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Subject[PATCH tty-next 22/22] 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

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 <>
Documentation/serial/driver | 6 ++++--
drivers/tty/serial/serial_core.c | 8 +++++++-
2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/Documentation/serial/driver b/Documentation/serial/driver
index c3a7689..b1a22d4 100644
--- a/Documentation/serial/driver
+++ b/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).

@@ -246,7 +248,7 @@ 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

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 15212d7..5ed5a46 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -427,7 +427,7 @@ uart_get_divisor(struct uart_port *port, unsigned int baud)


-/* 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)
@@ -1162,11 +1162,15 @@ uart_ioctl(struct tty_struct *tty, unsigned int cmd,

+ down_write(&tty->termios_rwsem);
ret = uart_set_info_user(tty, state, uarg);
+ up_write(&tty->termios_rwsem);

+ down_write(&tty->termios_rwsem);
ret = uart_do_autoconfig(tty, state);
+ up_write(&tty->termios_rwsem);

case TIOCSERGWILD: /* obsolete */
@@ -1268,7 +1272,9 @@ static void uart_set_termios(struct tty_struct *tty,

+ 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;


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