--- zzzz-none-000/linux-3.10.107/drivers/usb/misc/appledisplay.c	2017-06-27 09:49:32.000000000 +0000
+++ scorpion-7490-727/linux-3.10.107/drivers/usb/misc/appledisplay.c	2021-02-04 17:41:59.000000000 +0000
@@ -81,6 +81,7 @@
 	struct delayed_work work;
 	int button_pressed;
 	spinlock_t lock;
+	struct mutex sysfslock;		/* concurrent read and write */
 };
 
 static atomic_t count_displays = ATOMIC_INIT(0);
@@ -110,7 +111,7 @@
 			__func__, status);
 		return;
 	default:
-		dev_dbg(dev, "%s - nonzero urb status received: %d/n",
+		dev_dbg(dev, "%s - nonzero urb status received: %d\n",
 			__func__, status);
 		goto exit;
 	}
@@ -144,6 +145,7 @@
 	struct appledisplay *pdata = bl_get_data(bd);
 	int retval;
 
+	mutex_lock(&pdata->sysfslock);
 	pdata->msgdata[0] = 0x10;
 	pdata->msgdata[1] = bd->props.brightness;
 
@@ -156,15 +158,17 @@
 		0,
 		pdata->msgdata, 2,
 		ACD_USB_TIMEOUT);
-
+	mutex_unlock(&pdata->sysfslock);
+	
 	return retval;
 }
 
 static int appledisplay_bl_get_brightness(struct backlight_device *bd)
 {
 	struct appledisplay *pdata = bl_get_data(bd);
-	int retval;
+	int retval, brightness;
 
+	mutex_lock(&pdata->sysfslock);
 	retval = usb_control_msg(
 		pdata->udev,
 		usb_rcvctrlpipe(pdata->udev, 0),
@@ -174,11 +178,13 @@
 		0,
 		pdata->msgdata, 2,
 		ACD_USB_TIMEOUT);
+	brightness = pdata->msgdata[1];
+	mutex_unlock(&pdata->sysfslock);
 
 	if (retval < 0)
 		return retval;
 	else
-		return pdata->msgdata[1];
+		return brightness;
 }
 
 static const struct backlight_ops appledisplay_bl_data = {
@@ -241,6 +247,7 @@
 
 	spin_lock_init(&pdata->lock);
 	INIT_DELAYED_WORK(&pdata->work, appledisplay_work);
+	mutex_init(&pdata->sysfslock);
 
 	/* Allocate buffer for control messages */
 	pdata->msgdata = kmalloc(ACD_MSG_BUFFER_LEN, GFP_KERNEL);
@@ -322,7 +329,7 @@
 					pdata->urbdata, pdata->urb->transfer_dma);
 			usb_free_urb(pdata->urb);
 		}
-		if (pdata->bd && !IS_ERR(pdata->bd))
+		if (!IS_ERR(pdata->bd))
 			backlight_device_unregister(pdata->bd);
 		kfree(pdata->msgdata);
 	}