Subject: Re: [PATCH v3 3/3] powerpc/pseries/dlpar: Add device tree nodes for DLPAR IO add

From: Michael Ellerman <[email-masked]>
Date: Wed, 28 Aug 2024 18:12:52 +1000

Hi Haren,

One query below about the of_node refcounting.

Haren Myneni <[email-masked]> writes:
> In the powerpc-pseries specific implementation, the IO hotplug
> event is handled in the user space (drmgr tool). For the DLPAR
> IO ADD, the corresponding device tree nodes and properties will
> be added to the device tree after the device enable. The user
> space (drmgr tool) uses configure_connector RTAS call with the
> DRC index to retrieve the device nodes and updates the device
> tree by writing to /proc/ppc64/ofdt. Under system lockdown,
> /dev/mem access to allocate buffers for configure_connector RTAS
> call is restricted which means the user space can not issue this
> RTAS call and also can not access to /proc/ppc64/ofdt. The
> pseries implementation need user interaction to power-on and add
> device to the slot during the ADD event handling. So adds
> complexity if the complete hotplug ADD event handling moved to
> the kernel.
>
> To overcome /dev/mem access restriction, this patch extends the
> /sys/kernel/dlpar interface and provides ‘dt add index <drc_index>’
> to the user space. The drmgr tool uses this interface to update
> the device tree whenever the device is added. This interface
> retrieves device tree nodes for the corresponding DRC index using
> the configure_connector RTAS call and adds new device nodes /
> properties to the device tree.
>
> Signed-off-by: Scott Cheloha <[email-masked]>
> Signed-off-by: Haren Myneni <[email-masked]>
> ---
>  arch/powerpc/platforms/pseries/dlpar.c | 130 +++++++++++++++++++++++++
>  1 file changed, 130 insertions(+)
>
> diff --git a/arch/powerpc/platforms/pseries/dlpar.c b/arch/powerpc/platforms/pseries/dlpar.c
> index 1b49b47c4a4f..6f0bc3ddbf85 100644
> --- a/arch/powerpc/platforms/pseries/dlpar.c
> +++ b/arch/powerpc/platforms/pseries/dlpar.c
...
> @@ -330,6 +345,118 @@ int dlpar_unisolate_drc(u32 drc_index)
>  	return 0;
>  }
>  
> +static struct device_node *
> +get_device_node_with_drc_index(u32 index)
> +{
> +	struct device_node *np = NULL;
> +	u32 node_index;
> +	int rc;
> +
> +	for_each_node_with_property(np, "ibm,my-drc-index") {
> +		rc = of_property_read_u32(np, "ibm,my-drc-index",
> +					     &node_index);
> +		if (rc) {
> +			pr_err("%s: %pOF: of_property_read_u32 %s: %d\n",
> +			       __func__, np, "ibm,my-drc-index", rc);
> +			of_node_put(np);
> +			return NULL;
> +		}
> +
> +		if (index == node_index)
> +			break;

Here we return with np's refcount elevated.

> +	}
> +
> +	return np;
> +}
...
> +
> +static int dlpar_hp_dt_add(u32 index)
> +{
> +	struct device_node *np, *nodes;
> +	struct of_changeset ocs;
> +	int rc;
> +
> +	/*
> +	 * Do not add device node(s) if already exists in the
> +	 * device tree.
> +	 */
> +	np = get_device_node_with_drc_index(index);
> +	if (np) {
> +		pr_err("%s: Adding device node for index (%d), but "
> +				"already exists in the device tree\n",
> +				__func__, index);
> +		rc = -EINVAL;
> +		goto out;
 
In the error case you drop the reference on np (at out).

> +	}

> +	np = get_device_node_with_drc_info(index);
> 
But in the success case np is reassigned, so the refcount is leaked.
I think that's unintentional, but I'm not 100% sure.

> +	if (!np)
> +		return -EIO;
> +
> +	/* Next, configure the connector. */
> +	nodes = dlpar_configure_connector(cpu_to_be32(index), np);
> +	if (!nodes) {
> +		rc = -EIO;
> +		goto out;
> +	}
> +
> +	/*
> +	 * Add the new nodes from dlpar_configure_connector() onto
> +	 * the device-tree.
> +	 */
> +	of_changeset_init(&ocs);
> +	rc = dlpar_changeset_attach_cc_nodes(&ocs, nodes);
> +
> +	if (!rc)
> +		rc = of_changeset_apply(&ocs);
> +	else
> +		dlpar_free_cc_nodes(nodes);
> +
> +	of_changeset_destroy(&ocs);
> +
> +out:
> +	of_node_put(np);
> +	return rc;
> +}
> +
>  static int changeset_detach_node_recursive(struct of_changeset *ocs,
>  					struct device_node *node)
>  {

cheers