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

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Message-ID: <1386255952.30495.192.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 05 Dec 2013 07:05:52 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: introduce dev_consume_skb_any()

On Thu, 2013-12-05 at 06:45 -0800, Eric Dumazet wrote:
> On Thu, 2013-12-05 at 15:13 +0100, Hannes Frederic Sowa wrote:
> > 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?
> 
> Please take a look at kfree_skb() implementation.
> 
> If you think a comment is needed there, please feel free to add it.
> 

My understanding of this (old) barrier here is an implicit wmb in
skb_get()

This probably needs something like :

static inline struct sk_buff *skb_get(struct sk_buff *skb)
{
	smp_mb__before_atomic_inc(); /* check {consume|kfree}_skb() */
	atomic_inc(&skb->users);
}



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