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

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Message-ID: <540A19B8.4010907@hurleysoftware.com>
Date:	Fri, 05 Sep 2014 16:14:48 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Marc Gauthier <marc@...ence.com>,
	"paulmck@...ux.vnet.ibm.com" <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" <linux-alpha@...r.kernel.org>
Subject: Re: bit fields && data tearing

On 09/05/2014 03:38 PM, Marc Gauthier wrote:
> 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:
>>>> 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?
>>
>>>> in order to be supported by the Linux kernel.  (Michael Cree
>>>> has agreed to the resulting non-support of pre-EV56 Alpha CPUs:
>>>> https://lkml.org/lkml/2014/9/5/143.
> [...]
> 
>>>> +     and 64-bit systems, respectively.  Note that this means that the
>>>> +     Linux kernel does not support pre-EV56 Alpha CPUs, because these
>>>> +     older CPUs do not provide one-byte and two-byte loads and stores.
>>>                                  ^
>>>                             non-atomic
>>
>> I took this, thank you!
> 
> Eum, am I totally lost, or aren't both of these supposed to say "atomic" ?
> 
> Can't imagine requiring a CPU to provide non-atomic loads and stores
> (i.e. requiring old Alpha behavior?).

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."

Regards,
Peter Hurley

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