Applying RCU to the task structure broke oprofile, because
free_task_notify() can now be called from softirq.  This means that the
task_mortuary lock must be acquired with irq disabled in order to avoid
intermittent self-deadlock.  Since irq is now disabled, the critical
section within process_task_mortuary() has been restructured to be O(1) in
order to maximize scalability and minimize realtime latency degradation.

Kudos to Wu Fengguang for finding this problem!

CC: Wu Fengguang <wfg@mail.ustc.edu.cn>
Cc: Philippe Elie <phil.el@wanadoo.fr>
Cc: John Levon <levon@movementarian.org>
Signed-off-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>