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

Tyrel Datwyler [off-list ref] writes:

On 09/20/2017 04:39 AM, Michael Ellerman wrote:

quoted

Rob Herring [off-list ref] writes:

quoted

On Fri, Sep 15, 2017 at 6:04 AM, abdul [off-list ref] wrote:

quoted

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