[PATCH 3.16 131/136] mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge_cgroup()

[Ben Hutchings]

3.16.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Roman Gushchin <guro@xxxxxx>

commit 0362f326d86c645b5e96b7dbc3ee515986ed019d upstream.

An exiting task might belong to an offline cgroup.  In this case an
attempt to grab a cgroup reference from the task can end up with an
infinite loop in hugetlb_cgroup_charge_cgroup(), because neither the
cgroup will become online, neither the task will be migrated to a live
cgroup.

Fix this by switching over to css_tryget().  As css_tryget_online()
can't guarantee that the cgroup won't go offline, in most cases the
check doesn't make sense.  In this particular case users of
hugetlb_cgroup_charge_cgroup() are not affected by this change.

A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
css_tryget() instead of css_tryget_online() in task_get_css()").

Link: http://lkml.kernel.org/r/20191106225131.3543616-2-guro@xxxxxx
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/hugetlb_cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/hugetlb_cgroup.c
+++ b/mm/hugetlb_cgroup.c
@@ -181,7 +181,7 @@ int hugetlb_cgroup_charge_cgroup(int idx
 again:
 	rcu_read_lock();
 	h_cg = hugetlb_cgroup_from_task(current);
-	if (!css_tryget_online(&h_cg->css)) {
+	if (!css_tryget(&h_cg->css)) {
 		rcu_read_unlock();
 		goto again;
 	}