igb driver RX (was TX) hangs when out of mbuf clusters

[Jack Vogel]

I have been following this, and thinking about it. I still am working from a
theoretical
standpoint, but based on a patch I got quite a long time back and never
quite groked,
I believe now that I might have a solution.

The original PR and patch was kern/150516 from Beezar Liu,  I was never
quite comfortable
with the code changes, nor convinced that it was a real issue and not a
misunderstanding.
However I think now that this very report might be behind what we are seeing
today. I have
a slightly different approach to solving it, of course it remains to be seen
if it handles it
properly.

Please try the patch I've attached, I'm open to further correction or
polishing of the
changes. And thanks to Beezar for his original report and changes, this is
not for em,
but if this eliminates the problem its clearly needed in all drivers.

Jack
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