"spin lock sched lock held by 0xc63b7870 for > 5 seconds" at reboot

i386 SMP server, up-to-date current:

Uptime: 1d22h27m15s
Rebooting...
cpu_reset: Stopping other CPUs
spin lock sched lock held by 0xc63b7870 for > 5 seconds
panic: spin lock held too long
cpuid = 0
KDB: enter: panic
[thread pid 99832 tid 100162 ]
Stopped at      kdb_enter+0x30: leave
db> wh
Tracing pid 99832 tid 100162 td 0xc84d5360
kdb_enter(c072025f,0,c071f552,ef2dab08,c84d5360) at kdb_enter+0x30
panic(c071f552,c63b7870,c63b7870,c84d5360,c0786c30) at panic+0x13f
_mtx_lock_spin(c0786c30,c84d5360,2,c071bf1d,cb) at _mtx_lock_spin+0xbd
_mtx_lock_spin_flags(c0786c30,2,c071bf1d,cb,0) at _mtx_lock_spin_flags+0xe7
hardclock_cpu(0,c0786c30,2,c0721c41,c07fcd40) at hardclock_cpu+0x3c
hardclock(0,c06e0b97,0,ef2dac18,c06c81ae) at hardclock+0x18
lapic_handle_timer(8,28,28,f4240,0) at lapic_handle_timer+0x7e
Xtimerint() at Xtimerint+0x1e
--- interrupt, eip = 0xc06e0b97, esp = 0xef2dabe4, ebp = 0xef2dac18 ---
DELAY(f4240,c63b0220,ef2dac34,c052f63d,f4240) at DELAY+0x9f
cpu_reset(f4240,ef2dac64,c052f4fa,0,0) at cpu_reset+0xce
shutdown_reset(0,0,c0720283,195,0) at shutdown_reset+0x23
boot(0,0,c0720283,a2,ef2dad30) at boot+0x65a
reboot(c84d5360,ef2dad04,4,445,c6926420) at reboot+0x47
syscall(3b,3b,3b,0,0) at syscall+0x2ea
Xint0x80_syscall() at Xint0x80_syscall+0x1f
--- syscall (55, FreeBSD ELF32, reboot), eip = 0x280a9f5b, esp = 0xbfbfe80c, ebp = 0xbfbfe858 ---
db> show locks
exclusive sleep mutex Giant r = 0 (0xc0786c48) locked _at_ kern/vfs_syscalls.c:126
db> show alllocks
Process 99832 (reboot) thread 0xc84d5360 (100162)
exclusive sleep mutex Giant r = 0 (0xc0786c48) locked _at_ kern/vfs_syscalls.c:126
db> show allpcpu
Current CPU: 0

cpuid        = 0
curthread    = 0xc84d5360: pid 99832 "reboot"
curpcb       = 0xef2dad90
fpcurthread  = none
idlethread   = 0xc63b7a20: pid 13 "idle: cpu0"
APIC ID      = 0
currentldt   = 0x50
spin locks held:

cpuid        = 1
curthread    = 0xc63b7870: pid 12 "idle: cpu1"
curpcb       = 0xea3f5d90
fpcurthread  = none
idlethread   = 0xc63b7870: pid 12 "idle: cpu1"
APIC ID      = 1
currentldt   = 0x50
spin locks held:

^---- Note: allegedly no spin locks held, contrary to panic

cpuid        = 2
curthread    = 0xc63b76c0: pid 11 "idle: cpu2"
curpcb       = 0xea3f2d90
fpcurthread  = none
idlethread   = 0xc63b76c0: pid 11 "idle: cpu2"
APIC ID      = 2
currentldt   = 0x50
spin locks held:

cpuid        = 3
curthread    = 0xc63b7510: pid 10 "idle: cpu3"
curpcb       = 0xea3efd90
fpcurthread  = none
idlethread   = 0xc63b7510: pid 10 "idle: cpu3"
APIC ID      = 3
currentldt   = 0x50
spin locks held:

db> wh 12
Tracing pid 12 tid 100004 td 0xc63b7870
cpustop_handler(ea3f5b90,c06dcd98,1,c,c63b7870) at cpustop_handler+0x2c
ipi_nmi_handler(1,c,c63b7870,ea3f5b28,ea3f5b28) at ipi_nmi_handler+0x29
trap(c0520008,c63b0028,ea3f0028,6a0,c078cc58) at trap+0x3c
calltrap() at calltrap+0x5
--- trap 0x13, eip = 0xc05254bf, esp = 0xea3f5bd8, ebp = 0xea3f5bf8 ---
_mtx_lock_spin_flags(c078cc58,0,c07242fa,6a0,c0786c30) at _mtx_lock_spin_flags+0x47
witness_lock_list_get(0,c0786c30,c0796458,c63b7870,c0786c30) at witness_lock_list_get+0x3d
witness_lock(c0786c30,a,c071bf26,cb,c0721c41) at witness_lock+0xcd
_mtx_lock_spin_flags(c0786c30,2,c071bf1d,cb,c07fcd6c) at _mtx_lock_spin_flags+0x104
hardclock_cpu(0,ea3f5c9c,2,ea3f5cdc,c06c81ae) at hardclock_cpu+0x3c
lapic_handle_timer(8,ea3f0028,c0540028,0,fffffffd) at lapic_handle_timer+0xa3
Xtimerint() at Xtimerint+0x1e
--- interrupt, eip = 0xc06d507f, esp = 0xea3f5cdc, ebp = 0xea3f5cdc ---
mp_grab_cpu_hlt(ea3f5d04,c0516ac2,c0786c48,2,c071d59e) at mp_grab_cpu_hlt+0x16
cpu_idle(c0786c48,2,c071d59e,70,c63b68d0) at cpu_idle+0x8
idle_proc(0,ea3f5d38,c071d434,31a,0) at idle_proc+0x92
fork_exit(c0516a30,0,ea3f5d38) at fork_exit+0xc5
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xea3f5d6c, ebp = 0 ---
db> wh 11
Tracing pid 11 tid 100005 td 0xc63b76c0
cpustop_handler(ea3f2c8c,c06dcd98,c055c1c7,c07d2c68,0) at cpustop_handler+0x2c
ipi_nmi_handler(c055c1c7,c07d2c68,0,c0786c30,c0796458) at ipi_nmi_handler+0x29
trap(ea3f0008,c06c0028,c06c0028,0,fffffffb) at trap+0x3c
calltrap() at calltrap+0x5
--- trap 0x13, eip = 0xc0546bb8, esp = 0xea3f2cd4, ebp = 0xea3f2cd4 ---
runq_check(c0788640,4,ea3f2d04,c0516ac7,c0786c48) at runq_check+0xf
sched_runnable(c0786c48,2,c071d59e,70,c63b6b04) at sched_runnable+0x13
idle_proc(0,ea3f2d38,c071d434,31a,0) at idle_proc+0x97
fork_exit(c0516a30,0,ea3f2d38) at fork_exit+0xc5
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xea3f2d6c, ebp = 0 ---
db> wh 10
Tracing pid 10 tid 100006 td 0xc63b7510
cpustop_handler(ea3efc94,c06dcd98,c055c1c7,c07d2c68,0) at cpustop_handler+0x2c
ipi_nmi_handler(c055c1c7,c07d2c68,0,c0786c30,c0796458) at ipi_nmi_handler+0x29
trap(8,ea3e0028,c0540028,0,fffffff7) at trap+0x3c
calltrap() at calltrap+0x5
--- trap 0x13, eip = 0xc06d507f, esp = 0xea3efcdc, ebp = 0xea3efcdc ---
mp_grab_cpu_hlt(ea3efd04,c0516ac2,c0786c48,2,c071d59e) at mp_grab_cpu_hlt+0x16
cpu_idle(c0786c48,2,c071d59e,70,c63b6d38) at cpu_idle+0x8
idle_proc(0,ea3efd38,c071d434,31a,0) at idle_proc+0x92
fork_exit(c0516a30,0,ea3efd38) at fork_exit+0xc5
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xea3efd6c, ebp = 0 ---
db>