Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index d503d1a..ca19b4c 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-only
 #include "cgroup-internal.h"
 
 #include <linux/sched/cputime.h>
@@ -87,7 +88,6 @@
 						   struct cgroup *root, int cpu)
 {
 	struct cgroup_rstat_cpu *rstatc;
-	struct cgroup *parent;
 
 	if (pos == root)
 		return NULL;
@@ -115,8 +115,8 @@
 	 * However, due to the way we traverse, @pos will be the first
 	 * child in most cases. The only exception is @root.
 	 */
-	parent = cgroup_parent(pos);
-	if (parent && rstatc->updated_next) {
+	if (rstatc->updated_next) {
+		struct cgroup *parent = cgroup_parent(pos);
 		struct cgroup_rstat_cpu *prstatc = cgroup_rstat_cpu(parent, cpu);
 		struct cgroup_rstat_cpu *nrstatc;
 		struct cgroup **nextp;
@@ -140,9 +140,12 @@
 		 * updated stat.
 		 */
 		smp_mb();
+
+		return pos;
 	}
 
-	return pos;
+	/* only happens for @root */
+	return NULL;
 }
 
 /* see cgroup_rstat_flush() */