linux-kernel - Re: [PATCH 4/4] TTY: call tty_driver_lookup

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Message-ID: <4E95FFCA.2090604@gmail.com>
Date:	Wed, 12 Oct 2011 22:59:54 +0200
From:	Jiri Slaby <jirislaby@...il.com>
To:	Jiri Slaby <jslaby@...e.cz>
CC:	gregkh@...e.de, linux-kernel@...r.kernel.org,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Alan Cox <alan@...hat.com>
Subject: Re: [PATCH 4/4] TTY: call tty_driver_lookup_tty unconditionally

On 10/12/2011 11:32 AM, Jiri Slaby wrote:
> Commit 4a2b5fddd5 (Move tty lookup/reopen to caller) made the call to
> tty_driver_lookup_tty conditional in tty_open. It doesn't look like it
> was an intention. Or if it was, it was not documented in the changelog
> and the code now looks weird. For example there would be no need to
> remember the tty driver and tty index. Further the condition depends
> on a tty which we drop a reference of already.
> 
> If I'm looking correctly, this should not matter thanks to the locking
> currently done there. Thus, tty_driver->ttys[idx] cannot change under
> our hands. But anyway, it makes sense to change that to the old
> behaviour.

Well, this doesn't work for ptys. devpts lookup code expects an inode to
be one of devpts filesystem (/dev/pts/*), not something on tmpfs like
/dev/tty. So it looks like the change was intentional, but very
undocumented and leaving there some unused code.

Now, I don't know what to do with the tty being around with a dropped
reference. Maybe nothing in the presence of the BTM which I *think*
prevents tty to go now.

Also I have just found that having ptm_unix98_lookup is pointless. For
master, we have ptmx_open which doesn't call lookup. And if it did, it
would trigger BUG_ON in devpts_get_tty called from ptm_unix98_lookup.

thanks,
-- 
js
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