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

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Date:	Fri, 7 Nov 2008 16:27:23 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
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()

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 
> 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?
> 

write a cannot happen before read a (same variable).
write a must happen after read b because it depends on the b value. It
makes sure the the side-effect of "write a" is seen by other CPUs
*after* we have read the b value.

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

It makes sense. The hardware clock has no cache coherency problem.. so
it could be seen as doing wmb() after each data update.

Mathieu

> -- Steve

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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