Ok, nm about the sosreport - I got the info from some older emails from
axino, nova is using qemu-nbd to locally mount images and access the
partitions inside them.  I was able to trivially reproduce this simply
by creating an image, attaching it with qemu-nbd to /dev/nbd0,
partitioning it and mkfs its p1 and then mounting it, then while copying
a file to it, performing qemu-nbd -d to un-attach it to /dev/nbd0.  That
causes the spam of "Attempted..." error messages.

So this appears to be a simple case of nova calling qemu-nbd -d while
there is still I/O to the image.  The right thing to do is simply
ratelimit the error messages (and they really should be anyway, as
they're printing directly inside a loop).  The messages themselves do
not indicate any kernel error, simply that the nbd device was removed
while being written to.

Can you try this kernel PPA to see if it fixes the problem?  You will
still see the error messages, but only a few lines since they'll be

Of course there is still the (probably more serious) problem of the serial port 
driver hanging a cpu and eating up memory; that probably deserves its own bug, 
since it's caused by this, but a separate issue.

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  Soft lockup with "block nbdX: Attempted send on closed socket" spam

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