serial: core: Initialise spin lock before use in uart_configure_port()

The comment near to uart_port_spin_lock_init() says:

  Ensure that the serial console lock is initialised early.
  If this port is a console, then the spinlock is already initialised.

and there is nothing about enabled or disabled consoles. The commit
a3cb39d ("serial: core: Allow detach and attach serial device
for console") made a change, which follows the comment, and also to
prevent reinitialisation of the lock in use, when user detaches and
attaches back the same console device. But this change discovers
another issue, that uart_add_one_port() tries to access a spin lock
that now may be uninitialised. This happens when a driver expects
the serial core to register a console on its behalf. In this case
we must initialise a spin lock before use.

Fixes: a3cb39d ("serial: core: Allow detach and attach serial device for console")
Reported-by: Marc Zyngier <maz@kernel.org>
Reported-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: Anatoly Pugachev <matorola@gmail.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20200706214903.56148-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1914,6 +1914,12 @@ static inline bool uart_console_enabled(struct uart_port *port)
 	return uart_console(port) && (port->cons->flags & CON_ENABLED);
 }
 
+static void __uart_port_spin_lock_init(struct uart_port *port)
+{
+	spin_lock_init(&port->lock);
+	lockdep_set_class(&port->lock, &port_lock_key);
+}
+
 /*
  * Ensure that the serial console lock is initialised early.
  * If this port is a console, then the spinlock is already initialised.
@@ -1923,8 +1929,7 @@ static inline void uart_port_spin_lock_init(struct uart_port *port)
 	if (uart_console(port))
 		return;
 
-	spin_lock_init(&port->lock);
-	lockdep_set_class(&port->lock, &port_lock_key);
+	__uart_port_spin_lock_init(port);
 }
 
 #if defined(CONFIG_SERIAL_CORE_CONSOLE) || defined(CONFIG_CONSOLE_POLL)
@@ -2370,6 +2375,13 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state,
 		/* Power up port for set_mctrl() */
 		uart_change_pm(state, UART_PM_STATE_ON);
 
+		/*
+		 * If this driver supports console, and it hasn't been
+		 * successfully registered yet, initialise spin lock for it.
+		 */
+		if (port->cons && !(port->cons->flags & CON_ENABLED))
+			__uart_port_spin_lock_init(port);
+
 		/*
 		 * Ensure that the modem control lines are de-activated.
 		 * keep the DTR setting that is set in uart_set_options()