[ 3.5.y.z extended stable ] Patch "TTY: Fix tty miss restart after we turn off flow-control" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 871f1f47995e6724b742f5bacb86111aee56523a Mon Sep 17 00:00:00 2001
From: Wang YanQing <udknight at gmail.com>
Date: Thu, 9 May 2013 14:16:47 +0800
Subject: [PATCH] TTY: Fix tty miss restart after we turn off flow-control

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:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=465823

Signed-off-by: Wang YanQing <udknight at gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
[ luis: adjust context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 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 26f0d0e..72d3ff8 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1529,6 +1529,14 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
 			tty->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);
--
1.8.1.2