cgroup: Remove duplicates in cgroup v1 tasks file

One PID may appear multiple times in a preloaded pidlist.
(Possibly due to PID recycling but we have reports of the same
task_struct appearing with different PIDs, thus possibly involving
transfer of PID via de_thread().)

Because v1 seq_file iterator uses PIDs as position, it leads to
a message:
> seq_file: buggy .next function kernfs_seq_next did not update position index

Conservative and quick fix consists of removing duplicates from `tasks`
file (as opposed to removing pidlists altogether). It doesn't affect
correctness (it's sufficient to show a PID once), performance impact
would be hidden by unconditional sorting of the pidlist already in place
(asymptotically).

Link: https://lore.kernel.org/r/20230823174804.23632-1-mkoutny@suse.com/Suggested-by: Firo Yang <firo.yang@suse.com>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org

kernel/cgroup/cgroup-v1.c

@@ -360,10 +360,9 @@ static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
 	}
 	css_task_iter_end(&it);
 	length = n;
-	/* now sort & (if procs) strip out duplicates */
+	/* now sort & strip out duplicates (tgids or recycled thread PIDs) */
 	sort(array, length, sizeof(pid_t), cmppid, NULL);
-	if (type == CGROUP_FILE_PROCS)
-		length = pidlist_uniq(array, length);
+	length = pidlist_uniq(array, length);
 
 	l = cgroup_pidlist_find_create(cgrp, type);
 	if (!l) {