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

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Date:	Fri, 07 Nov 2008 11:03:35 +0000
From:	David Howells <dhowells@...hat.com>
To:	Nicolas Pitre <nico@....org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	dhowells@...hat.com,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

Nicolas Pitre <nico@....org> wrote:

> > I mean, the darned thing is called from sched_clock(), which can be
> > concurrently called on separate CPUs and which can be called from
> > interrupt context (with an arbitrary nesting level!) while it was running
> > in process context.
> 
> Yes!  And this is so on *purpose*.  Please take some time to read the 
> comment that goes along with it, and if you're still not convinced then 
> look for those explanation emails I've already posted.

I agree with Nicolas on this.  It's abominably clever, but I think he's right.

The one place I remain unconvinced is over the issue of preemption of a process
that is in the middle of cnt32_to_63(), where if the preempted process is
asleep for long enough, I think it can wind time backwards when it resumes, but
that's not a problem for the one place I want to use it (sched_clock()) because
that is (almost) always called with preemption disabled in one way or another.

The one place it isn't is a debugging case that I'm not too worried about.

> > /*
> >  * Caller must provide locking to protect *caller_state
> >  */
> 
> NO!  This is meant to be LOCK FREE!

Absolutely.

David
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