[PATCH v3] TTY: Fix tty miss restart after we turn off flow-control

[Wang YanQing <udknight@gmail.com>]

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>
---
 Changes v1-v2:
 1: move the fix into n_tty_set_termios instead of change TCXONC's behavior suggested by Peter Hurley
 2: rewrite some ChangeLog and use more reasonable subject.
 Changes v2-v3:
 1: Fix potential NULL pointer dereference.

 drivers/tty/n_tty.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 19083ef..9365978 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1587,6 +1587,14 @@ 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);
-- 
1.7.12.4.dirty