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

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Date:	Tue, 19 Aug 2008 13:44:23 +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 Tue, 19 Aug 2008 09:54:14 +0200 Ico Doornekamp <lkml@...v.nl> wrote:
> 
> > 
> > 
> > * 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.
> > 
> 
> Well damn.  Are you sure?  The code looks solid to me.
> 
> At least, it does after 
> 
> Author: Alan Cox <alan@...hat.com>  2008-08-15 02:39:38
> Committer: Linus Torvalds <torvalds@...ux-foundation.org>  2008-08-15 10:34:07
> Parent: 000b9151d7851cc1e490b2a76d0206e524f43cca (Fix race/oops in tty layer after BKL pushdown)
> Branches: git-cifs, git-ia64, git-nfs, git-powerpc-merge, linux-next, remotes/origin/master
> Follows: v2.6.27-rc3
> Precedes: next-20080818
> 
>     tty: remove resize window special case
> 
> perhaps you're still running a kernel which is earlier than that?

I reported the behaviour on 2.6.26.2, but I was not aware this issue was
adressed already in the 2.6.27 tree. I will update to the latest 2.6.27
rc and report if the problem still persists.

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