Subject: Re: [RFC PATCH 00/12] Implement XDP bpf_redirect vairants

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 11 Jul 2017 16:23:44 +0200

On Mon, 10 Jul 2017 17:59:17 -0700
John Fastabend <[email-masked]> wrote:

> On 07/10/2017 11:30 AM, Jesper Dangaard Brouer wrote:
> > On Sat, 8 Jul 2017 21:06:17 +0200
> > Jesper Dangaard Brouer <[email-masked]> wrote:
> >   
> >> On Sat, 08 Jul 2017 10:46:18 +0100 (WEST)
> >> David Miller <[email-masked]> wrote:
> >>  
> >>> From: John Fastabend <[email-masked]>
> >>> Date: Fri, 07 Jul 2017 10:48:36 -0700
> >>>     
> >>>> On 07/07/2017 10:34 AM, John Fastabend wrote:      
> >>>>> This series adds two new XDP helper routines bpf_redirect() and
> >>>>> bpf_redirect_map(). The first variant bpf_redirect() is meant
> >>>>> to be used the same way it is currently being used by the cls_bpf
> >>>>> classifier. An xdp packet will be redirected immediately when this
> >>>>> is called.      
> >>>>
> >>>> Also other than the typo in the title there ;) I'm going to CC
> >>>> the driver maintainers working on XDP (makes for a long CC list but)
> >>>> because we would want to try and get support in as many as possible in
> >>>> the next merge window.
> >>>>
> >>>> For this rev I just implemented on ixgbe because I wrote the
> >>>> original XDP support there. I'll volunteer to do virtio as well.      
> >>>
> >>> I went over this series a few times and it looks great to me.
> >>> You didn't even give me some coding style issues to pick on :-)    
> >>
> >> We (Daniel, Andy and I) have been reviewing and improving on this
> >> patchset the last couple of weeks ;-).  We had some stability issues,
> >> which is why it wasn't published earlier. My plan is to test this
> >> latest patchset again, Monday and Tuesday. I'll try to assess stability
> >> and provide some performance numbers.  
> > 
> > 
> > Damn, I though it was stable, I have been running a lot of performance
> > tests, and then this just happened :-(  
> 
> Thanks, I'll take a look through the code and see if I can come up with
> why this might happen. I haven't hit it on my tests yet though.

I've figured out why this happens, and I have a fix, see patch below
with some comments with questions.

The problem is that we can leak map_to_flush in an error path, the fix:

diff --git a/net/core/filter.c b/net/core/filter.c
index 2ccd6ff09493..7f1f48668dcf 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2497,11 +2497,14 @@ int xdp_do_redirect_map(struct net_device *dev, struct xdp_buff *xdp,
        ri->map = NULL;
 
        trace_xdp_redirect(dev, fwd, xdp_prog, XDP_REDIRECT);
-
+       // Q: Should we also trace "goto out" (failed lookup)?
+       //    like bpf_warn_invalid_xdp_redirect();
        return __bpf_tx_xdp(fwd, map, xdp, index);
 out:
        ri->ifindex = 0;
-       ri->map = NULL;
+       // XXX: here we could leak ri->map_to_flush, which could be
+       //      picked up later by xdp_do_flush_map()
+       xdp_do_flush_map(); /* Clears ri->map_to_flush + ri->map */
        return -EINVAL;

While debugging this, I noticed that we can have packets in-flight,
while the XDP RX rings are being reconfigured.  I wonder if this is a
ixgbe driver XDP-bug?  I think it would be best to add some
RCU-barrier, after ixgbe_setup_tc().

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index ed97aa81a850..4872fbb54ecd 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -9801,7 +9804,18 @@ static int ixgbe_xdp_setup(struct net_device *dev, struct bpf_prog *prog)
 
        /* If transitioning XDP modes reconfigure rings */
        if (!!prog != !!old_prog) {
-               int err = ixgbe_setup_tc(dev, netdev_get_num_tc(dev));
+               // XXX: Warn pkts can be in-flight in old_prog
+               //      while ixgbe_setup_tc() calls ixgbe_close(dev).
+               //
+               // Should we avoid these in-flight packets?
+               // Would it be enough to add an synchronize_rcu()
+               // or rcu_barrier()?
+               // or do we need an napi_synchronize() call here?
+               //
+               int err;
+               netdev_info(dev,
+                           "Calling ixgbe_setup_tc() to reconfig XDP rings\n");
+               err = ixgbe_setup_tc(dev, netdev_get_num_tc(dev));
 
                if (err) {
                        rcu_assign_pointer(adapter->xdp_prog, old_prog);

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