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

[Tejun Heo <tj@kernel.org>]

Hello, Sebastian.

Sorry about late reply. Have been sick for a bit.

On Mon, Nov 11, 2024 at 06:04:50PM +0100, Sebastian Andrzej Siewior wrote:
...
> Let me check if I understood. We have three users of kernfs:
> 
> - cgroup
>   cgroup1_rename(): parent check (would get the new KERNFS_ROOT flag)
> 
> - resctrl
>   rdtgroup_rename(): seems to allow any "mon group" directory
>   different parent possible.
> 
> - sysfs
>   sysfs_move_dir_ns(): reame to a different parent

I'm not sure about resctrl but here we just need to add that flag to cgroup,
right?

> That new RCU interface would be used by cgroup only and sysfs/ resctrl
> would remain using the "old" code?
> If so, would you  prefer 
> |struct kernfs_node {
> |	…
> |	union {
> |		const char              name;
> |		const char              __rcu *name_rcu;
> |	}
> 
> to avoid patching resctrl + sysfs for for the rcu_derference name
> lookup?

As replied on the patches, it probably is cleaner to always use __rcu and
apply the additional locking on the reader side if renaming across different
parents is allowed.

Thanks.

-- 
tejun