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>

1 files changed, 6 insertions, 2 deletions

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 0f02351c9239..ece2049bd270 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/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.
          */
-        spin_lock_init(&port->lock);
-        lockdep_set_class(&port->lock, &port_lock_key);
+        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));