usb: cdc-acm: move ldisc dcd notification outside of acm's read lock

dcd_change notification call moved outside of the acm->read_lock
to protect any future tty ldisc that calls wait_serial_change()
Signed-off-by: Dan Drown <dan-netdev@drown.org>
Acked-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/ZN1zV/zjPgpGlHXo@vps3.drown.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -319,23 +319,24 @@ static void acm_process_notification(struct acm *acm, unsigned char *buf)
 		}
 
 		difference = acm->ctrlin ^ newctrl;
+
+		if ((difference & USB_CDC_SERIAL_STATE_DCD) && acm->port.tty) {
+			struct tty_ldisc *ld = tty_ldisc_ref(acm->port.tty);
+			if (ld) {
+				if (ld->ops->dcd_change)
+					ld->ops->dcd_change(acm->port.tty, newctrl & USB_CDC_SERIAL_STATE_DCD);
+				tty_ldisc_deref(ld);
+			}
+		}
+
 		spin_lock_irqsave(&acm->read_lock, flags);
 		acm->ctrlin = newctrl;
 		acm->oldcount = acm->iocount;
 
 		if (difference & USB_CDC_SERIAL_STATE_DSR)
 			acm->iocount.dsr++;
-		if (difference & USB_CDC_SERIAL_STATE_DCD) {
-			if (acm->port.tty) {
-				struct tty_ldisc *ld = tty_ldisc_ref(acm->port.tty);
-				if (ld) {
-					if (ld->ops->dcd_change)
-						ld->ops->dcd_change(acm->port.tty, newctrl & USB_CDC_SERIAL_STATE_DCD);
-					tty_ldisc_deref(ld);
-				}
-			}
+		if (difference & USB_CDC_SERIAL_STATE_DCD)
 			acm->iocount.dcd++;
-		}
 		if (newctrl & USB_CDC_SERIAL_STATE_BREAK) {
 			acm->iocount.brk++;
 			tty_insert_flip_char(&acm->port, 0, TTY_BREAK);