Some platforms need to grab mm->page_table_lock during switch_mm().
On the other hand code like swap_out() in mm/vmscan.c needs to hold
mm->page_table_lock during wakeups which needs to grab the runqueue
lock.  This creates a conflict and the resolution chosen here is to
not hold the runqueue lock during context_switch().

The implementation is specifically a "frozen" state implemented as a
spinlock, which is held around the context_switch() call.  This allows
the runqueue lock to be dropped during this time yet prevent another cpu
from running the "not switched away from yet" task.