panic: spin lock held too long (while rebooting)

I just had this panic, on an SMP machine

Rebooting...
cpu_reset: Stopping other CPUs
spin lock sched lock held by 0xc16de480 for > 5 seconds
panic: spin lock held too long
cpuid = 0
KDB: enter: panic
[thread pid 18843 tid 100041 ]
Stopped at      kdb_enter+0x2b: nop
db> trace
Tracing pid 18843 tid 100041 td 0xc182e600
kdb_enter(c0880e62) at kdb_enter+0x2b
panic(c088029b,c094cdf0,c16de480,c0880272,c08803ee) at panic+0x127
_mtx_lock_spin(c094cdf0,c182e600,2,c087cbfa,cb) at _mtx_lock_spin+0xa9
_mtx_lock_spin_flags(c094cdf0,2,c087cbfa,cb) at _mtx_lock_spin_flags+0x9e
hardclock_cpu(0,c09cdb20,0,c72a2ba0,c08069cd) at hardclock_cpu+0x25
hardclock(0,c0816772) at hardclock+0x13
lapic_handle_timer(8,28,c16b0028,c16ba00c,0) at lapic_handle_timer+0x71
Xtimerint() at Xtimerint+0x1e
--- interrupt, eip = 0xc0816772, esp = 0xc72a2be8, ebp = 0xc72a2c0c ---
DELAY(f4240) at DELAY+0x92
cpu_reset(f4240,c0881049,c72a2c64,c0653a0f,0) at cpu_reset+0xc8
shutdown_reset(0,0,c16ba00c,0,c0880e86) at shutdown_reset+0x1c
boot(0,c094ce0c,0,c0880e86,a1) at boot+0x553
reboot(c182e600,c72a2d04,c182e600,0,2) at reboot+0x32
syscall(3b,3b,3b,0,2) at syscall+0x27e
Xint0x80_syscall() at Xint0x80_syscall+0x1f
--- syscall (55, FreeBSD ELF32, reboot), eip = 0x280adc5f, esp = 0xbfbfedcc, 
ebp = 0xbfbfee18 ---