netdev - Re: [PATCH 1/2 v2] af-packet: Use existing netdev reference for bound sockets.

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Message-Id: <20110527.161542.568477840432205227.davem@davemloft.net>
Date:	Fri, 27 May 2011 16:15:42 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	greearb@...delatech.com, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2 v2] af-packet: Use existing netdev reference for
 bound sockets.

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 27 May 2011 22:08:41 +0200

> Le jeudi 26 mai 2011 à 21:11 -0700, Ben Greear a écrit :
>> On 05/26/2011 08:42 PM, Eric Dumazet wrote:
>> > Le jeudi 26 mai 2011 à 16:55 -0700, greearb@...delatech.com a écrit :
>> 
>> >>   out_free:
>> >>   	kfree_skb(skb);
>> >>   out_unlock:
>> >> -	if (dev)
>> >> +	if (dev&&  need_rls_dev)
>> >>   		dev_put(dev);
>> >>   out:
>> >>   	return err;
>> >
>> > Hmmm, I wonder why you want this Ben.
>> >
>> > IMHO this is buggy, because we can sleep in this function.
>> >
>> > We must take a ref on device (its really cheap these days, now we have a
>> > percpu device refcnt)
>> 
>> Why must you take the reference?  And if we must, why isn't the
>> current code that assigns the prot_hook.dev without taking a
>> reference OK?
>> 
> 
> If we sleep, device can disappear under us.
> 
> The only way to not take a reference is to hold rcu_read_lock(), but
> you're not allowed to sleep under rcu_read_lock().

You still have not addresses Ben's point.

Why is it ok for the po->prot_hook.dev handling to not take a
reference?  It's been doing this forever.  Ben is just borrowing this
behavior for his uses.

After some more research I think it happens to be OK because
->prot_hook.dev is used _only_ for pointer comparisons, it is never
actually dereferenced or used in any other way.  Probably, we should
just use ->ifindex for this.

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