linux-kernel - [PATCH 2/2] cgroup: Move rcu_head up near the top of cgroup

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Message-ID: <20241202101102.91106-2-siddh.raman.pant@oracle.com>
Date: Mon,  2 Dec 2024 15:41:02 +0530
From: Siddh Raman Pant <siddh.raman.pant@...cle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Sasha Levin <sashal@...nel.org>, stable <stable@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, cgroups@...r.kernel.org,
        Shivani Agarwal <shivani.agarwal@...adcom.com>
Subject: [PATCH 2/2] cgroup: Move rcu_head up near the top of cgroup_root

From: Waiman Long <longman@...hat.com>

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@...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>
Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>
Reviewed-by: Michal Koutný <mkoutny@...e.com>
Signed-off-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Fedor Pchelkin <pchelkin@...ras.ru>
[Shivani: Modified to apply on v5.4.y]
Signed-off-by: Shivani Agarwal <shivani.agarwal@...adcom.com>
Reviewed-by: Siddh Raman Pant <siddh.raman.pant@...cle.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 c64f11674850..f0798d98be8e 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -506,6 +506,10 @@ 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;
+
 	/* The root cgroup.  Root is destroyed on its release. */
 	struct cgroup cgrp;
 
@@ -515,10 +519,6 @@ 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;
 
-- 
2.45.2