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

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Message-ID: <1297707868.5226.189.camel@laptop>
Date:	Mon, 14 Feb 2011 19:24:28 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Newton <will.newton@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jbaron@...hat.com>,
	Mathieu Desnoyers <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, 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

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?



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