From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Howells <dhowells@redhat.com>
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
Date: Fri, 07 Nov 2008 10:55:12 +0000
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To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: dhowells@redhat.com, Linus Torvalds <torvalds@linux-foundation.org>, akpm@linux-foundation.org, Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-kernel@vger.kernel.org, Nicolas Pitre <nico@cam.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

Mathieu Desnoyers <mathieu.desnoyers@polymtl.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