Re: TIOCGWINSZ retuns old pty size after receiving SIGWINCH

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Javeed Shaikh

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Aug 11, 2008, 10:51:49 PM8/11/08
to linux-...@vger.kernel.org
Ico Doornekamp wrote:
> Hello,
>
> Recently my X terminals showed annoying behaviour where the application
> in the terminal was not resized properly to the actual size of the X
> terminal emulator window, resulting in a lot of misaligned text on the
> screen. Hunting the issue down from the windowmanager and the terminal
> emulator program, I suspect the problem might lie in the kernel. I'm
> running 2.6.26 on a dual core i386.
>
> What I see is this: the userspace application receives a SIGWINCH signal
> and acquires the terminal size usign the TIOCGWINSZ ioctl. It seems that
> in some cases the old instead of the new terminal size is returned.
> A small delay before the ioctl seems to 'fix' this behaviour.
>
> I noticed some changes involving locking in the the pty code in the last
> kernel verions, could one of these changes cause the above behaviour ? If
> so, wouldn't this affect much more users ?
>
> Ico

Hi,

I've been experiencing similar issues.
Here's a screenshot: http://omploader.org/vbzA1 .
Note how emacs is only taking up half of the terminal's total height.

However, my diagnostics are different from yours. Sending SIGWINCH
(in this case, to emacs) fixes the issue, at least temporarily. However,
sending SIGWINCH to the shell process under which emacs is running
(using a negative PID, to specify the "process group") has no visible effect.
I believe the issue has to do with sending SIGWINCH to process groups,
though I do not have enough experience in this area to be sure.

git-bisect tracked it down to this:
46151122e0a2e80e5a6b2889f595e371fe2b600d is first bad commit
commit 46151122e0a2e80e5a6b2889f595e371fe2b600d
Author: Mike Galbraith <efa...@gmx.de>
Date: Thu May 8 17:00:42 2008 +0200

sched: fix weight calculations

Starting at v2.6.26, I wasn't able to easily revert the commit. However,
the following "patch" seems to have alleviated the problem.

=== begin patch ===
diff --git a/kernel/sched_features.h b/kernel/sched_features.h
index 1c7283c..0e269ed 100644
--- a/kernel/sched_features.h
+++ b/kernel/sched_features.h
@@ -1,4 +1,4 @@
-SCHED_FEAT(NEW_FAIR_SLEEPERS, 1)
+SCHED_FEAT(NEW_FAIR_SLEEPERS, 0)
SCHED_FEAT(WAKEUP_PREEMPT, 1)
SCHED_FEAT(START_DEBIT, 1)
SCHED_FEAT(AFFINE_WAKEUPS, 1)
=== end patch ===

This is merely a workaround, however.

I was able to easily revert commit 46151122e0a2e80e5a6b2889f595e371fe2b600d on
the latest git version of the kernel. However, it did not alleviate the problem.

As in v2.6.26, the above patch (disabling NEW_FAIR_SLEEPERS) seems to work well.
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Javeed Shaikh

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Aug 12, 2008, 12:03:37 AM8/12/08
to linux-...@vger.kernel.org
I spoke too soon. The problem still exists with my workaround on the
latest git kernel,
but it seems to be harder to reproduce.

Javeed Shaikh

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Aug 12, 2008, 7:58:36 PM8/12/08
to linux-...@vger.kernel.org
I appear to have fixed it.

It seems that SIGWINCH was being fired off before the tty's size was
updated, as Ico hypothesized.

The patch follows.
Please comment!

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 7501310..8e2fa3c 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -3021,6 +3021,9 @@ static int tiocswinsz(struct tty_struct *tty,
struct tty_struct *real_tty,
rpgrp = get_pid(real_tty->pgrp);
spin_unlock_irqrestore(&tty->ctrl_lock, flags);

+ tty->winsize = tmp_ws;
+ real_tty->winsize = tmp_ws;
+
if (pgrp)
kill_pgrp(pgrp, SIGWINCH, 1);
if (rpgrp != pgrp && rpgrp)
@@ -3029,8 +3032,6 @@ static int tiocswinsz(struct tty_struct *tty,
struct tty_struct *real_tty,
put_pid(pgrp);
put_pid(rpgrp);

- tty->winsize = tmp_ws;
- real_tty->winsize = tmp_ws;
done:
mutex_unlock(&tty->termios_mutex);
return 0;

Alan Cox

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Aug 13, 2008, 5:37:45 AM8/13/08
to Javeed Shaikh, linux-...@vger.kernel.org
On Tue, 12 Aug 2008 19:58:20 -0400
"Javeed Shaikh" <syscr...@gmail.com> wrote:

> I appear to have fixed it.

You've moved the race.

> It seems that SIGWINCH was being fired off before the tty's size was
> updated, as Ico hypothesized.

There is what I believe to be a correct fix in the -next tree. It is a
bit invasive for 2.6.27 given we are in the -rc tree but I could push the
relevant pieces if the problem justifies the risk

Alan

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