zpool export: umount fails, device inexplicably busy (lsof finds nothing)

[Graham Perrin]

On 25/11/2020 17:35, Graham Perrin wrote:
> …
>> root at momh167-gjp4-8570p:~ # zpool export Transcend
>> cannot unmount '/Volumes/t500': umount failed
>> root at momh167-gjp4-8570p:~ # umount /Volumes/t500
>> umount: unmount of /Volumes/t500 failed: Device busy
>> root at momh167-gjp4-8570p:~ # lsof /Volumes/t500/
>> root at momh167-gjp4-8570p:~ #
>>
>> ----
>>
>> When this last occurred, I could export the pool after signing out 
>> from KDE.
>>
> The problem seemed to be consistently reproducible but (fingers 
> crossed) no problem yesterday or today.
>
> Previously: most often using OpenZFS in base.
>
> Now using OpenZFS from ports, with r367936.
>
> If the problem recurs: is it _ever_ sane to get nothing listed by lsof 
> for a volume that can not be unmounted?

I spoke too soon, the problem recurred this afternoon:

root at mowa219-gjp4-8570p:~ # zpool export Transcend
cannot unmount '/Volumes/t500/VirtualBox': umount failed
root at mowa219-gjp4-8570p:~ # lsof /Volumes/t500/VirtualBox
lsof: WARNING: device cache mismatch: /dev/drm/121
lsof: WARNING: /root/.lsof_mowa219-gjp4-8570p was updated.
root at mowa219-gjp4-8570p:~ # lsof /Volumes/t500/VirtualBox
root at mowa219-gjp4-8570p:~ # zpool export Transcend
cannot unmount '/Volumes/t500/VirtualBox': umount failed
root at mowa219-gjp4-8570p:~ # uname -v
FreeBSD 13.0-CURRENT #72 r367936: Sun Nov 22 21:46:00 GMT 2020 
root at mowa219-gjp4-8570p:/usr/obj/usr/src/amd64.amd64/sys/GENERIC-NODEBUG
root at mowa219-gjp4-8570p:~ # zfs --version
zfs-0.8.0-1
zfs-kmod-v2020110200-zfs_8583540c6
root at mowa219-gjp4-8570p:~ #