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

[Tejun Heo]

Hello, Sebastian.

On Fri, Nov 08, 2024 at 11:24:06PM +0100, Sebastian Andrzej Siewior wrote:
> Instead of using kernfs_rename_lock for lookups of ::name use RCU for
> lookup. Rely on kn's kernfs_root::kernfs_rwsem for update
> synchronisation.
> 
> The .*_locked() have been moved into the callers.
> The lock in kernfs_get_parent() has been dropped, the parent node should
> node vanish underneath us. The RCU read-lock and atomic_inc_not_zero()
> is a safety net in case it does.
> kernfs_fop_readdir() no longer drops kernfs_root::kernfs_rwsem to ensure
> the name pointer does not vanish while the page fault is handled.
> kernfs_notify_workfn() gained the lock for the same reason.

I owe an apology. I was just thinking about cgroups. Sysfs, I think, does
allow moving node a different parent, which IIRC is used by netdevs. How
about something like this:

- Add a KERNFS_ROOT flag indicating that parent-changing moves aren't
  allowed.

- Update kernfs_rename_ns() to trigger warning and fail if the above flag is
  set and new_parent is different from the old one.

- Create a separate interface which uses RCU instead of rename lock for name
  / path lookups. The RCU ones should trigger warning if used when the above
  KERNFS_ROOT flag is not set.

Thanks.

-- 
tejun