shutdown -r hangs after update to 8.1

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I have two machines where shutdown -r just failed to actually reboot.
Both were recently updated from 6.1 to 8.1, my impression is that it worked 
before.
Both display "syncing disks...1 done" and then hang.
On both, I'm able to enter ddb.

On the first one, the backtrace (minus the interrupt) is
mutex_enter
ffs_sync
VFS_SYNC
sched_sync
which strikes me as odd since the sync has claimed to finish.
entering "reboot" in ddb then actually reboots.

On the second one, the backtrace is
x86_stihlt
acpicpu_cstate_idle_entry
acpicpu_cstate_idle
idle_loop
and entering "reboot" doesn't actually reboot.

Both machines were wedged with mst probably the vnode lock on the root of an 
NFS import held, but that appears unrelated to me.

Any clues?