From 38db89799bdf11625a831c5af33938dcb11908b6 Mon Sep 17 00:00:00 2001 From: Alan Cox Date: Thu, 11 Jun 2009 12:44:17 +0100 Subject: tty: throttling race fix The tty throttling code can race due to the lock drops. It takes very high loads but this has been observed and verified by Rob Duncan. The basic problem is that on an SMP box we can go CPU #1 CPU #2 need to throttle ? suppose we should buffer space cleared are we throttled yes ? - unthrottle call throttle method This changeet take the termios lock to protect against this. The termios lock isn't the initial obvious candidate but many implementations of throttle methods already need to poke around their own termios structures (and nobody really locks them against a racing change of flow control). This does mean that anyone who is setting tty->low_latency = 1 and then calling tty_flip_buffer_push from their unthrottle method is going to end up collapsing in a pile of locks. However we've removed all the known bogus users of low_latency = 1 and such use isn't safe anyway for other reasons so catching it would be an improvement. Signed-off-by: Alan Cox Signed-off-by: Linus Torvalds --- drivers/char/tty_ioctl.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'drivers/char/tty_ioctl.c') diff --git a/drivers/char/tty_ioctl.c b/drivers/char/tty_ioctl.c index 6f4c7d0a53b..2401dbcbee9 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); -- cgit v1.2.3-18-g5258