Re: [PATCH 2/4] time: add a notifier chain for when the system timeis stepped

From: John Stultz
Date: Wed Jun 19 2013 - 12:52:16 EST

On 06/19/2013 08:25 AM, David Vrabel wrote:
From: David Vrabel <david.vrabel@xxxxxxxxxx>

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.


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