This has been needed for a long time, but now with the advent of a
reference counted struct pid there are real consequences for getting this
wrong.

Someone I think it was Oleg Nesterov pointed out that this construct was
missing locking, when I introduced struct pid.  After taking time to review
the locking construct already present I figured out which lock needs to be
taken.  The other paths that access f_owner.pid take either the f_owner
read or the write lock.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>