cgroup: fix a subtle bug in descendant pre-order walk
When cgroup_next_descendant_pre() initiates a walk, it checks whether
the subtree root doesn't have any children and if not returns NULL.
Later code assumes that the subtree isn't empty. This is broken
because the subtree may become empty inbetween, which can lead to the
traversal escaping the subtree by walking to the sibling of the
subtree root.
There's no reason to have the early exit path. Remove it along with
the later assumption that the subtree isn't empty. This simplifies
the code a bit and fixes the subtle bug.
While at it, fix the comment of cgroup_for_each_descendant_pre() which
was incorrectly referring to ->css_offline() instead of
->css_online().
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: stable@vger.kernel.org
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 38b1365..31e9ef3 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2954,11 +2954,8 @@
WARN_ON_ONCE(!rcu_read_lock_held());
/* if first iteration, pretend we just visited @cgroup */
- if (!pos) {
- if (list_empty(&cgroup->children))
- return NULL;
+ if (!pos)
pos = cgroup;
- }
/* visit the first child if exists */
next = list_first_or_null_rcu(&pos->children, struct cgroup, sibling);
@@ -2966,14 +2963,14 @@
return next;
/* no child, visit my or the closest ancestor's next sibling */
- do {
+ while (pos != cgroup) {
next = list_entry_rcu(pos->sibling.next, struct cgroup,
sibling);
if (&next->sibling != &pos->parent->children)
return next;
pos = pos->parent;
- } while (pos != cgroup);
+ }
return NULL;
}