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

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Message-ID: <20111016192057.GB14883@us.ibm.com>
Date:	Sun, 16 Oct 2011 12:20:57 -0700
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	Jiri Slaby <jslaby@...e.cz>, gregkh@...e.de,
	linux-kernel@...r.kernel.org,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Alan Cox <alan@...hat.com>, hpa@...or.com
Subject: Re: [PATCH 4/4] TTY: call tty_driver_lookup_tty unconditionally

Ccing H.Peter Anvin.

Jiri Slaby [jirislaby@...il.com] wrote:
| 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.

Yes this was intentional - even though the tty_driver_lookup() was
unconditional in tty_init_dev() I believe it did not do anything useful
when called from ptmx_open(). ptmx_open() would be setting up a new pty and
the lookup would not find a tty.

Would the following change to tty_open() help ?

got_driver:
+	/* check if we are opening a new pty or reopening an existing tty */
        if (!tty) {
-	/* check whether we're reopening an existing tty */
                tty = tty_driver_lookup_tty(driver, inode, index);

I am not sure about the unused code you refer to above. Can you please
clarify ?

Sukadev

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