2015 at 08:43:44AM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2015-07-14 at 00:15 +0200, Peter Zijlstra wrote:
> 
> > > This is instead the sequence that is of concern:
> > > 
> > > 	store a
> > > 	unlock M
> > > 	lock N
> > > 	load b
> > 
> > So its late and that table didn't parse, but that should be ordered too.
> > The load of b should not be able to escape the lock N.
> > 
> > If only because LWSYNC is a valid RMB and any LOCK implementation must
> > load the lock state to observe it unlocked.
> 
> What happens is that the load passes the store conditional, though it
> doesn't pass the load with reserve. However, both store A and unlock M
> being just stores with an lwsync, can pass a load, so they can pass the
> load with reserve. And thus inside the LL/SC loop, our store A has
> passed our load B.

Ah cute.. Thanks, clearly I wasn't awake enough anymore :-)
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