Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates
From: Peter Zijlstra <peterz@infradead.org>
To: Will Newton <will.newton@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Jason Baron <jbaron@redhat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>, hpa@zytor.com,
        mingo@elte.hu, tglx@linutronix.de, andi@firstfloor.org,
        roland@redhat.com, rth@redhat.com, masami.hiramatsu.pt@hitachi.com,
        fweisbec@gmail.com, avi@redhat.com, davem@davemloft.net,
        sam@ravnborg.org, ddaney@caviumnetworks.com, michael@ellerman.id.au,
        linux-kernel@vger.kernel.org, Mike Frysinger <vapier@gentoo.org>,
        Chris Metcalf <cmetcalf@tilera.com>, dhowells <dhowells@redhat.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        "heiko.carstens" <heiko.carstens@de.ibm.com>,
        benh <benh@kernel.crashing.org>
In-Reply-To: <AANLkTi=jz+mGru--+YA62EeER17Tuq2DocrkKNaU2k+1@mail.gmail.com>
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Date: Mon, 14 Feb 2011 19:24:28 +0100
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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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