moxart_mac_start_xmit() doesn't care where the tail is, head can catch up and pass the tail multiple times without ever calling moxart_tx_finished() (called from IRQ handler).

This is especially bad under congestion when dev_kfree_skb_irq() is not called for large numbers of skbs, see slabinfo comparison below:

[root@zurkon ~]# cat /proc/slabinfo > /root/slabinfo
    
[  140.280000] moxart-ethernet 90900000.ethernet eth0: moxart_mac_start_xmit tx_head=63 tx_tail=47
.. (1-62 printed, moxart_tx_finished() never called)
[  140.290000] moxart-ethernet 90900000.ethernet eth0: moxart_mac_start_xmit tx_head=0 tx_tail=47
.. (1-62 printed, moxart_tx_finished() never called)
[  140.880000] moxart-ethernet 90900000.ethernet eth0: moxart_mac_start_xmit tx_head=63 tx_tail=47
[  140.890000] moxart-ethernet 90900000.ethernet eth0: moxart_mac_start_xmit tx_head=0 tx_tail=47
.. (1-38 printed, moxart_tx_finished() never called)
[  141.260000] moxart-ethernet 90900000.ethernet eth0: moxart_mac_start_xmit tx_head=39 tx_tail=47
[  141.270000] moxart-ethernet 90900000.ethernet eth0: moxart_tx_finished: 47
.. (48-38 moxart_tx_finished())
[  141.270000] moxart-ethernet 90900000.ethernet eth0: moxart_tx_finished: 39
[  141.290000] moxart-ethernet 90900000.ethernet eth0: moxart_mac_start_xmit tx_head=40 tx_tail=40
    
[root@zurkon ~]# cat /root/slabinfo | grep kmallo
kmalloc-8192           4      4   8192    4    8 : tunables    0    0    0 : slabdata      1      1      0
kmalloc-4096          15     16   4096    8    8 : tunables    0    0    0 : slabdata      2      2      0
kmalloc-2048          50     56   2048    8    4 : tunables    0    0    0 : slabdata      7      7      0
kmalloc-1024          44     48   1024    8    2 : tunables    0    0    0 : slabdata      6      6      0
kmalloc-512          173    176    512    8    1 : tunables    0    0    0 : slabdata     22     22      0
kmalloc-256          112    112    256   16    1 : tunables    0    0    0 : slabdata      7      7      0
kmalloc-192          222    252    192   21    1 : tunables    0    0    0 : slabdata     12     12      0
kmalloc-128          138    192    128   32    1 : tunables    0    0    0 : slabdata      6      6      0
kmalloc-96           400    420     96   42    1 : tunables    0    0    0 : slabdata     10     10      0
kmalloc-64           366    384     64   64    1 : tunables    0    0    0 : slabdata      6      6      0
kmalloc-32          1985   2048     32  128    1 : tunables    0    0    0 : slabdata     16     16      0
    
[root@zurkon ~]# cat /proc/slabinfo | grep kmalloc
kmalloc-8192           4      4   8192    4    8 : tunables    0    0    0 : slabdata      1      1      0
kmalloc-4096          15     16   4096    8    8 : tunables    0    0    0 : slabdata      2      2      0
kmalloc-2048        2259   2288   2048    8    4 : tunables    0    0    0 : slabdata    286    286      0
kmalloc-1024          55     56   1024    8    2 : tunables    0    0    0 : slabdata      7      7      0
kmalloc-512          175    176    512    8    1 : tunables    0    0    0 : slabdata     22     22      0
kmalloc-256          112    112    256   16    1 : tunables    0    0    0 : slabdata      7      7      0
kmalloc-192         4446   4494    192   21    1 : tunables    0    0    0 : slabdata    214    214      0
kmalloc-128          141    224    128   32    1 : tunables    0    0    0 : slabdata      7      7      0
kmalloc-96           400    420     96   42    1 : tunables    0    0    0 : slabdata     10     10      0
kmalloc-64           402    512     64   64    1 : tunables    0    0    0 : slabdata      8      8      0
kmalloc-32          1976   2048     32  128    1 : tunables    0    0    0 : slabdata     16     16      0

https://bitbucket.org/Kasreyn/linux-next/commits/746afa1beb9768a6f4082afcfb28ee2ea1d24864