process hang in zfs->io_ ?

I'm running current as of 20080130 on an amd64 box. I can make
processes stuck in zfs->io_ (output truncated in ddb and top) when I
make some packages via ports tinderbox. The ports tinderbox access
local disk via nfs (I also tried nullfs).

ddb output of ps and alltrace can be found at

http://www.rafan.org/FreeBSD/zfs/textdump.zfs.20090130.txt

Any ideas?

Thanks,
Rong-En Fan