Commit a7fb0423 authored Dec 07, 2023 by Waiman Long Committed by Tejun Heo Dec 07, 2023

cgroup: Move rcu_head up near the top of cgroup_root



Commit d23b5c57 ("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 77070eeb ("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: d23b5c57 ("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>

parent 3232e7aa

include/linux/cgroup-defs.h

+4 −4

Original line number	Diff line number	Diff line
		@@ -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;