linux-kernel - Re: TIOCGWINSZ retuns old pty size after receiving SIGWINCH

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Message-ID: <20080819075414.GV29842@pruts.nl>
Date:	Tue, 19 Aug 2008 09:54:14 +0200
From:	Ico Doornekamp <lkml@...v.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: TIOCGWINSZ retuns old pty size after receiving SIGWINCH



* On 2008-08-19 Andrew Morton <akpm@...ux-foundation.org> wrote  :

> On Sun, 10 Aug 2008 17:08:59 +0200 Ico Doornekamp <lkml@...v.nl> wrote:
> 
> > 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 ?
> 
> hm, that code is pretty simple and although it does the SIGWINCH and
> the window-size setting in a peculiar order, it looks to be race-free.
> 
> Approximately what proportion of the time does it go wrong?

I guess about 10 to 20% of the resizes. I happen to be using a tiling
window manager which causes resizing more often and more agressive then
'normal' window managers, I guess this helps triggering the problem.

I temporary worked around this issue this by changing the order of the
signal and the updating of the pty size in tty_io.c's tiocswinsz(), but
this is not much of a real fix.

Ico

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