From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <shemminger@vyatta.com>
Subject: Re: unregister_netdev() deadlock
Date: Wed, 27 Feb 2008 08:46:24 -0800
Message-ID: <20080227084624.163c762b@extreme>
References: <1204127094.11318.0.camel@vglwks010.vgl2.office.vaioni.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Cc: netdev@vger.kernel.org, matthew.hattersley@vaioni.com
To: Jeba Anandhan <jeba.anandhan@vaioni.com>
Return-path: <netdev-owner@vger.kernel.org>
Received: from mail.vyatta.com ([216.93.170.194]:59174 "EHLO mail.vyatta.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753150AbYB0Qq2 (ORCPT <rfc822;netdev@vger.kernel.org>);
	Wed, 27 Feb 2008 11:46:28 -0500
In-Reply-To: <1204127094.11318.0.camel@vglwks010.vgl2.office.vaioni.com>
Sender: netdev-owner@vger.kernel.org
List-ID: <netdev.vger.kernel.org>

On Wed, 27 Feb 2008 15:44:54 +0000
Jeba Anandhan <jeba.anandhan@vaioni.com> wrote:

> Hi all,
> I have doubts about unregister the netdevice.
> 
> Scenario
> I have two netdevice entities. I wish to unregister one entity.
> 
> struct net_device *dev1;
> struct net_device *dev2;
> 
> dev1->timer = XXX;
> dev2->timer = YYY;
> 
> dev1->ioctl() {
> 
> ...
> case delete_other:
>             unregister_netdev(dev2);
>             free_netdev(dev2);
>             return 0;
> ...
> }
> 
> The dev2->refcnt is not zero in this case. unregister_netdev(dev2) cause
> deadlock. Could you tell me why it happens?.
> 
> 
> It works smoothly when we call the unregister_netdev(dev2) from
> cleanup_module(). dev2->refcnt is not zero in this case.  Still it is
> able to unregister.
> 
> 
> Thanks
> Jeba
> 

You need to properly manage device refcounts (or delete both).
Also since dev1->ioctl is called with RTNL you can't:
    use unregister_netdev() recursive locking (use unregister_netdevice)
    or  call free_netdev because of netdevice could still be in use (use dev->destructor instead).