What was happening, was that the main mfs loop was sleeping, and when it was
being awoken by a wakeup when it was supposed to process some IO requests.
The problem was that if it was being woken out of the tsleep() by a signal
at shutdown, it was going straight into dounmount() without servicing any
pending IO requests, causing dounmount() to fail because there were busy
buffers (and they could not be "processed" because the processing loop was
trying to unmount rather than dispatching into mfs_doio()).
This (dare I say it :-) appears to be a layering problem....
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