mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge_cgroup()

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 18fa84a ("cgroup: Use
css_tryget() instead of css_tryget_online() in task_get_css()").

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Roman Gushchin <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/hugetlb_cgroup.c

@@ -196,7 +196,7 @@ int hugetlb_cgroup_charge_cgroup(int idx, unsigned long nr_pages,
 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;
 	}