2023-10-09 13:58:39

by Michal Koutný

Subject: [PATCH] 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/[email protected]/
Suggested-by: Firo Yang <[email protected]>
Signed-off-by: Michal Koutný <[email protected]>
---
kernel/cgroup/cgroup-v1.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index c487ffef6652..76db6c67e39a 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/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) {
--
2.42.0

2023-10-09 16:33:06

by Tejun Heo

Subject: Re: [PATCH] cgroup: Remove duplicates in cgroup v1 tasks file

On Mon, Oct 09, 2023 at 03:58:11PM +0200, Michal Koutn? wrote:
> 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/[email protected]/
> Suggested-by: Firo Yang <[email protected]>
> Signed-off-by: Michal Koutn? <[email protected]>

Applied to cgroup/for-6.6-fixes.

Thanks.

--
tejun