scsi: ch: fixup refcounting imbalance for SCSI devices

The SCSI device is required to be present during ch_probe() and ch_open().
But the SCSI device itself is only checked during ch_open(), so it's anyones
guess if it had been present during ch_probe(). And consequently we can't
reliably detach it during ch_release(), as ch_remove() might have been
called first.  So initialize the changer device during ch_probe(), and take
a reference to the SCSI device during both ch_probe() and ch_open().

[mkp: fixed checkpatch warning]

Link: https://lore.kernel.org/r/20200213153207.123357-2-hare@suse.deReviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/ch.c

@@ -569,6 +569,7 @@ static void ch_destroy(struct kref *ref)
 {
 	scsi_changer *ch = container_of(ref, scsi_changer, ref);
 
+	ch->device = NULL;
 	kfree(ch->dt);
 	kfree(ch);
 }
@@ -594,14 +595,17 @@ ch_open(struct inode *inode, struct file *file)
 	spin_lock(&ch_index_lock);
 	ch = idr_find(&ch_index_idr, minor);
 
-	if (NULL == ch || scsi_device_get(ch->device)) {
+	if (ch == NULL || !kref_get_unless_zero(&ch->ref)) {
 		spin_unlock(&ch_index_lock);
 		mutex_unlock(&ch_mutex);
 		return -ENXIO;
 	}
-	kref_get(&ch->ref);
 	spin_unlock(&ch_index_lock);
-
+	if (scsi_device_get(ch->device)) {
+		kref_put(&ch->ref, ch_destroy);
+		mutex_unlock(&ch_mutex);
+		return -ENXIO;
+	}
 	file->private_data = ch;
 	mutex_unlock(&ch_mutex);
 	return 0;
@@ -938,6 +942,12 @@ static int ch_probe(struct device *dev)
 
 	ch->minor = ret;
 	sprintf(ch->name,"ch%d",ch->minor);
+	ret = scsi_device_get(sd);
+	if (ret) {
+		sdev_printk(KERN_WARNING, sd, "ch%d: failed to get device\n",
+			    ch->minor);
+		goto remove_idr;
+	}
 
 	class_dev = device_create(ch_sysfs_class, dev,
 				  MKDEV(SCSI_CHANGER_MAJOR, ch->minor), ch,
@@ -946,7 +956,7 @@ static int ch_probe(struct device *dev)
 		sdev_printk(KERN_WARNING, sd, "ch%d: device_create failed\n",
 			    ch->minor);
 		ret = PTR_ERR(class_dev);
-		goto remove_idr;
+		goto put_device;
 	}
 
 	mutex_init(&ch->lock);
@@ -964,6 +974,8 @@ static int ch_probe(struct device *dev)
 	return 0;
 destroy_dev:
 	device_destroy(ch_sysfs_class, MKDEV(SCSI_CHANGER_MAJOR, ch->minor));
+put_device:
+	scsi_device_put(sd);
 remove_idr:
 	idr_remove(&ch_index_idr, ch->minor);
 free_ch:
@@ -977,9 +989,11 @@ static int ch_remove(struct device *dev)
 
 	spin_lock(&ch_index_lock);
 	idr_remove(&ch_index_idr, ch->minor);
+	dev_set_drvdata(dev, NULL);
 	spin_unlock(&ch_index_lock);
 
 	device_destroy(ch_sysfs_class, MKDEV(SCSI_CHANGER_MAJOR,ch->minor));
+	scsi_device_put(ch->device);
 	kref_put(&ch->ref, ch_destroy);
 	return 0;
 }