s390/dasd: Fix locking issue when changing RO attribute

The function dasd_ro_store() calls set_disk_ro() to set the device in
question read-only. Since set_disk_ro() might sleep, we can't call it
while holding a lock. However, we also can't simply check if the device,
block, and gdp references are valid before we call set_disk_ro() because
an offline processing might have been started in the meanwhile which
will destroy those references.

In order to reliably call set_disk_ro() we have to ensure several
things:

- Still check validity of the mentioned references but additionally
  check if offline processing is running and bail out accordingly. Also,
  do this while holding the device lock.
- To ensure that the block device is still safe after the lock, increase
  the open_count while still holding the device lock.
Reviewed-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dasd_devmap.c

@@ -761,6 +761,7 @@ dasd_ro_store(struct device *dev, struct device_attribute *attr,
 {
 	struct ccw_device *cdev = to_ccwdev(dev);
 	struct dasd_device *device;
+	unsigned long flags;
 	unsigned int val;
 	int rc;
 
@@ -775,10 +776,22 @@ dasd_ro_store(struct device *dev, struct device_attribute *attr,
 	if (IS_ERR(device))
 		return PTR_ERR(device);
 
+	spin_lock_irqsave(get_ccwdev_lock(cdev), flags);
 	val = val || test_bit(DASD_FLAG_DEVICE_RO, &device->flags);
-	if (device->block && device->block->gdp)
+
+	if (!device->block || !device->block->gdp ||
+	    test_bit(DASD_FLAG_OFFLINE, &device->flags)) {
+		spin_unlock_irqrestore(get_ccwdev_lock(cdev), flags);
+		goto out;
+	}
+	/* Increase open_count to avoid losing the block device */
+	atomic_inc(&device->block->open_count);
+	spin_unlock_irqrestore(get_ccwdev_lock(cdev), flags);
+
 	set_disk_ro(device->block->gdp, val);
+	atomic_dec(&device->block->open_count);
 
+out:
 	dasd_put_device(device);
 
 	return count;