iucv: fix cpu hotplug

If the iucv module is compiled in/loaded but no user is registered cpu
hot remove doesn't work. Reason for that is that the iucv cpu hotplug
notifier on CPU_DOWN_PREPARE checks if the iucv_buffer_cpumask would
be empty after the corresponding bit would be cleared. However the bit
was never set since iucv wasn't enable. That causes all cpu hot unplug
operations to fail in this scenario.
To fix this use iucv_path_table as an indicator wether iucv is enabled
or not.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NUrsula Braun <ursula.braun@de.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/iucv/iucv.c

@@ -517,6 +517,7 @@ static int iucv_enable(void)
 	size_t alloc_size;
 	int cpu, rc;
 
+	get_online_cpus();
 	rc = -ENOMEM;
 	alloc_size = iucv_max_pathid * sizeof(struct iucv_path);
 	iucv_path_table = kzalloc(alloc_size, GFP_KERNEL);
@@ -524,19 +525,17 @@ static int iucv_enable(void)
 		goto out;
 	/* Declare per cpu buffers. */
 	rc = -EIO;
-	get_online_cpus();
 	for_each_online_cpu(cpu)
 		smp_call_function_single(cpu, iucv_declare_cpu, NULL, 1);
 	if (cpus_empty(iucv_buffer_cpumask))
 		/* No cpu could declare an iucv buffer. */
-		goto out_path;
+		goto out;
 	put_online_cpus();
 	return 0;
-
-out_path:
-	put_online_cpus();
-	kfree(iucv_path_table);
 out:
+	kfree(iucv_path_table);
+	iucv_path_table = NULL;
+	put_online_cpus();
 	return rc;
 }
 
@@ -551,8 +550,9 @@ static void iucv_disable(void)
 {
 	get_online_cpus();
 	on_each_cpu(iucv_retrieve_cpu, NULL, 1);
-	put_online_cpus();
 	kfree(iucv_path_table);
+	iucv_path_table = NULL;
+	put_online_cpus();
 }
 
 static int __cpuinit iucv_cpu_notify(struct notifier_block *self,
@@ -589,10 +589,14 @@ static int __cpuinit iucv_cpu_notify(struct notifier_block *self,
 	case CPU_ONLINE_FROZEN:
 	case CPU_DOWN_FAILED:
 	case CPU_DOWN_FAILED_FROZEN:
+		if (!iucv_path_table)
+			break;
 		smp_call_function_single(cpu, iucv_declare_cpu, NULL, 1);
 		break;
 	case CPU_DOWN_PREPARE:
 	case CPU_DOWN_PREPARE_FROZEN:
+		if (!iucv_path_table)
+			break;
 		cpumask = iucv_buffer_cpumask;
 		cpu_clear(cpu, cpumask);
 		if (cpus_empty(cpumask))