5.4-RELEASE-p4: shutdown hangs after rebuiding gmirror

[Michiel Boland]

[recap: machine hangs on shutdown - see original message for more details]
> Syncing disks, vnodes remaining...1 1 0 1 1 0 0 0 done
> No buffers busy after final sync
> Uptime: 55m39s
> GEOM_MIRROR: Device raid1: provider mirror/raid1 destroyed.
> GEOM_MIRROR: Device raid1 destroyed.

FWIW the hangs occur regardless of whether I rebuild a mirror. The problem 
is that the box sometimes hangs and sometimes not. Of course this makes 
the whole gmirror totally useless (as opposed to a bit of a nuisance)

It appears that the hang occurs somewhere between destruction of raid1 and 
'raid1.sync'. (whatever that is)

Does anyone have any clue as to what is going on or how I can debug this 
further? Please don't make me put solaris on this box. :)

Cheers
Michiel