cgroup: check if cgroup root is alive in cgroupstats_show()

euler inclusion
category: bugfix
bugzilla: 8883
CVE: NA

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

If a cgroup root is dying, show its hierarchy_id and num_cgroups
as 0.
Signed-off-by: NZefan Li <lizefan@huawei.com>
Tested-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NHanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: NBixuan Cui <cuibixuan@huawei.com>
Reviewed-by: NMiao Xie <miaoxie@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

kernel/cgroup/cgroup-v1.c

@@ -686,6 +686,7 @@ int proc_cgroupstats_show(struct seq_file *m, void *v)
 {
 	struct cgroup_subsys *ss;
 	int i;
+	bool dead;
 
 	seq_puts(m, "#subsys_name\thierarchy\tnum_cgroups\tenabled\n");
 	/*
@@ -695,11 +696,13 @@ int proc_cgroupstats_show(struct seq_file *m, void *v)
 	 */
 	mutex_lock(&cgroup_mutex);
 
-	for_each_subsys(ss, i)
+	for_each_subsys(ss, i) {
+		dead = percpu_ref_is_dying(&ss->root->cgrp.self.refcnt);
 		seq_printf(m, "%s\t%d\t%d\t%d\n",
-			   ss->legacy_name, ss->root->hierarchy_id,
-			   atomic_read(&ss->root->nr_cgrps),
+			   ss->legacy_name, dead ? 0 : ss->root->hierarchy_id,
+			   dead ? 0 : atomic_read(&ss->root->nr_cgrps),
 			   cgroup_ssid_enabled(i));
+	}
 
 	mutex_unlock(&cgroup_mutex);
 	return 0;