From: Alan Cox If you send a priority character (as is done for flow control) then the tty driver can either have its own method for "jumping the queue" or the characrer can be queued normally. In the latter case we call the write method but without the atomic_write_lock taken elsewhere. Make this consistent. Note that the send_xchar method if implemented remains outside of the lock as it can jump ahead of a current write so must not be locked out by it. Signed-off-by: Alan Cox Signed-off-by: Andrew Morton --- drivers/char/tty_ioctl.c | 18 +++++++++++------- 1 files changed, 11 insertions(+), 7 deletions(-) diff -puN drivers/char/tty_ioctl.c~fix-locking-for-tty-drivers-when-doing-urgent-characters drivers/char/tty_ioctl.c --- a/drivers/char/tty_ioctl.c~fix-locking-for-tty-drivers-when-doing-urgent-characters +++ a/drivers/char/tty_ioctl.c @@ -423,24 +423,28 @@ static int set_ltchars(struct tty_struct * * Send a high priority character to the tty even if stopped * - * Locking: none - * - * FIXME: overlapping calls with start/stop tty lose state of tty + * Locking: none for xchar method, write ordering for write method. */ -static void send_prio_char(struct tty_struct *tty, char ch) +static int send_prio_char(struct tty_struct *tty, char ch) { int was_stopped = tty->stopped; if (tty->driver->send_xchar) { tty->driver->send_xchar(tty, ch); - return; + return 0; } + + if (mutex_lock_interruptible(&tty->atomic_write_lock)) + return -ERESTARTSYS; + if (was_stopped) start_tty(tty); tty->driver->write(tty, &ch, 1); if (was_stopped) stop_tty(tty); + mutex_unlock(&tty->atomic_write_lock); + return 0; } int n_tty_ioctl(struct tty_struct * tty, struct file * file, @@ -514,11 +518,11 @@ int n_tty_ioctl(struct tty_struct * tty, break; case TCIOFF: if (STOP_CHAR(tty) != __DISABLED_CHAR) - send_prio_char(tty, STOP_CHAR(tty)); + return send_prio_char(tty, STOP_CHAR(tty)); break; case TCION: if (START_CHAR(tty) != __DISABLED_CHAR) - send_prio_char(tty, START_CHAR(tty)); + return send_prio_char(tty, START_CHAR(tty)); break; default: return -EINVAL; _