Panic - blockable sleep lock

Just got this:

panic: blockable sleep lock (sleep mutex) sellck _at_
/usr/src/sys/kern/sys_generic.c:1190
Debugger("panic")
Stopped at      Debugger+0x54:  xchgl   %ebx,in_Debugger.0
db> t
Debugger(c029bc34,c02de7a0,c029e847,df12da20,1) at Debugger+0x54
panic(c029e847,c02a9469,c029f08e,c029f065,4a6) at panic+0xab
witness_lock(c03041e0,8,c029f065,4a6,c029cf40) at witness_lock+0xaf
_mtx_lock_flags(c03041e0,0,c029f065,4a6,c6594030) at _mtx_lock_flags+0xb1
selwakeup(c6594004,c6594030,73,c6594030,df12dad0) at selwakeup+0x2f
ptcwakeup(c6594030,1,df12dadc,c01b0beb,c6594030) at ptcwakeup+0x2e
ptsstart(c6594030,df12daf4,c01b2984,c6594030,c6594030) at ptsstart+0x37
ttstart(c6594030,c6594030,7,df12dbf4,df12db10) at ttstart+0x1b
tputchar(73,c6594030,1,c02af00f,c21abab0) at tputchar+0x54
putchar(73,df12dbf4,0,c02e1b10,0) at putchar+0x6b
kvprintf(c02af00e,c019c650,df12dbf4,a,df12dc14) at kvprintf+0x7d
printf(c02af00e,7,c02dfdc0,7,296) at printf+0x57
sched_ithd(7,c02df540,c21aa84c,c21abab0,df12dc58) at sched_ithd+0x63
i386_unpend(570cb146,c02dcae0,c21a9180,df12dcb4,c0184585) at i386_unpend+0xc7
cpu_unpend(a8,df12dcc0,c0184354,c21abab0,df12dce0) at cpu_unpend+0x2a
cpu_critical_exit(c21abab0,df12dce0,c017533d,c02dcae0,1) at cpu_critical_exit+0x15
critical_exit(c02dcae0,1,c029b144,1ba,c21a9180) at critical_exit+0x24
_mtx_unlock_spin_flags(c02dcae0,0,c0299a15,231,c21aa790) at _mtx_unlock_spin_flags+0xbd
ithread_loop(c21a9180,df12dd48,c02998d2,30c,0) at ithread_loop+0x27a
fork_exit(c016b200,c21a9180,df12dd48) at fork_exit+0xc0
fork_trampoline() at fork_trampoline+0x1a
--- trap 0x1, eip = 0, esp = 0xdf12dd7c, ebp = 0 ---

-- 
Tis a wise thing to know what is wanted, wiser still to know when
it has been achieved and wisest of all to know when it is unachievable
for then striving is folly. [Magician]