Commit d23b5c577715 ("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 77070eeb8821 ("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: d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU safe")
Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/20231207143806.114e0a74@xxxxxxxxxxxxxxxx/
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Acked-by: Yafang Shao <laoar.shao@xxxxxxxxx>
---
include/linux/cgroup-defs.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 5a97ea95b564..ea48c861cd36 100644
--- a/include/linux/cgroup-defs.h
+++ b/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;
--
2.39.3
