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

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Message-ID: <1436827050.3948.236.camel@kernel.crashing.org>
Date:	Tue, 14 Jul 2015 08:37:30 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Mon, 2015-07-13 at 17:54 +0200, Peter Zijlstra wrote:

> 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.

The problem is if you have a load after the second lwsync, that load can
go up pass the store release. This has caused issues when N or M is what
you are trying to order against. That's why we had to add a sync to
spin_is_locked or similar.

Ben.

> 
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