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

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Message-ID: <1306526921.2533.7.camel@edumazet-laptop>
Date:	Fri, 27 May 2011 22:08:41 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 1/2 v2] af-packet:  Use existing netdev reference for
 bound sockets.

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().

> It seems a waste to do the lookup and free if we don't have to,
> and with thousands of devices, the lookup might take a reasonable
> amount of effort?

I understand you want to avoid the lookup, this part is fine for me, but
you need to take a reference on the device before eventual sleep.

Nowadays its a single "inc" instruction on x86, without even a lock
prefix (this on a percpu integer)

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