dump_task() should use find_lock_task_mm() too. It is necessary for
protecting task-exiting race.

dump_tasks() currently filters any task that does not have an attached
->mm since it incorrectly assumes that it must either be in the process of
exiting and has detached its memory or that it's a kernel thread;
multithreaded tasks may actually have subthreads that have a valid ->mm
pointer and thus those threads should actually be displayed.  This change
finds those threads, if they exist, and emit their information along with
the rest of the candidate tasks for kill.
Signed-off-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NDavid Rientjes <rientjes@google.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>