debugging rmmod failures

[Steve French]

Any suggestions on how to debug why rmmod fails with EBUSY?

I was trying to debug a problem where rmmod fails during an xfstest
run but couldn't find useful information on how to get ideas 'why'
rmmod is failing (in this case with EBUSY) and I could only find two
places where EBUSY would plausibly be returned but don't see the
pr_debug message in the log (e.g. "already dying") that I would expect
to get in those cases.

It also fails in this test scenario (which takes a while to reproduce
so isn't trivial to repro) with EBUSY when doing "rmmod --verbose
--force"  and the --verbose didn't display any additional info.

I also tried "trace-cmd record -e *module*" which showed it (one call)
returning 0xFFFFFFF0 but nothing useful that I could see.

Any ideas on how to debug *why* an rmmod fails?
-- 
Thanks,

Steve