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

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Message-ID: <9473.1226101838@redhat.com>
Date:	Fri, 07 Nov 2008 23:50:38 +0000
From:	David Howells <dhowells@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	dhowells@...hat.com,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"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:

> > I use smp_rmb() to do this on SMP systems (hrm, actually, a rmb() could
> > be required so it works also on UP systems safely wrt interrupts).
> 
> smp_rmb turns into a compiler barrier on UP and should prevent the below
> description.

Note that that does not guarantee that the two reads will be done in the order
you want.  The compiler barrier _only_ affects the compiler.  It does not stop
the CPU from doing the reads in any order it wants.  You need something
stronger than smp_rmb() if you need the reads to be so ordered.

David
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