fix the "remove task_ppid_nr_ns" commit

Commit 84427ea (remove task_ppid_nr_ns)
moved the task_tgid_nr_ns(task->real_parent) outside of lock_task_sighand().
This is wrong, ->real_parent could be freed/reused.

Both ->parent/real_parent point to nothing after __exit_signal() because
we remove the child from ->children list, and thus the child can't be
reparented when its parent exits.

rcu_read_lock() protects ->parent/real_parent, but _only_ if we know it was
valid before we take rcu lock.

Revert this part of the patch.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/array.c

@@ -426,7 +426,6 @@ static int do_task_stat(struct task_struct *task, char *buffer, int whole)
 	cgtime = gtime = cputime_zero;
 
 	rcu_read_lock();
-	ppid = task_tgid_nr_ns(task->real_parent, ns);
 	if (lock_task_sighand(task, &flags)) {
 		struct signal_struct *sig = task->signal;
 
@@ -465,6 +464,7 @@ static int do_task_stat(struct task_struct *task, char *buffer, int whole)
 		}
 
 		sid = task_session_nr_ns(task, ns);
+		ppid = task_tgid_nr_ns(task->real_parent, ns);
 		pgid = task_pgrp_nr_ns(task, ns);
 
 		unlock_task_sighand(task, &flags);