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

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Date:	Fri, 7 Nov 2008 16:04:48 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	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>,
	linux-arch@...r.kernel.org
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()


On Fri, 7 Nov 2008, Paul E. McKenney wrote:
> > > Would that make more sense ?
> > > 
> > 
> > Oh, actually, I got things reversed in this email : the readl(io_addr)
> > must be done _after_ the __m_cnt_hi read.
> > 
> > Therefore, two consecutive executions would look like :
> > 
> > barrier();  /* Make sure the compiler does not reorder __m_cnt_hi and
> >                previous mmio read. */
> > read __m_cnt_hi
> > smp_rmb();  /* Waits for every cached memory reads to complete */
> 
> If these are MMIO reads, then you need rmb() rather than smp_rmb(),
> at least on architectures that can reorder writes (Power, Itanium,
> and I believe also ARM, ...).

The read is from a clock source. The only writes that are happening is 
by the clock itself.

On a UP system, is a rmb still needed? That is, can you have two reads on 
the same CPU from the clock source that will produce a backwards clock? 
That to me sounds like the clock interface is broken.

-- Steve
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