Re: [PATCH v3] kernfs: Use RCU for kernfs_node::name and ::parent lookup.

[Michal Koutný]

Hello.

On Thu, Nov 21, 2024 at 06:52:50PM GMT, Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:

> - kernfs_rename_ns() is only using kernfs_rename_lock if the parents are
>   different. All users users use either RCU or kernfs_rwsem.
> - kernfs_fop_readdir() drops kernfs_root::kernfs_rwsem while holding a
>   reference to name and invoking dir_emit(). This has been changed and
>   lock is held.
> - kernfs_notify_workfn() access kernfs_node::name without any
>   protection. Added kernfs_root::kernfs_rwsem for the iteration.
> - kernfs_get_parent_dentry() acquires now kernfs_root::kernfs_rwsem
>   while accessing the parent node.
> - kernfs_node_dentry() acquires now kernfs_root::kernfs_rwsem while
>   parent is accessed and the name looked up.

Why is the kernfs_root::kernfs_rwsem newly R-taken? Shouldn't be RCU
read section sufficient for those users?

(Perhaps it's related to second observation I have -- why there is
sometimes kernfs_rcu_get_parent() whereas there are other call sites
with mere rcu_dereference(kn->parent)?)

Attachment: signature.asc
Description: PGP signature