serial: 8250: lock port for UART_IER access in omap8250_irq()

omap8250_irq() accesses UART_IER. This register is modified twice
by each console write (serial8250_console_write()) under the port
lock. omap8250_irq() must also take the port lock to guanentee
synchronized access to UART_IER.

Since the port lock is already being taken for the stop_rx() callback
and since it is safe to call cancel_delayed_work() while holding the
port lock, simply extend the port lock region to include UART_IER
access.

Fixes: 1fe0e1fa ("serial: 8250_omap: Handle optional overrun-throttle-ms property")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20230525093159.223817-8-john.ogness@linutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_omap.c

@@ -659,17 +659,18 @@ static irqreturn_t omap8250_irq(int irq, void *dev_id)
 	if ((lsr & UART_LSR_OE) && up->overrun_backoff_time_ms > 0) {
 		unsigned long delay;
 
+		/* Synchronize UART_IER access against the console. */
+		spin_lock(&port->lock);
 		up->ier = port->serial_in(port, UART_IER);
 		if (up->ier & (UART_IER_RLSI | UART_IER_RDI)) {
-			spin_lock(&port->lock);
 			port->ops->stop_rx(port);
-			spin_unlock(&port->lock);
 		} else {
 			/* Keep restarting the timer until
 			 * the input overrun subsides.
 			 */
 			cancel_delayed_work(&up->overrun_backoff);
 		}
+		spin_unlock(&port->lock);
 
 		delay = msecs_to_jiffies(up->overrun_backoff_time_ms);
 		schedule_delayed_work(&up->overrun_backoff, delay);