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

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Message-ID: <20140905210543.GC5001@linux.vnet.ibm.com>
Date:	Fri, 5 Sep 2014 14:05:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Marc Gauthier <marc@...ence.com>,
	Michael Cree <mcree@...on.net.nz>,
	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 10:48:34PM +0200, Thomas Gleixner wrote:
> On Fri, 5 Sep 2014, Paul E. McKenney wrote:
> > On Fri, Sep 05, 2014 at 01:34:52PM -0700, H. Peter Anvin wrote:
> > > On 09/05/2014 01:14 PM, Peter Hurley wrote:
> > > > 
> > > > Here's how I read the two statements.
> > > > 
> > > > First, the commit message:
> > > > 
> > > > "It [this commit] documents that CPUs [supported by the Linux kernel]
> > > > _must provide_ atomic one-byte and two-byte naturally aligned loads and stores."
> > > > 
> > > > 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."
> > > > 
> > > 
> > > Does this apply in general or only to SMP configurations?  I guess
> > > non-SMP configurations would still have problems if interrupted in the
> > > wrong place...
> > 
> > And preemption could cause problems, too.  So I believe that it needs
> > to be universal.
> 
> Well preemption is usually caused by an interrupt, except you have a
> combined load and preempt instruction :)

Fair point!  ;-)

							Thanx, Paul

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