From: john stultz <johnstul@us.ibm.com>

This patch fixes a race in the timer_interrupt code caused by
detect_lost_tick().  Since we're doing lost-tick compensation outside
timer->mark_offset, time can pass between time-source reads which can cause
gettimeofday inconsistencies.

Additionally detect_lost_tick() was broken for the PIT case, since the whole
point of detect_lost_tick() is to interpolate between two time sources to
find inconsistencies.  Additionally this could cause xtime_lock seq_lock
reader starvation which has been causing machine hangs for SMP boxes that use
the PIT as a time source.

This patch fixes the described race by removing detect_lost_tick() and
instead implementing the lost tick detection code inside mark_offset().

Some of the divs and mods being added here might concern folks, but by not
calling timer->get_offset() in detect_lost_tick() we eliminate much of the
same math.  I did some simple cycle counting and the new code comes out on
average equivalent or faster.