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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
This commit is contained in:
Michal Koutný 2023-10-09 15:58:11 +02:00 committed by Tejun Heo
parent 94f6f0550c
commit 1ca0b60515
1 changed files with 2 additions and 3 deletions
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3
kernel/cgroup/cgroup-v1.c
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@ -360,9 +360,8 @@ static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
} }
css_task_iter_end(&it); css_task_iter_end(&it);
length = n; 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); 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); l = cgroup_pidlist_find_create(cgrp, type);