Date: Tue, 7 May 2013 11:58:00 -0700
From: Greg KH <gregkh@linuxfoundation.org>
To: Wang YanQing <udknight@gmail.com>, jslaby@suse.cz,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH]TTY: Fix tty can't be restarted by TCXONC ioctl request
Message-ID: <20130507185800.GA10713@kroah.com>
References: <20130507184734.GA2459@udknight>
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On Wed, May 08, 2013 at 02:47:34AM +0800, Wang YanQing wrote:
> I meet emacs hang in start if I do the operation below:
>   1: echo 3 > /proc/sys/vm/drop_caches
>   2: emacs BigFile
>   3: Press CTRL-S follow 2 immediately
> 
> Then emacs hang on, CTRL-Q can't resume, the terminal
> hang on, you can do nothing with this terminal except
> close it.
> 
> The reason is before emacs takeover control the tty,
> we use CTRL-S to XOFF it. Then when emacs takeover the
> control, it may don't use the flow-control, so emacs hang.
> But after search the emacs's startup codes, I find they use TCXONC
> to workaround this situation:
> 
>   /* This code added to insure that, if flow-control is not to be used,
>      we have an unlocked terminal at the start. */
>   if (!tty_out->flow_control) ioctl (fileno (tty_out->input), TCXONC, 1);
>   ioctl (fileno (tty_out->input), TCXONC, 1);
> 
> But this workaround never work due the kernel's current code.
> This patch fix it.
> 
> Below is the ChangeLog introduce the tty->flow_stopped flag:
> 
> Thu Nov 21 10:05:22 1996  Theodre Ts'o  <tytso@localhost.mit.edu>
> 
>        * tty_ioctl.c (tty_wait_until_sent): Always check the driver
>                wait_until_ready routine, even if there are no characters
>                in the xmit buffer.  (There may be charactes in the device
>                FIFO.)
>                (n_tty_ioctl): Add new flag tty->flow_stopped which
>                indicates whether the tty is stopped due to a request by
>                the TCXONC ioctl (used by tcflow).  If so, don't let an
>                incoming XOFF character restart the tty.  The tty can only
>                be restarted by another TCXONC request.
> 
> So I think this patch make TCXONC can restart tty which stopped by
> STOP_CHAR don't break the original meaning.
> 
> This patch will fix some strange tty relation hang problem,
> I believe I meet it with vim in ssh, and also see below bug report:
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=465823

So this has been a problem since 1996?  Or is it newly introduced due to
some changes in this area in a newer kernel?

thanks,

greg k-h
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