linux-kernel - Re: [PATCH 2/4] time: add a notifier chain for when the system time is stepped

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Date:	Wed, 19 Jun 2013 09:52:06 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	xen-devel@...ts.xen.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/4] time: add a notifier chain for when the system time
 is stepped

On 06/19/2013 08:25 AM, David Vrabel wrote:
> From: David Vrabel <david.vrabel@...rix.com>
>
> The high resolution timer code gets notified of step changes to the
> system time with clock_was_set() or clock_was_set_delayed() calls.  If
> other parts of the kernel require similar notification there is no
> clear place to hook into.
>
> Add a clock_was_set atomic notifier chain
> (clock_was_set_notifier_list) and call this in place of
> clock_was_set().  If the timekeeping locks are held, the calls are
> deferred to a new tasklet.
>
> The hrtimer code adds a notifier block to this chain and uses it to
> call (the now internal) clock_was_set().  Since the timekeeping code
> does not call the chain from the timer irq clock_was_set_delayed() and
> associated code can be removed.

So on my initial quick review, this *looks* pretty reasonable. I get a 
little worried about interface abuse (ie: random drivers trying to hook 
into clock_was_set_notifier_list), but we can move that into 
timekeeper_internal.h or something similar to limit that.

The other issue here is we've been burned pretty badly in the past with 
changes to clock_was_set(), as its key to keeping timers in line with 
timekeeping.  So this will need a fair amount of testing and run time 
before this gets merged, so 3.12 is what we'd be targeting at the 
earliest (its getting a bit late for taking changes for 3.11 anyway).

If you want to try to push patch 1/4 in for 3.11 via the Xen tree, I'll 
see about queuing the other three for hopefully 3.12.

thanks
-john

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