linux-kernel - Re: [RFC PATCH 13/22 -v2] handle accurate time keeping over long delays

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Date:	Thu, 10 Jan 2008 15:05:06 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Tony Luck <tony.luck@...el.com>, bob.picco@...com,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 13/22 -v2] handle accurate time keeping over long
	delays

On Thu, 2008-01-10 at 14:52 -0800, john stultz wrote:
> The issue is that the race isn't just between the readers and the
> writer, but that time races against writer as well. So if you don't lock
> the readers out during the write, I'm not sure how you can avoid the
> window for inconsistencies.

Maybe to state it more clearly, the issue is that in order to be atomic,
the writer must atomically access the clock, and make all the updates in
one step.

So if a reader accesses the clock after the writer accesses the clock,
but before the writer finishes the update, there is the possibility time
could go backwards.

Now, not to completely throw water on it, It is possible to set up a
bounds argument, and say as long as NTP adjustments are less then X, and
the window between the writer starting and finishing the update is less
then Y, the resulting inconsistency is limited to Z. And if Z is less
then a nanosecond, then you're ok.

However, items like virtualization and the realtime patch can cause Y to
be stretched quite a bit, so finding a way to handle that would be
needed as well.

thanks
-john

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