panic: sx_xlock() of destroyed sx _at_ /zoo/kris/src8/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_rlock.c:535

9.0 doing I/O to a zfs:

panic: sx_xlock() of destroyed sx _at_ 
/zoo/kris/src8/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_rlock.c:535
db> wh
Tracing pid 14 tid 100047 td 0xffffff000357c720
kdb_enter() at kdb_enter+0x3d
panic() at panic+0x17b
_sx_xlock() at _sx_xlock+0xe9
zfs_range_unlock() at zfs_range_unlock+0x38
zfs_get_data() at zfs_get_data+0xd7
zil_commit() at zil_commit+0x532
zfs_sync() at zfs_sync+0xa6
sync_fsync() at sync_fsync+0x13a
VOP_FSYNC_APV() at VOP_FSYNC_APV+0xb7
sync_vnode() at sync_vnode+0x157
sched_sync() at sched_sync+0x1d1
fork_exit() at fork_exit+0x12a
fork_trampoline() at fork_trampoline+0xe
--- trap 0, rip = 0, rsp = 0xffffff8125da0d30, rbp = 0 ---

This was essentially just doing make world + cvs update + tar creation 
in a loop and failed after about a week.

Kris