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

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Message-ID: <540E3207.7090007@hurleysoftware.com>
Date:	Mon, 08 Sep 2014 18:47:35 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	"H. Peter Anvin" <hpa@...or.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
CC:	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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"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 09/08/2014 01:59 PM, H. Peter Anvin wrote:
> On 09/08/2014 10:52 AM, One Thousand Gnomes wrote:
>> On Fri, 05 Sep 2014 08:41:52 -0700
>> "H. Peter Anvin" <hpa@...or.com> wrote:
>>
>>> On 09/05/2014 08:31 AM, Peter Hurley wrote:
>>>>
>>>> Which is a bit ironic because I remember when Digital had a team
>>>> working on emulating native x86 apps on Alpha/NT.
>>>>
>>>
>>> Right, because the x86 architecture was obsolete and would never scale...
>>
>> Talking about "not scaling" can anyone explain how a "you need to use
>> set_bit() and friends" bug report scaled into a hundred message plus
>> discussion about ambiguous properties of processors (and nobody has
>> audited all the embedded platforms we support yet, or the weirder ARMs)
>> and a propsal to remove Alpha support.
>>
>> Wouldn't it be *much* simpler to do what I suggested in the first place
>> and use the existing intended for purpose, deliberately put there,
>> functions for atomic bitops, because they are fast on sane processors and
>> they work on everything else.
>>
>> I think the whole "removing Alpha EV5" support is basically bonkers. Just
>> use set_bit in the tty layer. Alpha will continue to work as well as it
>> always has done and you won't design out support for any future processor
>> that turns out not to do byte aligned stores.
>>
>> Alan
>>
> 
> Is *that* what we are talking about?  I was added to this conversation
> in the middle where it had already generalized, so I had no idea.

No, this is just what brought this craziness to my attention.

For example, byte- and short-sized circular buffers could not possibly
be safe either, when the head nears the tail.

Who has audited global storage and ensured that _every_ byte-sized write
doesn't happen to be adjacent to some other storage that may not happen
to be protected by the same (or any) lock?

Regards,
Peter Hurley



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