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

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Date:	Fri, 07 Nov 2008 10:55:12 +0000
From:	David Howells <dhowells@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	dhowells@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, Nicolas Pitre <nico@....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()

Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> Assume the time source is a global clock which insures that time will never
> *ever* go backward. Use a smp_rmb() to make sure the cnt_lo value is read before
> __m_cnt_hi.

If you have an smp_rmb(), then don't you need an smp_wmb()/smp_mb() to match
it to make it work?  And is your assumption valid that smp_rmb() will affect
memory vs the I/O access to read the clock?  There's no requirement that
cnt_lo will have been read from an MMIO location at all.

David
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