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Date: Sat, 08 Nov 2008 21:54:04 -0500 (EST)
From: Nicolas Pitre <nico@cam.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Russell King <rmk+lkml@arm.linux.org.uk>,
       David Howells <dhowells@redhat.com>,
       Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       lkml <linux-kernel@vger.kernel.org>, Ralf Baechle <ralf@linux-mips.org>,
       benh@kernel.crashing.org, paulus@samba.org,
       David Miller <davem@davemloft.net>, Ingo Molnar <mingo@redhat.com>,
       Thomas Gleixner <tglx@linutronix.de>,
       Steven Rostedt <rostedt@goodmis.org>, linux-arch@vger.kernel.org
Subject: Re: [PATCH] clarify usage expectations for cnt32_to_63()
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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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