zfs upgrade hang upgrading from v3 to v5

[Darren Pilgrim]

On 1/4/2014 2:54 PM, Darren Pilgrim wrote:
> Later today I'll try setting a
> mountpoint on it see if the upgrade will succeed then.

Well, later that day didn't happen, but this morning did.

To recap:

When I upgraded the system from 8.3 to 9.2, I upgraded the zfs pool to 
feature flags and all filesystems to version 5.  Everything upgraded 
fine, except z0, the base filesystem in the pool.  z0 is the only 
filesystem with mountpoint=none.  When upgrading z0, zfs upgrade hangs 
in tx->tx_sync_done_cv.  The rest of the system apparently hangs on disk 
I/O to an extent requiring a hard reboot (NMI reset).

Setting a mountpoint made the upgrade work:

# zfs get mountpoint z0
NAME  PROPERTY    VALUE       SOURCE
z0    mountpoint  none        local
# mkdir /z0 &&  zfs set mountpoint=/z0 z0
# zfs list z0
NAME   USED  AVAIL  REFER  MOUNTPOINT
z0    84.3G   361G    30K  /z0
# zfs upgrade z0
1 filesystems upgraded