s390/dasd: fix panic during offline processing

A DASD device consists of the device itself and a discipline with a
corresponding private structure. These fields are set up during online
processing right after the device is created and before it is processed by
the state machine and made available for I/O.
During offline processing the discipline pointer and the private data gets
freed within the state machine and without protection of the existing
reference count. This might lead to a kernel panic because a function might
have taken a device reference and accesses the discipline pointer and/or
private data of the device while this is already freed.

Fix by freeing the discipline pointer and the private data after ensuring
that there is no reference to the device left.

Reviewed-by: Peter Oberparleiter <[e-mail]>
Signed-off-by: Stefan Haberland <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

drivers/s390/block/dasd.c

···
 {
 	/* Disable extended error reporting for this device. */
 	dasd_eer_disable(device);
-	/* Forget the discipline information. */
-	if (device->discipline) {
-		if (device->discipline->uncheck_device)
-			device->discipline->uncheck_device(device);
-		module_put(device->discipline->owner);
-	}
-	device->discipline = NULL;
-	if (device->base_discipline)
-		module_put(device->base_discipline->owner);
-	device->base_discipline = NULL;
 	device->state = DASD_STATE_NEW;
 
 	if (device->block)
···
 	return 0;
 }
 EXPORT_SYMBOL_GPL(dasd_generic_probe);
+
+void dasd_generic_free_discipline(struct dasd_device *device)
+{
+	/* Forget the discipline information. */
+	if (device->discipline) {
+		if (device->discipline->uncheck_device)
+			device->discipline->uncheck_device(device);
+		module_put(device->discipline->owner);
+		device->discipline = NULL;
+	}
+	if (device->base_discipline) {
+		module_put(device->base_discipline->owner);
+		device->base_discipline = NULL;
+	}
+}
+EXPORT_SYMBOL_GPL(dasd_generic_free_discipline);
 
 /*
  * This will one day be called from a global not_oper handler.

drivers/s390/block/dasd_devmap.c

···
 	/* Wait for reference counter to drop to zero. */
 	wait_event(dasd_delete_wq, atomic_read(&device->ref_count) == 0);
 
+	dasd_generic_free_discipline(device);
 	/* Disconnect dasd_device structure from ccw_device structure. */
 	cdev = device->cdev;
 	device->cdev = NULL;

drivers/s390/block/dasd_int.h

···
 int  dasd_cancel_req(struct dasd_ccw_req *);
 int dasd_flush_device_queue(struct dasd_device *);
 int dasd_generic_probe (struct ccw_device *, struct dasd_discipline *);
+void dasd_generic_free_discipline(struct dasd_device *);
 void dasd_generic_remove (struct ccw_device *cdev);
 int dasd_generic_set_online(struct ccw_device *, struct dasd_discipline *);
 int dasd_generic_set_offline (struct ccw_device *cdev);