diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 09475695effd..e7882611da0e 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2275,6 +2275,10 @@ int serial8250_do_startup(struct uart_port *port)
 
 	if (port->irq && !(up->port.flags & UPF_NO_THRE_TEST)) {
 		unsigned char iir1;
+
+		if (port->irqflags & IRQF_SHARED)
+			disable_irq_nosync(port->irq);
+
 		/*
 		 * Test for UARTs that do not reassert THRE when the
 		 * transmitter is idle and the interrupt has already
@@ -2284,8 +2288,6 @@ int serial8250_do_startup(struct uart_port *port)
 		 * allow register changes to become visible.
 		 */
 		spin_lock_irqsave(&port->lock, flags);
-		if (up->port.irqflags & IRQF_SHARED)
-			disable_irq_nosync(port->irq);
 
 		wait_for_xmitr(up, UART_LSR_THRE);
 		serial_port_out_sync(port, UART_IER, UART_IER_THRI);
@@ -2297,9 +2299,10 @@ int serial8250_do_startup(struct uart_port *port)
 		iir = serial_port_in(port, UART_IIR);
 		serial_port_out(port, UART_IER, 0);
 
+		spin_unlock_irqrestore(&port->lock, flags);
+
 		if (port->irqflags & IRQF_SHARED)
 			enable_irq(port->irq);
-		spin_unlock_irqrestore(&port->lock, flags);
 
 		/*
 		 * If the interrupt is not reasserted, or we otherwise