TTY: Fix tty miss restart after we turn off flow-control

I meet emacs hang in start if I do the operation below:
  1: echo 3 > /proc/sys/vm/drop_caches
  2: emacs BigFile
  3: Press CTRL-S follow 2 immediately

Then emacs hang on, CTRL-Q can't resume, the terminal
hang on, you can do nothing with this terminal except
close it.

The reason is before emacs takeover control the tty,
we use CTRL-S to XOFF it. Then when emacs takeover the
control, it may don't use the flow-control, so emacs hang.
This patch fix it.

This patch will fix a kind of strange tty relation hang problem,
I believe I meet it with vim in ssh, and also see below bug report:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=465823

Signed-off-by: Wang YanQing <udknight@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index d655416..6c7fe90 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1573,6 +1573,14 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
                        ldata->real_raw = 0;
        }
        n_tty_set_room(tty);
+       /*
+        * Fix tty hang when I_IXON(tty) is cleared, but the tty
+        * been stopped by STOP_CHAR(tty) before it.
+        */
+       if (!I_IXON(tty) && old && (old->c_iflag & IXON) && !tty->flow_stopped) {
+               start_tty(tty);
+       }
+
        /* The termios change make the tty ready for I/O */
        wake_up_interruptible(&tty->write_wait);
        wake_up_interruptible(&tty->read_wait);