linux-kernel - Re: bit fields && data tearing

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Message-ID: <20140905212727.GA6169@omega>
Date:	Sat, 6 Sep 2014 09:27:27 +1200
From:	Michael Cree <mcree@...on.net.nz>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Marc Gauthier <marc@...ence.com>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Laight <David.Laight@...LAB.COM>,
	Jakub Jelinek <jakub@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Miroslav Franc <mfranc@...hat.com>,
	Richard Henderson <rth@...ddle.net>,
	"linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>
Subject: Re: bit fields && data tearing

On Fri, Sep 05, 2014 at 05:12:28PM -0400, Peter Hurley wrote:
> On 09/05/2014 04:39 PM, Michael Cree wrote:
> > On Fri, Sep 05, 2014 at 04:14:48PM -0400, Peter Hurley wrote:
> >> Second, in the body of the document:
> >>
> >> "The Linux kernel no longer supports pre-EV56 Alpha CPUs, because these
> >> older CPUs _do not provide_ atomic one-byte and two-byte loads and stores."
> > 
> > Let's be clear here, the pre-EV56 Alpha CPUs do provide an atomic
> > one-byte and two-byte load and store; it's just that one must use
> > locked load and store sequences to achieve atomicity.  The point,
> > I think, is that the pre-EV56 Alpha CPUs provide non-atomic one-byte
> > and two-byte load and stores as the norm, and that is the problem.
> 
> I'm all for an Alpha expert to jump in here and meet the criteria;
> which is that byte stores cannot corrupt adjacent storage (nor can
> aligned short stores).
> 
> To my mind, a quick look at Documentation/circular-buffers.txt will
> pretty much convince anyone that trying to differentiate by execution
> context is undoable.
> 
> If someone wants to make Alphas do cmpxchg loops for every byte store,
> then ok. Or any other solution that doesn't require subsystem code
> changes.

I am not suggesting that anyone do that work.  I'm certainly not going
to do it.

All I was pointing out is that the claim that "_do not provide_" made
above with emphasis is, strictly interpreted, not true, thus should not
be committed to the documentation without further clarification.

Cheers
Michael.
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