cgroup: Move rcu_head up near the top of cgroup_root

commit a7fb0423c201ba12815877a0b5a68a6a1710b23a upstream.

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@canb.auug.org.au>
Closes: https://lore.kernel.org/lkml/20231207143806.114e0a74@canb.auug.org.au/
Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name cgroup-move-rcu_head-up-near-the-top-of-cgroup_root.patch

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index e0e03c68000f02b98cc035e3937acf129df337bf..c1bf210fe718949115cdfb267978f66b2656f19f 100644 (file)
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -501,6 +501,10 @@ 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.  Root is destroyed on its release. */
        struct cgroup cgrp;
 
@@ -510,10 +514,6 @@ 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;