Guaranteed kernel panic with ZFS + nullfs

[Andriy Gapon]

on 17/03/2011 14:34 Luke Marsden said the following:
> On Thu, 2011-03-17 at 13:37 +0200, Kostik Belousov wrote:
>> [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)

I believe that it's a bad idea to forcefully unmount a filesystem under a nullfs
mount.  Without -f the unmounting wouldn't succeed?

>>> sudo zfs mount $POOL/foo # PANIC!
>>>
>>> Can anyone suggest a patch which fixes this? Preferably against
>>> 8-STABLE :-)
>> Please show the backtrace.
>>
> 
> Here you go:
> 
> http://lukemarsden.net/zfs-panic-1.png
> http://lukemarsden.net/zfs-panic-2.png

IMO this is expected.  I am not sure if this is ZFS specific or if it can happen
with any kind of an underlying filesystem.

Maybe Edward would be interested in fixing this behavior? :)

-- 
Andriy Gapon