linux-kernel - Re: TTY: tty

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Message-ID: <4F5C861F.2000507@nod.at>
Date:	Sun, 11 Mar 2012 12:01:51 +0100
From:	Richard Weinberger <richard@....at>
To:	Jiri Slaby <jslaby@...e.cz>
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

Am 11.03.2012 00:21, schrieb Richard Weinberger:
> Am 10.03.2012 23:51, schrieb Jiri Slaby:
>> 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?
>>
> 
> "lsof | grep tty" does not show anything else than tty0. :-\
> 

BTW: When I start the kernel with /bin/sh as init, opening and writing to any TTY works fine.
It looks like Fedora 16's userspace does something that makes the TTYs unhappy.

Thanks,
//richard


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