2 files changed, 17 insertions, 2 deletions

diff --git a/drivers/char/tty_ioctl.c b/drivers/char/tty_ioctl.c
index 6f4c7d0a53bf..2401dbcbee9c 100644
--- a/drivers/char/tty_ioctl.c
+++ b/drivers/char/tty_ioctl.c
@@ -97,14 +97,19 @@ EXPORT_SYMBOL(tty_driver_flush_buffer);
  *      @tty: terminal
  *
  *      Indicate that a tty should stop transmitting data down the stack.
+ *      Takes the termios mutex to protect against parallel throttle/unthrottle
+ *      and also to ensure the driver can consistently reference its own
+ *      termios data at this point when implementing software flow control.
  */
 
 void tty_throttle(struct tty_struct *tty)
 {
+        mutex_lock(&tty->termios_mutex);
         /* check TTY_THROTTLED first so it indicates our state */
         if (!test_and_set_bit(TTY_THROTTLED, &tty->flags) &&
             tty->ops->throttle)
                 tty->ops->throttle(tty);
+        mutex_unlock(&tty->termios_mutex);
 }
 EXPORT_SYMBOL(tty_throttle);
 
@@ -113,13 +118,21 @@ EXPORT_SYMBOL(tty_throttle);
  *      @tty: terminal
  *
  *      Indicate that a tty may continue transmitting data down the stack.
+ *      Takes the termios mutex to protect against parallel throttle/unthrottle
+ *      and also to ensure the driver can consistently reference its own
+ *      termios data at this point when implementing software flow control.
+ *
+ *      Drivers should however remember that the stack can issue a throttle,
+ *      then change flow control method, then unthrottle.
  */
 
 void tty_unthrottle(struct tty_struct *tty)
 {
+        mutex_lock(&tty->termios_mutex);
         if (test_and_clear_bit(TTY_THROTTLED, &tty->flags) &&
             tty->ops->unthrottle)
                 tty->ops->unthrottle(tty);
+        mutex_unlock(&tty->termios_mutex);
 }
 EXPORT_SYMBOL(tty_unthrottle);
 
diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
index bcba84ea2d86..3566129384a4 100644
--- a/include/linux/tty_driver.h
+++ b/include/linux/tty_driver.h
@@ -127,7 +127,8 @@
  *      the line discipline are close to full, and it should somehow
  *      signal that no more characters should be sent to the tty.
  *
- *      Optional: Always invoke via tty_throttle();
+ *      Optional: Always invoke via tty_throttle(), called under the
+ *      termios lock.
  * 
  * void (*unthrottle)(struct tty_struct * tty);
  *
@@ -135,7 +136,8 @@
  *      that characters can now be sent to the tty without fear of
  *      overrunning the input buffers of the line disciplines.
  * 
- *      Optional: Always invoke via tty_unthrottle();
+ *      Optional: Always invoke via tty_unthrottle(), called under the
+ *      termios lock.
  *
  * void (*stop)(struct tty_struct *tty);
  *