linux-kernel - [PATCH-cgroup v2] cgroup: Move rcu_head up near the top of cgroup

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Message-Id: <20231207134614.882991-1-longman@redhat.com>
Date:   Thu,  7 Dec 2023 08:46:14 -0500
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Yafang Shao <laoar.shao@...il.com>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH-cgroup v2] cgroup: Move rcu_head up near the top of cgroup_root

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@...b.auug.org.au>
Closes: https://lore.kernel.org/lkml/20231207143806.114e0a74@canb.auug.org.au/
Signed-off-by: Waiman Long <longman@...hat.com>
Acked-by: Yafang Shao <laoar.shao@...il.com>
---
 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