linux-kernel - Re: [PATCH] clarify usage expectations for cnt32_to

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Date:	Sat, 08 Nov 2008 21:54:04 -0500 (EST)
From:	Nicolas Pitre <nico@....org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Russell King <rmk+lkml@....linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	lkml <linux-kernel@...r.kernel.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>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH] clarify usage expectations for cnt32_to_63()

On Sat, 8 Nov 2008, Mathieu Desnoyers wrote:

> > I used a rmb() so this is also safe for mixed usages in and out of 
> > interrupt context.  On the architecture I care about this is turned into
> > a simple compiler barrier and therefore doesn't make a difference, while 
> > smp_rmb() is a noop which isn't right.
> > 
> 
> Hum ? smp_rmb() is turned into a compiler barrier on !SMP architectures.
> Turning it into a NOP would be broken. Actually, ARM defines it as a
> barrier().

Oh, right. I got confused somehow with read_barrier_depends().

> I *think* that smp_rmb() would be enough, supposing the access to memory
> is done in program order wrt local interrupts in UP. This is basically
> Steven's question, which has not received any clear answer yet. I'd like
> to know what others think about it.

In the mean time a pure rmb() is the safest thing to do now.  Once we 
can convince ourselves that out-of-order reads are always rolled back 
upon the arrival of an interrupt then this could be relaxed.


Nicolas
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