[Cake] Lockup at high speeds

[Toke Høiland-Jørgensen]

As you may have seen, I hit a lockup bug when testing Cake at high
speeds. I am not quite sure what is happening, but we seem to be looping
forever either in cake_dequeue() or cake_enqueue(), causing deadlocks.

Unfortunately, I have no more time to debug this for now. As far as I
can tell, it only seems to be happening as the shaped rate approaches or
exceeds the link speed, or when running in unlimited mode. So for all I
know it might be possible to trigger it at lower speeds as well...

Anyone with any bright ideas?

-Toke

P.S. While testing this, I did manage to get some high speed results
before everything blew up on me:

Summary of tcp_4up test run 'Cake 40gbps' (at 2018-06-01 17:36:19.216149):

                           avg       median          # data pts
 Ping (ms) ICMP :         0.36         0.12 ms              197
 TCP upload avg :      9563.03      9563.95 Mbits/s         151
 TCP upload sum :     38252.11     38255.80 Mbits/s         151
 TCP upload::1  :      9686.85      9674.40 Mbits/s         150
 TCP upload::2  :      9444.94      9460.09 Mbits/s         150
 TCP upload::3  :      9438.95      9461.36 Mbits/s         150
 TCP upload::4  :      9681.37      9675.69 Mbits/s         150