linux-kernel - Re: [PATCH 0/2] jump label: 2.6.38 updates

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Message-ID: <20110214223755.436e7cf4@mfleming-mobl1.ger.corp.intel.com>
Date:	Mon, 14 Feb 2011 22:37:55 +0000
From:	Matt Fleming <matt@...sole-pimps.org>
To:	David Miller <davem@...emloft.net>
Cc:	rostedt@...dmis.org, peterz@...radead.org, will.newton@...il.com,
	jbaron@...hat.com, mathieu.desnoyers@...ymtl.ca, hpa@...or.com,
	mingo@...e.hu, tglx@...utronix.de, andi@...stfloor.org,
	roland@...hat.com, rth@...hat.com, masami.hiramatsu.pt@...achi.com,
	fweisbec@...il.com, avi@...hat.com, sam@...nborg.org,
	ddaney@...iumnetworks.com, michael@...erman.id.au,
	linux-kernel@...r.kernel.org, vapier@...too.org,
	cmetcalf@...era.com, dhowells@...hat.com, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com, benh@...nel.crashing.org
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates

On Mon, 14 Feb 2011 13:46:00 -0800 (PST)
David Miller <davem@...emloft.net> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> Date: Mon, 14 Feb 2011 16:39:36 -0500
> 
> > Thus it is not about global, as global is updated by normal means
> > and will update the caches. atomic_t is updated via the ll/sc that
> > ignores the cache and causes all this to break down. IOW... broken
> > hardware ;)
> 
> I don't see how cache coherency can possibly work if the hardware
> behaves this way.

Cache coherency is still maintained provided writes/reads both go
through the cache ;-)

The problem is that for read-modify-write operations the arbitration
logic that decides who "wins" and is allowed to actually perform the
write, assuming two or more CPUs are competing for a single memory
address, is not implemented in the cache controller, I think. I'm not a
hardware engineer and I never understood how the arbitration logic
worked but I'm guessing that's the reason that the ll/sc instructions
bypass the cache.

Which is why the atomic_t functions worked out really well for that
arch, such that any accesses to an atomic_t * had to go through the
wrapper functions.
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