Thomas Gleixner wrote:
>>> The question is, how the tick timer gets enqueued in the softirq queue.
>>> Can you isolate the codepath, where this happens ?
> 
> The TIMER_SOFTIRQ runs the hrtimers during bootup until a usable
> clocksource and clock event sources are registered. The switch to high
> resolution mode happens inside of the TIMER_SOFTIRQ, but runs the
> softirq afterwards. That way the tick emulation timer, which was set up
> in the switch to highres might be executed in the softirq context, which
> is a BUG. The rbtree has not to be touched by the softirq after the
> highres switch.
> 

And an additional request, just to make it explicit that we should not
have any NO_SOFTIRQ callbacks in the tree; BUG out if we encounter such
a thing.

Signed-off-by: Andres Salomon <dilinger@debian.org>