Guaranteed kernel panic with ZFS + nullfs

[Kostik Belousov]

[Enormous Cc: list trimmed down]
On Thu, Mar 17, 2011 at 12:08:01AM -0400, Luke Marsden wrote:
> Hi all,
> 
> The following script seems to cause a guaranteed kernel panic on 8.1-R,
> 8.2-R and 8-STABLE as of today (2011-03-16), with both ZFS v14/15, and
> v28 on 8.2-R with mm@ patches from 2011-03. I suspect it may also affect
> 9-CURRENT but have not tested this yet.
> 
> #!/usr/local/bin/bash
> export POOL=hpool # change this to your pool name
> sudo zfs destroy -r $POOL/foo
> sudo zfs create $POOL/foo
> sudo zfs set mountpoint=/foo $POOL/foo
> sudo mount -t nullfs /foo /bar
> sudo touch /foo/baz
> ls /bar # should see baz
> sudo zfs umount -f $POOL/foo # seems okay (ls: /bar: Bad file
> descriptor)
> sudo zfs mount $POOL/foo # PANIC!
> 
> Can anyone suggest a patch which fixes this? Preferably against
> 8-STABLE :-)
Please show the backtrace.

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