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

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Message-ID: <20140109164936.4b17cf38@nehalam.linuxnetplumber.net>
Date:	Thu, 9 Jan 2014 16:49:36 -0800
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Cong Wang <xiyou.wangcong@...il.com>
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, 9 Jan 2014 16:30:12 -0800
Cong Wang <xiyou.wangcong@...il.com> wrote:

> On Thu, Jan 9, 2014 at 4:21 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> >
> >
> > 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.
> >
> 
> There is only one ingress queue, that is dev->ingress_queue, right?
> 
> And since on ingress, the only possible qdisc is sch_ingress,
> looking at ingress_enqueue(), I don't see anything so dangerous.
> 
> As I said in the cover letter, I may still miss something in the qdisc
> layer, but doesn't look like related with multiqueue. Mind to be more
> specific?
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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.
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