8250.c: port.lock is irq-safe

serial8250_startup() doesn't disable interrupts while taking the &up->port.lock
which might race against the interrupt handler serial8250_interrupt(), which
when entered, will deadlock waiting for the lock to be released.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Tested-by: Ingo Molnar <mingo@elte.hu>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/8250.c

@@ -1885,7 +1885,7 @@ static int serial8250_startup(struct uart_port *port)
 		 * the interrupt is enabled.  Delays are necessary to
 		 * allow register changes to become visible.
 		 */
-		spin_lock(&up->port.lock);
+		spin_lock_irqsave(&up->port.lock, flags);
 		if (up->port.flags & UPF_SHARE_IRQ)
 			disable_irq_nosync(up->port.irq);
 
@@ -1901,7 +1901,7 @@ static int serial8250_startup(struct uart_port *port)
 
 		if (up->port.flags & UPF_SHARE_IRQ)
 			enable_irq(up->port.irq);
-		spin_unlock(&up->port.lock);
+		spin_unlock_irqrestore(&up->port.lock, flags);
 
 		/*
 		 * If the interrupt is not reasserted, setup a timer to