linux-kernel - Re: [RFC patch 08/18] cnt32_to_63 should use smp

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Date:	Fri, 07 Nov 2008 16:22:59 -0500 (EST)
From:	Nicolas Pitre <nico@....org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	David Howells <dhowells@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	lkml <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>, benh@...nel.crashing.org,
	paulus@...ba.org, David Miller <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arch@...r.kernel.org
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

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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