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

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Message-ID: <20140905200926.GW5001@linux.vnet.ibm.com>
Date:	Fri, 5 Sep 2014 13:09:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Michael Cree <mcree@...on.net.nz>,
	"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
Subject: Re: bit fields && data tearing

On Fri, Sep 05, 2014 at 03:24:35PM -0400, Peter Hurley wrote:
> On 09/05/2014 03:05 PM, Paul E. McKenney wrote:
> > On Fri, Sep 05, 2014 at 02:50:31PM -0400, Peter Hurley wrote:
> >> On 09/05/2014 02:09 PM, Paul E. McKenney wrote:
> 
> [cut]
> 
> >>> ------------------------------------------------------------------------
> >>>
> >>> documentation: Record limitations of bitfields and small variables
> >>>
> >>> This commit documents the fact that it is not safe to use bitfields as
> >>> shared variables in synchronization algorithms.  It also documents that
> >>> CPUs must provide one-byte and two-byte load and store instructions
> >>                    ^
> >>                 atomic
> > 
> > Here you meant non-atomic?  My guess is that you are referring to the
> > fact that you could emulate a one-byte store on pre-EV56 Alpha CPUs
> > using the ll and sc atomic-read-modify-write instructions, correct?
> 
> Yes, that's what I meant. I must be tired and am misreading the commit
> message, or misinterpreting it's meaning.

Very good, got it!

							Thanx, Paul

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