Re: [RFC PATCH 2/3] cgroup: Rely on namespace_sem in current_cgns_cgroup_from_root explicitly

[Christian Brauner <brauner-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Tue, May 02, 2023 at 03:38:46PM +0200, Michal Koutný wrote:
> The function current_cgns_cgroup_from_root() expects a stable
> cgroup_root, which is currently ensured with RCU read side paired with
> cgroup_destroy_root() called after RCU period.
> 
> The particular current_cgns_cgroup_from_root() is called from VFS code
> and cgroup_root stability can be also ensured by namespace_sem. Mark it
> explicitly as a preparation for further rework.
> 
> Signed-off-by: Michal Koutný <mkoutny-IBi9RG/b67k@public.gmane.org>
> ---
>  fs/namespace.c         | 5 ++++-
>  include/linux/mount.h  | 4 ++++
>  kernel/cgroup/cgroup.c | 7 +++----
>  3 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 54847db5b819..0d2333832064 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -71,7 +71,10 @@ static DEFINE_IDA(mnt_group_ida);
>  static struct hlist_head *mount_hashtable __read_mostly;
>  static struct hlist_head *mountpoint_hashtable __read_mostly;
>  static struct kmem_cache *mnt_cache __read_mostly;
> -static DECLARE_RWSEM(namespace_sem);
> +DECLARE_RWSEM(namespace_sem);
> +#ifdef CONFIG_LOCKDEP
> +EXPORT_SYMBOL_GPL(namespace_sem);
> +#endif
>  static HLIST_HEAD(unmounted);	/* protected by namespace_sem */
>  static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
>  
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index 1ea326c368f7..6277435f6748 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -80,6 +80,10 @@ static inline struct mnt_idmap *mnt_idmap(const struct vfsmount *mnt)
>  	return smp_load_acquire(&mnt->mnt_idmap);
>  }
>  
> +#ifdef CONFIG_LOCKDEP
> +extern struct rw_semaphore namespace_sem;
> +#endif

Nope, we're not putting namespace_sem in a header. The code it protects
is massively sensitive and it interacts with mount_lock and other locks.
This stays private to fs/namespace.c as far as I'm concerned.