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

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Message-ID: <BLU0-SMTP7304A1572FCB87F54CCCFA96D00@phx.gbl>
Date:	Mon, 14 Feb 2011 13:53:49 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Will Newton <will.newton@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jbaron@...hat.com>, 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, davem@...emloft.net,
	sam@...nborg.org, ddaney@...iumnetworks.com,
	michael@...erman.id.au, linux-kernel@...r.kernel.org,
	Mike Frysinger <vapier@...too.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	dhowells <dhowells@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	"heiko.carstens" <heiko.carstens@...ibm.com>,
	benh <benh@...nel.crashing.org>
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Mon, 2011-02-14 at 17:50 +0000, Will Newton wrote:
> > 
> > It would observe a stale value, but that value would only be updated
> > when the cache line was reloaded from main memory which would have to
> > be triggered by either eviction or cache flushing. So it could get
> > pretty stale. Whilst that's probably within the spec. of atomic_read I
> > suspect it would lead to problems in practice. I could be wrong
> > though. 
> 
> Right, so the typical scenario that could cause pain is something like:
> 
> while (atomic_read(&foo) != n)
>   cpu_relax();
> 
> and the problem is that cpu_relax() doesn't know which particular
> cacheline to flush in order to make things go faster, hm?

As an information point, this is why I mapped "uatomic_read()" to
"CMM_LOAD_SHARED" in my userspace RCU library rather than just doing a
volatile access. On cache-coherent architectures, the arch-specific code
turns CMM_LOAD_SHARED into a simple volatile access, but for
non-cache-coherent architectures, it can call the required
architecture-level primitives to fetch the stale data.

FWIW, I also have "CMM_STORE_SHARED" which does pretty much the same
thing. I use these for rcu_assign_pointer() and rcu_dereference() (thus
replacing "ACCESS_ONCE()").

The more detailed comment and macros are found at
http://git.lttng.org/?p=userspace-rcu.git;a=blob;f=urcu/system.h

I hope this helps,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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