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

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Date:	Thu, 9 Jan 2014 17:09:11 -0800
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Linux Kernel Network Developers <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, Jan 9, 2014 at 4:49 PM, Stephen Hemminger
<stephen@...workplumber.org> wrote:
>
> I think what Eric is saying is that on a multi-queue NIC, multiple
> queues can be receiving packets and then sending them on to the ingress
> queue discipline. Up until your patch that code itself was protected
> by qdisc_lock and did not have to worry about any SMP issues. Now, any
> qdisc attached on ingress could run in parallel. This would break all
> the code in those queue disciplines. Think of the simplest case
> of policing.

I noticed that we have multiqueue for rx, which is dev->_rx[], so they
still share the name dev->ingress_queue. Packets on different CPU's
from the same device still need to run through the same ingress qdisc.

Except for ifb, ingress qdisc can only do filtering (with some actions),
therefore I still don't see the problem.

ifb device switches to its "egress" to do policing, which is safe since it
does not use ->ingress_queue any more.
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