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Date: Fri, 07 Nov 2008 16:22:59 -0500 (EST)
From: Nicolas Pitre <nico@cam.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: David Howells <dhowells@redhat.com>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       akpm@linux-foundation.org, Ingo Molnar <mingo@elte.hu>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       lkml <linux-kernel@vger.kernel.org>, Ralf Baechle <ralf@linux-mips.org>,
       benh@kernel.crashing.org, paulus@samba.org,
       David Miller <davem@davemloft.net>, Ingo Molnar <mingo@redhat.com>,
       Thomas Gleixner <tglx@linutronix.de>,
       Steven Rostedt <rostedt@goodmis.org>, linux-arch@vger.kernel.org
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
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On Fri, 7 Nov 2008, Mathieu Desnoyers wrote:

> First off, read hw cnt low _is_ an uncached memory read (this is the
> mmio read). __m_cnt_hi is a cached read, and therefore can be delayed if
> the cache-line is busy. And we have no control on how much time can pass
> between the two reads given the CPU may stall waiting for a cache-line.
> 
> So the scenario above happens if CPU A have __m_cnt_hi in its cacheline,
> but for come reason CPU B have to defer the cacheline read of __m_cnt_hi
> due to heavy cacheline traffic and decides to proceed to mmio read
> before the cacheline has been brought to the CPU because "hey, there is
> no data dependency between those two reads !".

OK that makes sense.


Nicolas
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