taskstats: add_del_listener() should ignore !valid listeners

When send_cpu_listeners() finds the orphaned listener it marks it as
!valid and drops listeners->sem.  Before it takes this sem for writing,
s->pid can be reused and add_del_listener() can wrongly try to re-use
this entry.

Change add_del_listener() to check ->valid = T.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Vasiliy Kulikov <segoon@openwall.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/taskstats.c

@@ -304,7 +304,7 @@ static int add_del_listener(pid_t pid, const struct cpumask *mask, int isadd)
 			listeners = &per_cpu(listener_array, cpu);
 			down_write(&listeners->sem);
 			list_for_each_entry(s2, &listeners->list, list) {
-				if (s2->pid == pid)
+				if (s2->pid == pid && s2->valid)
 					goto exists;
 			}
 			list_add(&s->list, &listeners->list);