linux-kernel - Re: [PATCH RFC] percpu: add data dependency barrier in percpu accessors and operations

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Date:	Tue, 17 Jun 2014 13:47:01 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH RFC] percpu: add data dependency barrier in percpu
 accessors and operations


On Tue, 17 Jun 2014, Linus Torvalds wrote:

> On Jun 17, 2014 6:28 AM, "Christoph Lameter" <cl@...two.org> wrote:
> >
> > Even alpha maintains the illusion of changes becoming visible in the
> > proper order for the currently executing thread. No barriers are needed.
>
> No Christoph, alpha really doesn't. Barriers needed.

Everythig breaks if a single hardware execution thread can no longer
observe a consistent view of the world of the data it modifies.

We are not talking about synchronization here between multiple hardware
threads. This is simply code running on a single hardware thread and all
architectures maintain the uniprocessor illusion there.


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