netdev - Re: Re: [B.A.T.M.A.N.] [PATCH] net: fix possible deadlocks in rtnl

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Date:	Mon, 3 Dec 2012 21:09:06 +0100
From:	Antonio Quartulli <ordex@...istici.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Simon Wunderlich <simon.wunderlich@...03.tu-chemnitz.de>,
	Sven Eckelmann <sven@...fation.org>,
	b.a.t.m.a.n@...ts.open-mesh.org, netdev@...r.kernel.org,
	davem@...emloft.net, Simon Wunderlich <siwu@....tu-chemnitz.de>
Subject: Re: Re: [B.A.T.M.A.N.] [PATCH] net: fix possible deadlocks in
 rtnl_trylock/unlock

On Sat, Dec 01, 2012 at 09:01:53PM +0100, Simon Wunderlich wrote:
> On Sat, Dec 01, 2012 at 11:44:34AM -0800, Eric Dumazet wrote:
> > On Sat, 2012-12-01 at 20:04 +0100, Sven Eckelmann wrote:
> > > On Saturday 01 December 2012 10:36:12 Eric Dumazet wrote:
> > >  
> > > > > diff --git a/net/bridge/br_sysfs_br.c b/net/bridge/br_sysfs_br.c
> > > > > index c5c0593..c122782 100644
> > > > > --- a/net/bridge/br_sysfs_br.c
> > > > > +++ b/net/bridge/br_sysfs_br.c
> > > > > @@ -142,7 +142,7 @@ static ssize_t store_stp_state(struct device *d,
> > > > > 
> > > > >  	if (!rtnl_trylock())
> > > > >  	
> > > > >  		return restart_syscall();
> > > > >  	
> > > > >  	br_stp_set_enabled(br, val);
> > > > > 
> > > > > -	rtnl_unlock();
> > > > > +	__rtnl_unlock();
> > > > > 
> > > > >  	return len;
> > > > >  
> > > > >  }
> > > > 
> > > > I have no idea of why you believe there is a problem here.
> > > > 
> > > > Could you explain how net_todo_list could be not empty ?
> > > > 
> > > > As long as no device is unregistered between
> > > > rtnl_trylock()/rtnl_unlock(), there is no possible deadlock.
> > > 
> > > I am not sure what "here" means for your. At least batman-adv tries to 
> > > unregister a device -> problem [1]. I will not make any judgements about the 
> > > other uses in the kernel/other parts patched by Simon.
> > > 
> > 
> > I was reacting to the change in net/bridge/br_sysfs_br.c
> > 
> > rtnl_trylock() could set a boolean flag to explicitly WARN_ON()
> > in case we try to unregister a device.
> 
> Well, I'm not sure if this can happen in the bridge code, but from looking at the
> code it doesn't appear to be impossible. It would be better to be sure that it can't
> deadlock IMHO.
> 
> (Although doing unlock/lock/unlock in an unlock function is also a little "uncommon").

But still we have the problem in batman-adv (as Sven pointed out in a previous
email) that tries to unregister a device in that "critical window".

Exporting __rtnl_unlock() would solve the issue in this case.

If you think the bridge code should not end up in such situation, what if Simon
resends the patch with only the __rtnl_unlock() exportation and the change in 
batman-adv?


Cheers,


-- 
Antonio Quartulli

..each of us alone is worth nothing..
Ernesto "Che" Guevara

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