linux-kernel - Re: [PATCH] software node: recursively unregister child swnodes

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Date:   Thu, 4 Jun 2020 22:15:23 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     jorhand@...ux.microsoft.com
Cc:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, linux-next@...r.kernel.org
Subject: Re: [PATCH] software node: recursively unregister child swnodes

On Thu, Jun 04, 2020 at 12:36:23PM -0700, jorhand@...ux.microsoft.com wrote:
> From: Jordan Hand <jorhand@...ux.microsoft.com>
> 
> If a child swnode is unregistered after it's parent, it can lead to
> undefined behavior.

Crashing the system is not really "undefined" :)

> When a swnode is unregistered, recursively free it's children to avoid
> this condition.

Are you sure?  Why would you be unregistering a child after it's parent?
Why not just do not do that?

> 
> Signed-off-by: Jordan Hand <jorhand@...ux.microsoft.com>
> ---
>  drivers/base/swnode.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/base/swnode.c b/drivers/base/swnode.c
> index e5eb27375416..e63093b1542b 100644
> --- a/drivers/base/swnode.c
> +++ b/drivers/base/swnode.c
> @@ -619,6 +619,8 @@ static void software_node_release(struct kobject *kobj)
>  		property_entries_free(swnode->node->properties);
>  		kfree(swnode->node);
>  	}
> +
> +	list_del(&kobj->entry);
>  	ida_destroy(&swnode->child_ids);
>  	kfree(swnode);
>  }
> @@ -712,11 +714,6 @@ EXPORT_SYMBOL_GPL(software_node_register_nodes);
>   * @nodes: Zero terminated array of software nodes to be unregistered
>   *
>   * Unregister multiple software nodes at once.
> - *
> - * NOTE: Be careful using this call if the nodes had parent pointers set up in
> - * them before registering.  If so, it is wiser to remove the nodes
> - * individually, in the correct order (child before parent) instead of relying
> - * on the sequential order of the list of nodes in the array.
>   */
>  void software_node_unregister_nodes(const struct software_node *nodes)
>  {
> @@ -839,10 +836,16 @@ EXPORT_SYMBOL_GPL(fwnode_create_software_node);
>  void fwnode_remove_software_node(struct fwnode_handle *fwnode)
>  {
>  	struct swnode *swnode = to_swnode(fwnode);
> +	struct swnode *child = NULL;
>  
>  	if (!swnode)
>  		return;
>  
> +	while (!list_empty(&swnode->children)) {
> +		child = list_first_entry_or_null(&swnode->children, struct swnode, entry);
> +		fwnode_remove_software_node(&child->fwnode);

You should document that you just changed the behaivor here, as you are
now really doing fwnode_remove_software_node_and_all_children().

but again, why?  Who wants to unregister a child before a parent?

thanks,

greg k-h