reboot hanging

I get this fairly often when rebooting an amd64 machine.  Looks like
it's hung trying to do a TLB shootdown.

hammer1# reboot
Aug  3 20:56:11 hammer1 reboot: rebooted by root
Aug  3 20:56:11 hammer1 syslogd: exiting on signal 15
Waiting (max 60 seconds) for system process `vnlru' to stop...done
Waiting (max 60 seconds) for system process `bufdaemon' to stop...done
Waiting (max 60 seconds) for system process `syncer' to stop...
Syncing disks, vnodes remaining...2 2 0 1 1 1 0 0 0 done
All buffers synced.

[hung here]

KDB: enter: Break sequence on console
[thread pid 16 tid 100005 ]
Stopped at      kdb_enter+0x31: leave
db> wh
Tracing pid 16 tid 100005 td 0xffffff00010c96a0
kdb_enter() at kdb_enter+0x31
siointr1() at siointr1+0xde
siointr() at siointr+0x31
intr_execute_handlers() at intr_execute_handlers+0x102
lapic_handle_intr() at lapic_handle_intr+0x39
Xapic_isr1() at Xapic_isr1+0x7f
--- interrupt, rip = 0xffffffff8039d376, rsp = 0xffffffffabc93b70, rbp = 0xffffffffabc93b80 ---
acpi_cpu_c1() at acpi_cpu_c1+0x6
acpi_cpu_idle() at acpi_cpu_idle+0x1c4
cpu_idle() at cpu_idle+0x29
sched_idletd() at sched_idletd+0x1b1
fork_exit() at fork_exit+0x14a
fork_trampoline() at fork_trampoline+0xe
--- trap 0, rip = 0, rsp = 0xffffffffabc93d30, rbp = 0 ---
db> show allpcpu
Current CPU: 1

cpuid        = 0
curthread    = 0xffffff00010ef9f0: pid 19 "swi4: clock sio"
curpcb       = 0xffffffffabcb1d40
fpcurthread  = none
idlethread   = 0xffffff00010c3000: pid 17 "idle: cpu0"

cpuid        = 1
curthread    = 0xffffff00010c96a0: pid 16 "idle: cpu1"
curpcb       = 0xffffffffabc93d40
fpcurthread  = none
idlethread   = 0xffffff00010c96a0: pid 16 "idle: cpu1"

cpuid        = 2
curthread    = 0xffffff00010c9350: pid 15 "idle: cpu2"
curpcb       = 0xffffffffabc8ed40
fpcurthread  = none
idlethread   = 0xffffff00010c9350: pid 15 "idle: cpu2"

cpuid        = 3
curthread    = 0xffffff00010c6350: pid 3 "g_up"
curpcb       = 0xffffffffabc7ad40
fpcurthread  = none
idlethread   = 0xffffff00010c9000: pid 14 "idle: cpu3"

cpuid        = 4
curthread    = 0xffffff00010c69f0: pid 13 "idle: cpu4"
curpcb       = 0xffffffffabc84d40
fpcurthread  = none
idlethread   = 0xffffff00010c69f0: pid 13 "idle: cpu4"

cpuid        = 5
curthread    = 0xffffff00010c6000: pid 12 "idle: cpu5"
curpcb       = 0xffffffffabc75d40
fpcurthread  = none
idlethread   = 0xffffff00010c6000: pid 12 "idle: cpu5"

cpuid        = 6
curthread    = 0xffffff00010c39f0: pid 11 "idle: cpu6"
curpcb       = 0xffffffffabc70d40
fpcurthread  = none
idlethread   = 0xffffff00010c39f0: pid 11 "idle: cpu6"

cpuid        = 7
curthread    = 0xffffff00010c36a0: pid 10 "idle: cpu7"
curpcb       = 0xffffffffabc6bd40
fpcurthread  = none
idlethread   = 0xffffff00010c36a0: pid 10 "idle: cpu7"

db> wh 3
Tracing pid 3 tid 100010 td 0xffffff00010c6350
cpustop_handler() at cpustop_handler+0x40
ipi_nmi_handler() at ipi_nmi_handler+0x34
trap() at trap+0x4d
nmi_calltrap() at nmi_calltrap+0x8
--- trap 0x13, rip = 0xffffffff803b01b2, rsp = 0xffffffffab19aff0, rbp = 0xffffffffabc7ab00 ---
smp_tlb_shootdown() at smp_tlb_shootdown+0xa2
smp_invlpg_range() at smp_invlpg_range+0x1e
pmap_invalidate_range() at pmap_invalidate_range+0x59
pmap_qremove() at pmap_qremove+0x65
swp_pager_async_iodone() at swp_pager_async_iodone+0x5f
bufdone() at bufdone+0x9e
swapgeom_done() at swapgeom_done+0x3d
biodone() at biodone+0x88
g_io_schedule_up() at g_io_schedule_up+0x122
g_up_procbody() at g_up_procbody+0x78
fork_exit() at fork_exit+0x14a
fork_trampoline() at fork_trampoline+0xe
--- trap 0, rip = 0, rsp = 0xffffffffabc7ad30, rbp = 0 ---
db>