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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Wed, 6 Dec 2023 20:51:46 -0800
Message-ID: <CAJD7tkZtt8xedBJyRns+6HpdXoBxadLUGuGNG5s1trEbRgb9hA@mail.gmail.com>
Subject: Re: [PATCH-cgroup] cgroup: Move rcu_head up near the top of cgroup_root
To:     Waiman Long <longman@redhat.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Yafang Shao <laoar.shao@gmail.com>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Stephen Rothwell <sfr@canb.auug.org.au>
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On Wed, Dec 6, 2023 at 8:38=E2=80=AFPM Waiman Long <longman@redhat.com> wro=
te:
>
> 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 use of kvfree_rcu(), however, has the limitation that the offset of
> the rcu_head structure within the larger data structure cannot exceed
> 4096 or the compilation will fail. By putting rcu_head below the cgroup
> structure, any change to the cgroup structure that makes it larger has
> the risk of build failure. Commit 77070eeb8821 ("cgroup: Avoid false
> cacheline sharing of read mostly rstat_cpu") happens to be the commit
> that breaks it even though it is not its fault. Fix it by moving the
> rcu_head structure up before the cgroup structure.
>
> Fixes: d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU=
 safe")
> Signed-off-by: Waiman Long <longman@redhat.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..45359969d8cf 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;
> +

Perhaps the comment should mention the placement requirements, and
maybe a pointer to wherever it is specified that the offset of struct
rcu_head should not exceed 4096?