netdev - Re: [RFC Patch net-next 4/4] net_sched: make ingress qdisc lockless

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Message-ID: <1389313313.31367.74.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 09 Jan 2014 16:21:53 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	netdev@...r.kernel.org, John Fastabend <john.fastabend@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [RFC Patch net-next 4/4] net_sched: make ingress qdisc lockless

On Thu, 2014-01-09 at 10:19 -0800, Cong Wang wrote:
> Cc: John Fastabend <john.fastabend@...il.com>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Cc: David S. Miller <davem@...emloft.net>
> Cc: Jamal Hadi Salim <jhs@...atatu.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  net/core/dev.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index ce01847..e357d05 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3376,10 +3376,8 @@ static int ing_filter(struct sk_buff *skb, struct netdev_queue *rxq)
>  
>  	q = rxq->qdisc;
>  	if (q != &noop_qdisc) {
> -		spin_lock(qdisc_lock(q));
>  		if (likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
>  			result = qdisc_enqueue_root(skb, q);
> -		spin_unlock(qdisc_lock(q));
>  	}
>  
>  	return result;


Really, you'll have to explain in the changelog why you think this is
safe, because I really do not see how this can be valid.

I think I already said it was not safe at all...

You could try a multiqueue NIC for some interesting effects.



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