Re: TIOCGWINSZ retuns old pty size after receiving SIGWINCH

[Javeed Shaikh]

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;


On Tue, Aug 12, 2008 at 12:03 AM, Javeed Shaikh <syscrash2k@xxxxxxxxx> wrote:
> I spoke too soon. The problem still exists with my workaround on the
> latest git kernel,
> but it seems to be harder to reproduce.
>
> On Mon, Aug 11, 2008 at 10:51 PM, Javeed Shaikh <syscrash2k@xxxxxxxxx> wrote:
>> 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 <efault@xxxxxx>
>> 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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