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

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Message-ID: <1297722030.23343.86.camel@gandalf.stny.rr.com>
Date:	Mon, 14 Feb 2011 17:20:30 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	David Miller <davem@...emloft.net>
Cc:	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, 2011-02-14 at 13:46 -0800, David Miller 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.
> 
> In cache aliasing situations, yes I can understand a L1 cache visibility
> issue being present, but with kernel only stuff that should never happen
> otherwise we have a bug in the arch cache flushing support.

I guess the issue is, if you use ll/sc on memory, you must always use
ll/sc on that memory, otherwise any normal read won't read the proper
cache.

The atomic_read() in this arch uses ll to read the memory directly and
skip the cache. If we make atomic_read() like the other archs:

#define atomic_read(v)	(*(volatile int *)&(v)->counter)

This pulls the counter into cache, and it will not be updated by a
atomic_inc() from another CPU.

Ideally, we would like a single atomic_read() but due to these wacky
archs, it may not be possible.

-- Steve


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