Subject: Re: [net-next V4 PATCH 3/5] bpf: cpumap xdp_buff to skb conversion and allocation

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 6 Oct 2017 14:11:40 +0200

On Thu, 05 Oct 2017 12:22:43 +0200
Daniel Borkmann <[email-masked]> wrote:

> On 10/04/2017 02:03 PM, Jesper Dangaard Brouer wrote:
> [...]
> >   static int cpu_map_kthread_run(void *data)
> >   {
> >   	struct bpf_cpu_map_entry *rcpu = data;
> >
> >   	set_current_state(TASK_INTERRUPTIBLE);
> >   	while (!kthread_should_stop()) {
> > +		unsigned int processed = 0, drops = 0;
> >   		struct xdp_pkt *xdp_pkt;
> >
> > -		schedule();
> > -		/* Do work */
> > -		while ((xdp_pkt = ptr_ring_consume(rcpu->queue))) {
> > -			/* For now just "refcnt-free" */
> > -			page_frag_free(xdp_pkt);
> > +		/* Release CPU reschedule checks */
> > +		if (__ptr_ring_empty(rcpu->queue)) {
> > +			schedule();
> > +		} else {
> > +			cond_resched();
> > +		}
> > +
> > +		/* Process packets in rcpu->queue */
> > +		local_bh_disable();
> > +		/*
> > +		 * The bpf_cpu_map_entry is single consumer, with this
> > +		 * kthread CPU pinned. Lockless access to ptr_ring
> > +		 * consume side valid as no-resize allowed of queue.
> > +		 */
> > +		while ((xdp_pkt = __ptr_ring_consume(rcpu->queue))) {
> > +			struct sk_buff *skb;
> > +			int ret;
> > +
> > +			skb = cpu_map_build_skb(rcpu, xdp_pkt);
> > +			if (!skb) {
> > +				page_frag_free(xdp_pkt);
> > +				continue;
> > +			}
> > +
> > +			/* Inject into network stack */
> > +			ret = netif_receive_skb_core(skb);  
> 
> Don't we need to hold RCU read lock for above netif_receive_skb_core()?

Yes, I guess we do, but I'll place it in netif_receive_skb_core() before
invoking __netif_receive_skb_core(), like netif_receive_skb() does
around __netif_receive_skb().

It looks like the RCU section protects:
 	rx_handler = rcu_dereference(skb->dev->rx_handler);

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer