netdev - Re: unregister_netdev() deadlock

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Date:	Wed, 27 Feb 2008 08:46:24 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Jeba Anandhan <jeba.anandhan@...oni.com>
Cc:	netdev@...r.kernel.org, matthew.hattersley@...oni.com
Subject: Re: unregister_netdev() deadlock

On Wed, 27 Feb 2008 15:44:54 +0000
Jeba Anandhan <jeba.anandhan@...oni.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).
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