linux-kernel - Re: mmotm 2010-11-23 - WARNING: at drivers/tty/tty

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Message-ID: <4CEE9280.9050504@suse.cz>
Date:	Thu, 25 Nov 2010 17:44:48 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Kyle McMartin <kyle@...artin.ca>
CC:	Valdis.Kletnieks@...edu, akpm@...ux-foundation.org,
	mm-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <gregkh@...e.de>
Subject: Re: mmotm 2010-11-23 - WARNING: at drivers/tty/tty_io.c:1331

On 11/25/2010 04:14 PM, Kyle McMartin wrote:
> On Tue, Nov 23, 2010 at 11:55:39PM -0500, Valdis.Kletnieks@...edu wrote:
>> On Tue, 23 Nov 2010 16:13:06 PST, akpm@...ux-foundation.org said:
>>> The mm-of-the-moment snapshot 2010-11-23-16-12 has been uploaded to
>>>
>>>    http://userweb.kernel.org/~akpm/mmotm/
>>
>> Seen during boot:
>>
>> [   23.015448] Modules linked in:
>> [   23.015453] Pid: 1207, comm: plymouthd Not tainted 2.6.37-rc3-mmotm1123 #3
>> [   23.015455] Call Trace:
> 
> I've been trying to figure this one out for a while, without much luck.
> (Users are seeing it in 2.6.36 as well.)
> 
> I *think* (I added a rawhide debugging patch to print the tty->name)
> that plymouth is always opening tty7 to cause this. My guess is the BKL
> removal has exposed some kind of race, but it's not obvious to me (and
> there's many other bugs to sort through too. :(
> 
> CC-ing Jiri since he seems to be the poor guy who's been poking this
> recently (there's a good few threads about this (though the others look
> like an ldisc attach race...)) I wouldn't think that's the case here
> since N_TTY is the default...

Ok, tty_reopen is called without TTY_LDISC set. For further
considerations, note tty_lock is held in tty_open. TTY_LDISC is cleared in:

1) __tty_hangup from tty_ldisc_hangup to tty_ldisc_enable. During this
section tty_lock is held.

2) tty_release via tty_ldisc_release till the end of tty existence. If
tty->count <= 1, tty_lock is taken, TTY_CLOSING bit set and then
tty_ldisc_release called. tty_reopen checks TTY_CLOSING before checking
TTY_LDISC.

3) tty_set_ldisc from tty_ldisc_halt to tty_ldisc_enable. We take
tty_lock, set TTY_LDISC_CHANGING, put tty_lock, do some other work, take
tty_lock, call tty_ldisc_enable, put tty_lock.

So the only option I see is 3) and we should do:
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1310,7 +1310,8 @@ static int tty_reopen(struct tty_struct *tty)
 {
        struct tty_driver *driver = tty->driver;

-       if (test_bit(TTY_CLOSING, &tty->flags))
+       if (test_bit(TTY_CLOSING, &tty->flags) ||
+                       test_bit(TTY_LDISC_CHANGING, &tty->flags))
                return -EIO;

        if (driver->type == TTY_DRIVER_TYPE_PTY &&

Alan, Greg?

thanks,
-- 
js
suse labs
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