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

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Date:	Thu, 5 Dec 2013 15:13:46 +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 04:45:08AM -0800, Eric Dumazet wrote:
> -		local_irq_save(flags);
> -		sd = &__get_cpu_var(softnet_data);
> -		skb->next = sd->completion_queue;
> -		sd->completion_queue = skb;
> -		raise_softirq_irqoff(NET_TX_SOFTIRQ);
> -		local_irq_restore(flags);
> +	if (likely(atomic_read(&skb->users) == 1)) {
> +		smp_rmb();

Could you give me a hint why this barrier is needed? IMHO the volatile
access in atomic_read should get rid of the control dependency so I
don't see a need for this barrier. Without the volatile access a
compiler-barrier would still suffice, I guess?


> +		atomic_set(&skb->users, 0);
> +	} else if (likely(!atomic_dec_and_test(&skb->users))) {
> +		return;

Or does this memory barrier deal with the part below this return?

Thanks,

  Hannes

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