Re: [PATCH v6 5/6] kernfs: Use RCU to access kernfs_node::parent.

[Tejun Heo]

On Thu, Jan 30, 2025 at 03:02:06PM +0100, Sebastian Andrzej Siewior wrote:
> kernfs_rename_lock is used to obtain stable kernfs_node::{name|parent}
> pointer. This is a preparation to access kernfs_node::parent under RCU
> and ensure that the pointer remains stable under the RCU lifetime
> guarantees.
> 
> For a complete path, as it is done in kernfs_path_from_node(), the
> kernfs_rename_lock is still required in order to obtain a stable parent
> relationship while computing the relevant node depth. This must not
> change while the nodes are inspected in order to build the path.
> If the kernfs user never moves the nodes (changes the parent) then the
> kernfs_rename_lock is not required and the RCU guarantees are
> sufficient. This "restriction" can be set with
> KERNFS_ROOT_INVARIANT_PARENT. Otherwise the lock is required.
> 
> Rename kernfs_node::parent to kernfs_node::__parent to denote the RCU
> access and use RCU accessor while accessing the node.
> Make cgroup use KERNFS_ROOT_INVARIANT_PARENT since the parent here can
> not change.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun