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

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Message-ID: <540DF17C.9080509@zytor.com>
Date:	Mon, 08 Sep 2014 11:12:12 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	paulmck@...ux.vnet.ibm.com,
	Peter Hurley <peter@...leysoftware.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jakub Jelinek <jakub@...hat.com>,
	Mikael Pettersson <mikpelinux@...il.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Richard Henderson <rth@...ddle.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Miroslav Franc <mfranc@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
	linux-ia64@...r.kernel.org
Subject: Re: bit fields && data tearing

On 09/07/2014 10:56 PM, James Bottomley wrote:
> On Sun, 2014-09-07 at 16:39 -0700, H. Peter Anvin wrote:
>> How many PARISC systems do we have that actually do real work on Linux?
> 
> I'd be very surprised if this problem didn't exist on all alignment
> requiring architectures, like PPC and Sparc as well.  I know it would be
> very convenient if all the world were an x86 ... but it would also be
> very boring as well.

I wouldn't be so sure about that.  That is a pretty aggressive
relaxation of ordering that PARISC has enacted here, kind of like the
Alpha "we don't need no stinking byte accesses".

> The rules for coping with it are well known and a relaxation of what we
> currently do in the kernel, so I don't see what the actual problem is.
> 
> In the context of this thread, PA can't do atomic bit sets (no atomic
> RMW except the ldcw operation) it can do atomic writes to fundamental
> sizes (byte, short, int, long) provided gcc emits the correct primitive
> (there are lots of gotchas in this, but that's not an architectural
> problem).  These atomicity guarantees depend on the underlying storage
> and are respected for main memory but not for any other type of bus.

So I'm not trying to push the "all the world is an x86"... certainly not
given that x86 has abnormally strict ordering rules and so is itself an
outlier.  What I *don't* really want to have to deal with is going
through more than causal effort to accommodate outliers which no longer
have any real value -- we have too much work to do.

	-hpa


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