Subject: Re: pktgen and spin_lock_bh in xmit path

From: Ben Greear <[email-masked]>
Date: Tue, 20 Oct 2009 10:37:36 -0700

On 10/19/2009 09:52 PM, Ben Greear wrote:
> Eric Dumazet wrote:
>> Ben Greear a écrit :
>>> I'm having strange issues when running pktgen on 10G interfaces while
>>> also running
>>> pktgen on mac-vlans on that interface, when the mac-vlan pktgen threads
>>> are on a different
>>> CPU.

I think I found the problem.  First, lockdep was not the issue, and mac-vlans
were properly setting up the lockdep keys.  I would have expected lockdep to
figure out I was trying to lock a non-valid lock, but maybe something else
kept that from happening.

Second:  I think the problem can only happen on my code tree because I
added code to allow mac-vlans to return NETDEV_TX_BUSY
when a hacked varient of dev_queue_xmit decided it could not immediately
transmit a packet.  Without my change, a packet would have to be created fresh
in this scenario, so it would not hit the bug.

However, I think pktgen might still need a similar fix because other drivers or
logic might also change the skb tx-queue map.

Here is the problem, or at least one of them:

pktgen tries to xmit, but gets NETDEV_TX_BUSY.  During the xmit attempt, the
skb queue map was changed to that of the underlying device, which was 4.  Note
that mac-vlans have only a single tx queue.
pktgen will retry this skb, but it never resets the skb queue back to 0.
This means that it will soon be accessing txq[4], which is corrupting
memory.  Things rapidly decline from here!

Here is a patch for comment, in case the pktgen folks would like to
apply something similar:

@@ -3991,11 +4001,26 @@ static void pktgen_xmit(struct pktgen_dev *pkt_dev, u64 now)
                 }
         }

-       if (!pkt_dev->skb) {
+       if ((!pkt_dev->skb) || (pkt_dev->clone_count <= 1)) {
+               /** If clone count is low, that might be because device is a layered
+                * virtual device, like mac-vlan.  In that case, the queue-map may be
+                * changed while transmitting out the lower levels, so we need to
+                * reset this here so we don't accidentally use a bogus queue.
+                */
+       reset_queue_map:
                 set_cur_queue_map(pkt_dev);
                 queue_map = pkt_dev->cur_queue_map;
         } else {
                 queue_map = skb_get_queue_mapping(pkt_dev->skb);
+               if (unlikely(queue_map >= odev->num_tx_queues)) {
+                       static int do_once = 1;
+                       if (do_once) {
+                               printk("pktgen ERROR:  queue_map range error, queue_map: %i  num_tx_queues: %i  iface: %s\n",
+                                      queue_map, odev->num_tx_queues, odev->name);
+                               WARN_ON(1);
+                       }
+                       goto reset_queue_map;
+               }
         }

         txq = netdev_get_tx_queue(odev, queue_map);

Thanks,
Ben

-- 
Ben Greear <[email-masked]>
Candela Technologies Inc  http://www.candelatech.com