sporadic hangs on generic/186

[bfields@fieldses.org (J. Bruce Fields)]

In the last couple days I've started getting hangs on xfstests
generic/186 on upstream.  I also notice the test completes after 10+
hours (usually it takes about 5 minutes).  Sometimes this is accompanied
by "nfs: RPC call returned error 12" on the client.

Test description is:

# Ensuring that copy on write in buffered mode works when free space
# is heavily fragmented.
#   - Create two files
#   - Reflink the odd blocks of the first file into a third file.
#   - Reflink the even blocks of the second file into the third file.
#   - Try to fragment the free space by allocating a huge file and
#     punching out every other block.
#   - CoW across the halfway mark.
#   - Check that the files are now different where we say they're
#   different.

so maybe it's really some xfs change, I don't know.  Or maybe it's a
problem with my particular test filesystem (which doesn't get recreated
for each test run).

The problem doesn't reproduce easily enough to bisect.

I may just turn off that test for now.

--b.