yet another uart race condition fix (sonic-net#75)

Signed-off-by: Guohan Lu <gulv@microsoft.com>

patch/driver-uart-fix-another-race-between-uart_put_char-and-uart_shutdow.patch

@@ -4,11 +4,11 @@ From: Guohan Lu <gulv@microsoft.com>
 
 this is complete the fix in a5ba1d95e46ecaea638ddd7cd144107c783acb5d
 ---
- drivers/tty/serial/serial_core.c |    6 ++++--
- 1 file changed, 4 insertions(+), 2 deletions(-)
+ drivers/tty/serial/serial_core.c |   12 ++++++++----
+ 1 file changed, 8 insertions(+), 4 deletions(-)
 
 diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
-index 53e6db8..ec1187d 100644
+index 53e6db8..c98a5d0 100644
 --- a/drivers/tty/serial/serial_core.c
 +++ b/drivers/tty/serial/serial_core.c
 @@ -530,10 +530,12 @@ static int uart_put_char(struct tty_struct *tty, unsigned char c)
@@ -26,3 +26,18 @@ index 53e6db8..ec1187d 100644
  	if (port && uart_circ_chars_free(circ) != 0) {
  		circ->buf[circ->head] = c;
  		circ->head = (circ->head + 1) & (UART_XMIT_SIZE - 1);
+@@ -567,10 +569,12 @@ static int uart_write(struct tty_struct *tty,
+ 	}
+
+ 	circ = &state->xmit;
+-	if (!circ->buf)
++	port = uart_port_lock(state, flags);
++	if (!circ->buf) {
++		uart_port_unlock(port, flags);
+ 		return 0;
++	}
+
+-	port = uart_port_lock(state, flags);
+ 	while (port) {
+ 		c = CIRC_SPACE_TO_END(circ->head, circ->tail, UART_XMIT_SIZE);
+ 		if (count < c)