netdev - Re: [PATCH net-next] net: introduce dev_consume_skb

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Date:	Thu, 5 Dec 2013 17:54:14 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: introduce dev_consume_skb_any()

On Thu, Dec 05, 2013 at 08:38:05AM -0800, Eric Dumazet wrote:
> On Thu, 2013-12-05 at 16:44 +0100, Hannes Frederic Sowa wrote:
> 
> > It makes some sense but I did not grasp the whole ->users dependency
> > picture, yet. I guess the barrier is only needed when refcount drops
> > down to 0 and we don't necessarily need one when incrementing ->users.
> 
> If you are the only user of this skb, really no smp barrier is needed at
> all.
> 
> The problem comes when another cpu is working on the skb, and finally
> releases its reference on it.
> 
> Before releasing its reference, it must commit all changes it might have
> done onto skb. Otherwise another cpu might read stale data.
> 
> The smp_wmb() is done by the atomic_dec_and_test(), as it contains a
> full barrier.
> 
> So the smp_rmb() pairs with the barrier done in atomic_dec_and_test()

Ha, it all makes sense now! Thanks, Eric!

(Sorry for the noise but I find this kind of problems very interesting)

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