linux-kernel - Re: TTY: tty

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Date:	Sat, 10 Mar 2012 23:51:53 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Richard Weinberger <richard@....at>
CC:	linux-kernel@...r.kernel.org, alan@...rguk.ukuu.org.uk,
	gregkh@...uxfoundation.org, Jiri Slaby <jirislaby@...il.com>
Subject: Re: TTY: tty_port questions

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On 03/10/2012 11:26 PM, Richard Weinberger wrote:
> Hi!
> 
> While moving UML's console driver to tty_port some strange things
> happened. So, I have a few questions. :-)
> 
> The original driver did not implement tty_operations->hangup(). If
> I implement it and call tty_port_hangup(), as Alan suggested, the
> login fails on all TTYs except tty0. It fails because the opened
> TTY returns EIO upon read()/write() after /bin/login called
> vhangup().
> 
> The call chain is: vhangup() -> tty_vhangup_self() -> tty_vhangup()
> -> __tty_hangup()
> 
> Within __tty_hangup() something happens that I don't fully
> understand:
> 
> if (cons_filp) { if (tty->ops->close) for (n = 0; n < closecount;
> n++) tty->ops->close(tty, cons_filp); } else if (tty->ops->hangup) 
> (tty->ops->hangup)(tty);
> 
> Login on tty0 works because cons_filp is not NULL and
> tty->ops->close() is called. On the other hand login fails on every
> other TTY because cons_filp remains NULL and the TTY hangs up.
> 
> Is there something missing in my hangup function?
> 
> If I omit tty_operations->hangup() and leave it, like the old
> driver, NULL non-tty0 TTYs cannot be opened. (getty terminates
> immediately because it cannot open any TTY.) open() retuns -EIO
> because the TTY_CLOSING is set in tty->flags.
> 
> How can this be?

Hmm, it looks like some process is sitting on a TTY which was hung.
And the system offers this hung TTY to others on further opens. Could
you check that there is no process with open TTY after the vhangup?

regards,
- -- 
js
suse labs
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