linux-kernel - Re: [PATCH]: disassociate tty locking fixups

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Date:	Sat, 14 Oct 2006 14:20:24 +0200
From:	Arjan van de Ven <arjan@...radead.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...l.org
Subject: Re: [PATCH]: disassociate tty locking fixups


> @@ -1364,14 +1366,16 @@ static void do_tty_hangup(void *data)
>  				p->signal->tty = NULL;
>  			if (!p->signal->leader)
>  				continue;
> +			mutex_unlock(&tty_mutex);
>  			group_send_sig_info(SIGHUP, SEND_SIG_PRIV, p);
>  			group_send_sig_info(SIGCONT, SEND_SIG_PRIV, p);
> +			mutex_lock(&tty_mutex);
>  			if (tty->pgrp > 0)
>  				p->signal->tty_old_pgrp = tty->pgrp;
>  		} while_each_task_pid(tty->session, PIDTYPE_SID, p);
>  	}
>  	read_unlock(&tasklist_lock);
> -
> +	mutex_unlock(&tty_mutex);

Hi,

what is the lock ordering rules between tasklist_lock and tty_mutex?
In the code above you first take tty_mutex then tasklist_lock, and later
on you drop tty_mutex, with the result that you then have a
tasklist_lock -> tty_mutex order.

This can deadlock if someone gets in the middle with a
mutex_lock(&tty_mutex);
write_lock(&tasklist_lock);
....

in addition, are you sure you don't need to revalidate anything after
retaking the lock?

Greetings,
   Arjan van de Ven

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