diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b964f9e406f13..5d40afebe6cd3 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2026,7 +2026,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	if (!group)
 		return -ENOMEM;
 	/* pre-allocate to guarantee space while iterating in rcu read-side. */
-	retval = flex_array_prealloc(group, 0, group_size - 1, GFP_KERNEL);
+	retval = flex_array_prealloc(group, 0, group_size, GFP_KERNEL);
 	if (retval)
 		goto out_free_group_list;