do_each_thread() is rcu-safe, and all tasks which use this ->mm must sleep
in wait_for_completion(&mm->core_done) at this point, so we can use RCU
locks.

Also, remove unneeded INIT_LIST_HEAD(new) before list_add(new, head).
Signed-off-by: NOleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>