serial: Fix locking for uart driver set_termios() method

The low-level uart driver may modify termios settings to override
settings that are not compatible with the uart, such as CRTSCTS.
Thus, callers of the low-level uart driver's set_termios() method must
hold termios_rwsem write lock to prevent concurrent access to termios,
in case such override occurs.

The termios_rwsem lock requirement does not extend to console setup
(ie., uart_set_options), as console setup cannot race with tty
operations. Nor does this lock requirement extend to functions which
cannot be concurrent with tty ioctls (ie., uart_port_startup() and
uart_resume_port()).

Further, always claim the port mutex to protect hardware
re-reprogramming in the set_termios() uart driver method. Note this
is unnecessary for console initialization in uart_set_options()
which cannot be concurrent with other uart operations.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/serial/driver

@@ -59,7 +59,9 @@ The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
 access to the info->tmpbuf bouncebuffer used for port writes.
 
 The port_sem semaphore is used to protect against ports being added/
-removed or reconfigured at inappropriate times.
+removed or reconfigured at inappropriate times. Since v2.6.27, this
+semaphore has been the 'mutex' member of the tty_port struct, and
+commonly referred to as the port mutex (or port->mutex).
 
 
 uart_ops
@@ -248,7 +250,7 @@ hardware.
 	Other flags may be used (eg, xon/xoff characters) if your
 	hardware supports hardware "soft" flow control.
 
-	Locking: none.
+	Locking: caller holds port->mutex
 	Interrupts: caller dependent.
 	This call must not sleep
 

drivers/tty/serial/serial_core.c

@@ -436,7 +436,7 @@ uart_get_divisor(struct uart_port *port, unsigned int baud)
 
 EXPORT_SYMBOL(uart_get_divisor);
 
-/* FIXME: Consistent locking policy */
+/* Caller holds port mutex */
 static void uart_change_speed(struct tty_struct *tty, struct uart_state *state,
 					struct ktermios *old_termios)
 {
@@ -1173,11 +1173,15 @@ uart_ioctl(struct tty_struct *tty, unsigned int cmd,
 		break;
 
 	case TIOCSSERIAL:
+		down_write(&tty->termios_rwsem);
 		ret = uart_set_info_user(tty, state, uarg);
+		up_write(&tty->termios_rwsem);
 		break;
 
 	case TIOCSERCONFIG:
+		down_write(&tty->termios_rwsem);
 		ret = uart_do_autoconfig(tty, state);
+		up_write(&tty->termios_rwsem);
 		break;
 
 	case TIOCSERGWILD: /* obsolete */
@@ -1278,7 +1282,9 @@ static void uart_set_termios(struct tty_struct *tty,
 		return;
 	}
 
+	mutex_lock(&state->port.mutex);
 	uart_change_speed(tty, state, old_termios);
+	mutex_unlock(&state->port.mutex);
 	/* reload cflag from termios; port driver may have overriden flags */
 	cflag = tty->termios.c_cflag;