linux-kernel - Re: 2.6.21-rc4-mm1: init hangs on tty

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Message-ID: <m1vegvlgyx.fsf@ebiederm.dsl.xmission.com>
Date:	Tue, 20 Mar 2007 15:53:26 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.21-rc4-mm1: init hangs on tty_mutex

Hugh Dickins <hugh@...itas.com> writes:

> I get a hang at startup or shutdown: tiocsctty() holds tty_mutex, and
> now with Eric's tty-in-tiocsctty-when-we-steal-a-tty-hang-it-up.patch
> it can end up calling con_close(), which tries to take tty_mutex.
> So best revert that in hot-fixes, until Eric provides a better.

Well I'm confused I know I tested this, and I even added a printk and
saw it trigger a couple of times.

Hmm.  Looking at the code it is clearly not safe to hold the tty_mutex
when calling tty_vhangup.  I thought the other caller in tty_io.c was doing
that but on a second glance it is clearly not.  And the hangup and close
methods do seem to take the tty_mutex.

How this worked in my testing is beyond me..

I will see if I can generate a better patch later.

Acked-by: Eric W. Biederman <ebiederm@...ssion.com>

>
> Signed-off-by: Hugh Dickins <hugh@...itas.com>
>
> --- 2.6.21-rc4-mm1/drivers/char/tty_io.c 2007-03-20 12:49:34.000000000 +0000
> +++ linux/drivers/char/tty_io.c	2007-03-20 18:14:53.000000000 +0000
> @@ -2980,7 +2980,9 @@ static int tiocsctty(struct tty_struct *
>  			/*
>  			 * Steal it away
>  			 */
> -			tty_vhangup(tty);
> +			read_lock(&tasklist_lock);
> +			session_clear_tty(tty->session);
> +			read_unlock(&tasklist_lock);
>  		} else {
>  			ret = -EPERM;
>  			goto unlock;
-
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