cgroup: Move rcu_head up near the top of cgroup_root

Commit d23b5c5 ("cgroup: Make operations on the cgroup root_list RCU
safe") adds a new rcu_head to the cgroup_root structure and kvfree_rcu()
for freeing the cgroup_root.

The current implementation of kvfree_rcu(), however, has the limitation
that the offset of the rcu_head structure within the larger data
structure must be less than 4096 or the compilation will fail. See the
macro definition of __is_kvfree_rcu_offset() in include/linux/rcupdate.h
for more information.

By putting rcu_head below the large cgroup structure, any change to the
cgroup structure that makes it larger run the risk of causing build
failure under certain configurations. Commit 77070ee ("cgroup:
Avoid false cacheline sharing of read mostly rstat_cpu") happens to be
the last straw that breaks it. Fix this problem by moving the rcu_head
structure up before the cgroup structure.

Fixes: d23b5c5 ("cgroup: Make operations on the cgroup root_list RCU safe")
Reported-by: Stephen Rothwell <[email protected]>
Closes: https://lore.kernel.org/lkml/[email protected]/
Signed-off-by: Waiman Long <[email protected]>
Acked-by: Yafang Shao <[email protected]>
Reviewed-by: Yosry Ahmed <[email protected]>
Reviewed-by: Michal Koutný <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: Waiman-Long

include/linux/cgroup-defs.h

@@ -562,6 +562,10 @@ struct cgroup_root {
 	/* Unique id for this hierarchy. */
 	int hierarchy_id;
 
+	/* A list running through the active hierarchies */
+	struct list_head root_list;
+	struct rcu_head rcu;	/* Must be near the top */
+
 	/*
 	 * The root cgroup. The containing cgroup_root will be destroyed on its
 	 * release. cgrp->ancestors[0] will be used overflowing into the
@@ -575,10 +579,6 @@ struct cgroup_root {
 	/* Number of cgroups in the hierarchy, used only for /proc/cgroups */
 	atomic_t nr_cgrps;
 
-	/* A list running through the active hierarchies */
-	struct list_head root_list;
-	struct rcu_head rcu;
-
 	/* Hierarchy-specific flags */
 	unsigned int flags;