[PATCH 34/75] TTY: Fix tty miss restart after we turn off flow-control

From: Kamal Mostafa
Date: Tue Jun 04 2013 - 13:05:39 EST -stable review patch. If anyone has any objections, please let me know.


From: Wang YanQing <udknight@xxxxxxxxx>

commit dab73b4eb9ef924a2b90dab84e539076d82b256f upstream.

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:

Signed-off-by: Wang YanQing <udknight@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
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;
+ /*
+ * 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 */

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