linux-kernel - Re: [PATCH] barriers: introduce smp_mb__release

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Message-ID: <20150916104314.GA3604@twins.programming.kicks-ass.net>
Date:	Wed, 16 Sep 2015 12:43:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] barriers: introduce smp_mb__release_acquire and update
 documentation

On Wed, Sep 16, 2015 at 11:29:08AM +0100, Will Deacon wrote:
> > Indeed, that is a hole in the definition, that I think we should close.

> I'm struggling to understand the hole, but here's my intuition. If an
> ACQUIRE on CPUx reads from a RELEASE by CPUy, then I'd expect CPUx to
> observe all memory accessed performed by CPUy prior to the RELEASE
> before it observes the RELEASE itself, regardless of this new barrier.
> I think this matches what we currently have in memory-barriers.txt (i.e.
> acquire/release are neither transitive or multi-copy atomic).

Ah agreed. I seem to have gotten my brain in a tangle.

Basically where a program order release+acquire relies on an address
dependency, a cross cpu release+acquire relies on causality. If we
observe the release, we must also observe everything prior to it etc.
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