[mainline][DLPAR][Oops] OF: ERROR: Bad of_node_put() on /cpus

[Michael Ellerman]

Tyrel Datwyler <tyreld at linux.vnet.ibm.com> writes:
> On 09/20/2017 04:39 AM, Michael Ellerman wrote:
>> Rob Herring <robh at kernel.org> writes:
>>> On Fri, Sep 15, 2017 at 6:04 AM, abdul <abdhalee at linux.vnet.ibm.com> wrote:
>>>>
>>>> Mainline kernel panics during DLPAR CPU add/remove operation.
>>>>
>>>> Machine Type: Power8 PowerVM LPAR
>>>> kernel 4.13.0
>>>
>>> Did 4.12 work or when was it last working? I'm not seeing anything
>>> recent in the DT code that looks suspicious.
>> 
>> I'm pretty sure it's:
>> 
>> int dlpar_attach_node(struct device_node *dn, struct device_node *parent)
>> {
>> 	int rc;
>> 
>> 	dn->parent = parent;
>> 
>> 	rc = of_attach_node(dn);
>> 	if (rc) {
>> 		printk(KERN_ERR "Failed to add device node %pOF\n", dn);
>> 		return rc;
>> 	}
>> 
>> 	of_node_put(dn->parent);
>> HERE    ^^^^^^^^^^
>> 
>> 	return 0;
>> }
>> 
>> 
>> Prior to 215ee763f8cb ("powerpc: pseries: remove dlpar_attach_node
>> dependency on full path"), we re-looked up the parent, and got another
>> reference on it. That meant the put before the return there was correct.
>> But now it's not because the caller has a reference to parent but it's
>> not ours to drop.
>> 
>> Testing a fix, will report back.
>
> So, that patch slipped past me. Not only is the parent reference not ours to drop, but
> when I went and looked at dlpar_cpu_add() I also noticed that of_node_put() was done on
> the parent prior to the call to dlpar_attach_node(). With the addition of "parent" to the
> dlpar_attach_node() parameter list dlpar_cpu_add() needs to be fixed up to hold the
> "parent" reference until after dlpar_attach_node() returns.

Yep. I wrote the same patch :)

Rob asked me to test it, which I did, but /cpus starts out with an
elevated ref count, so you have to do ~30 (on my system) DLPAR removes
to hit the bug, which I didn't do.

I've updated my test script to do roughly $(nproc) x 10 DLPAR removes,
which is hopefully sufficient to catch these bugs in future.

cheers