netdev - Re: [PATCH net-next V4 1/3] net: Add GRO support for UDP encapsulating protocols

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Date:	Tue, 14 Jan 2014 16:50:29 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Or Gerlitz <or.gerlitz@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Jerry Chu <hkchu@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Yan Burman <yanb@...lanox.com>,
	Shlomo Pongratz <shlomop@...lanox.com>
Subject: Re: [PATCH net-next V4 1/3] net: Add GRO support for UDP
 encapsulating protocols

On Tue, 2014-01-14 at 23:51 +0200, Or Gerlitz wrote:

> >> +       rcu_read_lock();
> >> +
> >> +       uo_priv = rcu_dereference(udp_offload_base);
> >> +       for (; uo_priv != NULL; uo_priv = rcu_dereference(uo_priv->next)) {
> >> +               if (uo_priv->offload->port == uh->dest &&
> >> +                   uo_priv->offload->callbacks.gro_complete)
> >> +                       goto found;
> >> +       }
> >> +
> >> +       rcu_read_unlock();
> >> +       return err;
> >> +
> >> +found:
> >> +       atomic_inc(&uo_priv->refcount);
> >
> > This is an expensive operation in the critical path.
> 
> I know, but I don't see how to get away without having the ref/unref
> wrapping, ideas welcome
> 
> > Can uo_priv be protected by rcu also?
> 
> uo_priv is the actual element which is rcu protected, not sure to
> follow on your question.
> 

Seems pretty easy : unlock rcu after calling gro_complete() as in :

found:
       err = uo_priv->offload->callbacks.gro_complete(skb, nhoff + sizeof(struct udphdr));
       rcu_read_unlock();
       return err;


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