serial_core: Don't re-initialize a previously initialized spinlock.

The uart_set_options() code unconditionally initalizes the spinlock
on the port. This can cause a deadlock in some situations.

One instance that exposed the problem, was when writing to
/sys/module/kgdboc/parameters/kgdboc to use ttyS0 when the console
is already running on ttyS0. If the spinlock is re-initialized
while the lock is held due to output to the console, there
is a deadlock.

Assume the spinlock is initialized if the port is a console.
Signed-off-by: Randy Witt <rewitt@declaratino.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1830,9 +1830,13 @@ uart_set_options(struct uart_port *port, struct console *co,
 	/*
 	 * Ensure that the serial console lock is initialised
 	 * early.
+	 * If this port is a console, then the spinlock is already
+	 * initialised.
 	 */
+	if (!(uart_console(port) && (port->cons->flags & CON_ENABLED))) {
 		spin_lock_init(&port->lock);
 		lockdep_set_class(&port->lock, &port_lock_key);
+	}
 
 	memset(&termios, 0, sizeof(struct ktermios));