linux-kernel - [PATCH 2/2] mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge

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Date:   Wed, 6 Nov 2019 14:51:31 -0800
From:   Roman Gushchin <guro@...com>
To:     <linux-mm@...ck.org>, Andrew Morton <akpm@...ux-foundation.org>
CC:     Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        Roman Gushchin <guro@...com>, <stable@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>
Subject: [PATCH 2/2] 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 18fa84a2db0e ("cgroup: Use
css_tryget() instead of css_tryget_online() in task_get_css()").

Signed-off-by: Roman Gushchin <guro@...com>
Cc: stable@...r.kernel.org
Cc: Tejun Heo <tj@...nel.org>
---
 mm/hugetlb_cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
index f1930fa0b445..2ac38bdc18a1 100644
--- a/mm/hugetlb_cgroup.c
+++ b/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;
 	}
-- 
2.17.1