Patch "tty: pty: Fix ldisc flush after userspace become aware of the data already" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

tty: pty: Fix ldisc flush after userspace become aware of the data already

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tty-pty-fix-ldisc-flush-after-userspace-become-aware-of-the-data-already.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wang YanQing <[email protected]>
Date: Wed, 22 Feb 2017 19:37:08 +0800
Subject: tty: pty: Fix ldisc flush after userspace become aware of the data already

From: Wang YanQing <[email protected]>

commit 77dae6134440420bac334581a3ccee94cee1c054 upstream.

While using emacs, cat or others' commands in konsole with recent kernels, I have met many times that CTRL-C freeze konsole. After konsole freeze I can't type anything, then I have to open a new one, it is very annoying.

See bug report:
https://bugs.kde.org/show_bug.cgi?id=175283

The platform in that bug report is Solaris, but now the pty in linux has the same problem or the same behavior as Solaris :)

It has high possibility to trigger the problem follow steps below:
Note: In my test, BigFile is a text file whose size is bigger than 1G 1:open konsole
1:cat BigFile
2:CTRL-C

After some digging, I find out the reason is that commit 1d1d14da12e7 ("pty: Fix buffer flush deadlock") changes the behavior of pty_flush_buffer.

Thread A Thread B
-------- --------
1:n_tty_poll return POLLIN
2:CTRL-C trigger pty_flush_buffer
tty_buffer_flush
n_tty_flush_buffer
3:attempt to check count of chars:
ioctl(fd, TIOCINQ, &available)
available is equal to 0

4:read(fd, buffer, avaiable)
return 0

5:konsole close fd

Yes, I know we could use the same patch included in the BUG report as a workaround for linux platform too. But I think the data in ldisc is belong to application of another side, we shouldn't clear it when we want to flush write buffer of this side in pty_flush_buffer. So I think it is better to disable ldisc flush in pty_flush_buffer, because its new hehavior bring no benefit except that it mess up the behavior between POLLIN, and TIOCINQ or FIONREAD.

Also I find no flush_buffer function in others' tty driver has the same behavior as current pty_flush_buffer.

---
 drivers/tty/pty.c |    7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

@@ -216,16 +216,11 @@ static int pty_signal(struct tty_struct
 static void pty_flush_buffer(struct tty_struct *tty)
 {
 	struct tty_struct *to = tty->link;
-	struct tty_ldisc *ld;
 
 	if (!to)
 		return;
 
-	ld = tty_ldisc_ref(to);
-	tty_buffer_flush(to, ld);
-	if (ld)
-		tty_ldisc_deref(ld);
-
+	tty_buffer_flush(to, NULL);
 	if (to->packet) {
 		spin_lock_irq(&tty->ctrl_lock);
 		tty->ctrl_status |= TIOCPKT_FLUSHWRITE;

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/tty-pty-fix-ldisc-flush-after-userspace-become-aware-of-the-data-already.patch