will_become_orphaned_pgrp: partially fix insufficient ->exit_state check

p->exit_state != 0 doesn't mean this process is dead, it may have
sub-threads.  Change the code to use "p->exit_state && thread_group_empty(p)"
instead.

Without this patch, ^Z doesn't deliver SIGTSTP to the foreground process
if the main thread has exited.

However, the new check is not perfect either.  There is a window when
exit_notify() drops tasklist and before release_task().  Suppose that
the last (non-leader) thread exits.  This means that entire group exits,
but thread_group_empty() is not true yet.

As Eric pointed out, is_global_init() is wrong as well, but I did not
dare to do other changes.

Just for the record, has_stopped_jobs() is absolutely wrong too.  But we
can't fix it now, we should first fix SIGNAL_STOP_STOPPED issues.

Even with this patch ^Z doesn't play well with the dead main thread.
The task is stopped correctly but do_wait(WSTOPPED) won't see it.  This
is another unrelated issue, will be (hopefully) fixed separately.
Signed-off-by: NOleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -214,20 +214,19 @@ struct pid *session_of_pgrp(struct pid *pgrp)
 static int will_become_orphaned_pgrp(struct pid *pgrp, struct task_struct *ignored_task)
 {
 	struct task_struct *p;
-	int ret = 1;
 
 	do_each_pid_task(pgrp, PIDTYPE_PGID, p) {
-		if (p == ignored_task
-				|| p->exit_state
-				|| is_global_init(p->real_parent))
+		if ((p == ignored_task) ||
+		    (p->exit_state && thread_group_empty(p)) ||
+		    is_global_init(p->real_parent))
 			continue;
+
 		if (task_pgrp(p->real_parent) != pgrp &&
-		    task_session(p->real_parent) == task_session(p)) {
-			ret = 0;
-			break;
-		}
+		    task_session(p->real_parent) == task_session(p))
+			return 0;
 	} while_each_pid_task(pgrp, PIDTYPE_PGID, p);
-	return ret;	/* (sighing) "Often!" */
+
+	return 1;
 }
 
 int is_current_pgrp_orphaned(void)