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

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Message-ID: <540A0DF3.8030802@hurleysoftware.com>
Date:	Fri, 05 Sep 2014 15:24:35 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	paulmck@...ux.vnet.ibm.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 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.

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