[PATCH v2] greybus/uart: Fix atomicity violation in get_serial_info()

[Qiu-ji Chen <chenqiuji666@gmail.com>]

Our static checker found a bug where set_serial_info() uses a mutex, but 
get_serial_info() does not. Fortunately, the impact of this is relatively 
minor. It doesn't cause a crash or any other serious issues. However, if a 
race condition occurs between set_serial_info() and get_serial_info(), 
there is a chance that the data returned by get_serial_info() will be 
meaningless.

Signed-off-by: Qiu-ji Chen <chenqiuji666@gmail.com>
Fixes: 0aad5ad563c8 ("greybus/uart: switch to ->[sg]et_serial()")
---
V2:
Modified the patch description to make it more concise and easier to understand.
Changed the fix code to ensure the logic is correct.
Thanks to Johan Hovold and Dan Carpenter for helpful suggestion.
---
 drivers/staging/greybus/uart.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c
index cdf4ebb93b10..8eab94cb06fa 100644
--- a/drivers/staging/greybus/uart.c
+++ b/drivers/staging/greybus/uart.c
@@ -596,11 +596,13 @@ static int get_serial_info(struct tty_struct *tty,
 	struct gb_tty *gb_tty = tty->driver_data;
 
 	ss->line = gb_tty->minor;
+	mutex_lock(&gb_tty->port.mutex);
 	ss->close_delay = jiffies_to_msecs(gb_tty->port.close_delay) / 10;
 	ss->closing_wait =
 		gb_tty->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
 		ASYNC_CLOSING_WAIT_NONE :
 		jiffies_to_msecs(gb_tty->port.closing_wait) / 10;
+	mutex_unlock(&gb_tty->port.mutex);
 
 	return 0;
 }
-- 
2.34.1