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

From: John Fastabend <[email-masked]>
Date: Tue, 11 Jul 2017 11:26:54 -0700

On 07/11/2017 07:23 AM, Jesper Dangaard Brouer wrote:
> 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.
> 

Awesome!

> 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();

Maybe another trace event? trace_xdp_redirect_failed()

>         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 */

+1 

ah map lookup failed and we need to do the flush nice catch.

>         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().
> 

Actually I think a synchronize_sched() is needed, after the IXGBE_DOWN bit
is set but before the xdp_tx queues are cleaned up. In practice the ixgbe_down/up
sequence has so many msleep() operations for napi cleanup and hardware sync
I would be really surprised we ever hit this. But for correctness we should
likely add it.	

> 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);
>