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.

This BUG was observed by Andres Salomon, who provided the information to
debug it.

Return early from the softirq, when the switch was sucessful.

[dilinger@debian.org: add debug warning]
[akpm@linux-foundation.org: make debug warning compile]
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andres Salomon <dilinger@debian.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andres Salomon <dilinger@debian.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>