shutdown not working with uart console

Dear coders,

I recognized one RELENG_8 problem when trying to shutdown via serial 
console. While shutdown via SSH works like expected, when called via 
serial console it keeps hanging forever.
I compiled a debug kernel for my embedded device and got the following 
backtrace:
~KDB: enter: Line break on console
[thread pid 1102 tid 100074 ]
Stopped at      kdb_enter+0x3b: movl    $0,kdb_why
db> bt
Tracing pid 1102 tid 100074 td 0xc36c26c0
kdb_enter(c08aca1d,c08b61ae,d5c5e894,c35a0180,0,...) at kdb_enter+0x3b
uart_intr(c35a0100,c36c26c0,c344a900,c347b0d0,4,...) at uart_intr+0x298
intr_event_handle(c344a900,d5c5e8ac,c36c26c0,c37a9ad0,1ce,...) at 
intr_event_handle+0x4b
intr_execute_handlers(c347b0d0,d5c5e8ac,0,d5c5e90c,c0852ed4,...) at 
intr_execute_handlers+0x48
lapic_handle_intr(38,d5c5e8ac) at lapic_handle_intr+0x3a
Xapic_isr1() at Xapic_isr1+0x34
--- interrupt, eip = 0xc05d0814, esp = 0xd5c5e8ec, ebp = 0xd5c5e90c ---
_lockmgr_assert(c37a9ad0,1,c08d41d6,1ce,d5c5e940,...) at 
_lockmgr_assert+0x13e
__lockmgr_args(c37a9ad0,100000,c37a9aec,0,0,...) at __lockmgr_args+0x16b
vop_stdunlock(d5c5e9ec,0,c36c26c0,80000,c37a9a78,...) at vop_stdunlock+0x55
VOP_UNLOCK_APV(c0912b40,d5c5e9ec,c091d71c,c09477e0,c37a9a78,...) at 
VOP_UNLOCK_APV+0x9a
_vn_lock(c37a9a78,80100,c08d4ec2,823,c08c5fec,...) at _vn_lock+0xbc
vget(c37a9a78,80100,c36c26c0,4f3,c37a9a78,...) at vget+0x61
devfs_revoke(d5c5eaa4,d5c5eaa4,c08c2b09,157,c36cf330,...) at 
devfs_revoke+0x210
exit1(c36c26c0,1,c08c78e8,ab6,3,...) at exit1+0x9ff
sigexit(c36c26c0,1,c08c78e8,a49,0,...) at sigexit+0x84
postsig(1,64,c08cc013,df,c36cf2a8,...) at postsig+0x34d
ast(d5c5ed38) at ast+0x42f
doreti_ast() at doreti_ast+0x17

I have no idea where the problem is, nor how to get more info.
Any help appreciated.

Thanks in advance,

-Harry