powerpc/pseries: Verify CPU doesn't exist before adding

When DLPAR adding a CPU we should verify that the CPU does not already
exist. Failure to do so can generate a kernel oops;

[    9.465585] kernel BUG at arch/powerpc/platforms/pseries/dlpar.c:382!
[    9.465796] Oops: Exception in kernel mode, sig: 5 [#1]

This oops can be generated by causing a probe to be performed on a cpu
by writing to the sysfs cpu probe file (/sys/devices/system/cpu/probe).
This patch adds a check for the existence of cpu prior to probing the cpu
so userspace doing the wrong thing won't trigger a BUG_ON().

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/pseries/dlpar.c

@@ -381,6 +381,32 @@ out:
 
 }
 
+static bool dlpar_cpu_exists(struct device_node *parent, u32 drc_index)
+{
+	struct device_node *child = NULL;
+	u32 my_drc_index;
+	bool found;
+	int rc;
+
+	/* Assume cpu doesn't exist */
+	found = false;
+
+	for_each_child_of_node(parent, child) {
+		rc = of_property_read_u32(child, "ibm,my-drc-index",
+					  &my_drc_index);
+		if (rc)
+			continue;
+
+		if (my_drc_index == drc_index) {
+			of_node_put(child);
+			found = true;
+			break;
+		}
+	}
+
+	return found;
+}
+
 static ssize_t dlpar_cpu_probe(const char *buf, size_t count)
 {
 	struct device_node *dn, *parent;
@@ -391,14 +417,23 @@ static ssize_t dlpar_cpu_probe(const char *buf, size_t count)
 	if (rc)
 		return -EINVAL;
 
-	rc = dlpar_acquire_drc(drc_index);
-	if (rc)
-		return -EINVAL;
-
 	parent = of_find_node_by_path("/cpus");
 	if (!parent)
 		return -ENODEV;
 
+	if (dlpar_cpu_exists(parent, drc_index)) {
+		of_node_put(parent);
+		printk(KERN_WARNING "CPU with drc index %x already exists\n",
+		       drc_index);
+		return -EINVAL;
+	}
+
+	rc = dlpar_acquire_drc(drc_index);
+	if (rc) {
+		of_node_put(parent);
+		return -EINVAL;
+	}
+
 	dn = dlpar_configure_connector(cpu_to_be32(drc_index), parent);
 	of_node_put(parent);
 	if (!dn) {