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

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Date:	Fri, 7 Nov 2008 15:08:12 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.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>,
	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, Mathieu Desnoyers wrote:
> 
> I want to make sure
> 
>   __m_cnt_hi
>  is read before
>   mmio cnt_lo read

Hmm, let me make sure I understand why there is no wmb.

Paul, can you verify this?

Mathieu, you do the following:

   read a
   smp_rmb
   reab b
   if (test b)
      write a

So the idea is that you must read b to test it. And since we must read a 
before reading b we can see that we write a before either?

The question remains, can the write happen before either of the reads?

But since the read b is reading the hw clock, perhaps that just implies a
wmb on the hardware side?

-- Steve
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