Re: [PATCH] powerpc/pseries: Fix dn reference error in dlpar_cpu_remove_by_index

[Tyrel Datwyler <tyreld@linux.vnet.ibm.com>]

On 02/19/2019 07:46 AM, Michael Bringmann wrote:
> powerpc/pseries: Fix dn reference error in dlpar_cpu_remove_by_index()
> 
> A reference to the device node of the CPU to be removed is released
> upon successful removal of the associated CPU device.  If the call
> to remove the CPU device fails, dlpar_cpu_remove_by_index() still
> frees the reference and this leads to miscomparisons and/or
> addressing errors later on.
> 
> This problem may be observed when trying to DLPAR 'hot-remove' a CPU
> from a system that has only a single CPU.  The operation will fail
> because there is no other CPU to which the kernel operations may be
> migrated, but the refcount will still be decremented.
> 
> Signed-off-by: Michael Bringmann <mwb@linux.vnet.ibm.com>
> 
> 
> diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> index 97feb6e..9537bb9 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> @@ -635,7 +635,8 @@ static int dlpar_cpu_remove_by_index(u32 drc_index)
>  	}
> 
>  	rc = dlpar_cpu_remove(dn, drc_index);
> -	of_node_put(dn);
> +	if (!rc)
> +		of_node_put(dn);
>  	return rc;
>  }
> 

NACK!

The logic here is wrong. Here is the full function.

static int dlpar_cpu_remove_by_index(u32 drc_index)
{
        struct device_node *dn;
        int rc;

        dn = cpu_drc_index_to_dn(drc_index);
        if (!dn) {
                pr_warn("Cannot find CPU (drc index %x) to remove\n",
                        drc_index);
                return -ENODEV;
        }

        rc = dlpar_cpu_remove(dn, drc_index);
        of_node_put(dn);
        return rc;
}

The call to cpu_drc_index_to_dn() returns a device_node with the reference count
incremented. So, regardless of the success or failure of the call to
dlpar_cpu_remove() you need to release that reference.

If there is a reference counting issue it is somewhere else.

-Tyrel