panics with today's current

Today's current panics whenever I try to build mail/procmail a little
after the following appears on the screen:

Jun 18 19:50:13 gothmog kernel: Acquiring lockmgr lock "ufs" with the following non-sleepable locks held:
Jun 18 19:50:13 gothmog kernel: exclusive sleep mutex vnode pollinfo r = 0 (0xc1ebc000) locked _at_ /usr/src/sys/kern/kern_event.c:881
Jun 18 19:50:13 gothmog kernel: KDB: stack backtrace:
Jun 18 19:50:13 gothmog kernel: kdb_backtrace(1,3001,c1e75c2c,c1bcb300,da92fa5c) at kdb_backtrace+0x29
Jun 18 19:50:13 gothmog kernel: witness_warn(5,c074cb78,c06cdc34,c06d670a) at witness_warn+0x18e
Jun 18 19:50:13 gothmog kernel: lockmgr(c1e75c08,3001,c1e75c2c,c1bcb300,da92fa88) at lockmgr+0x8d
Jun 18 19:50:13 gothmog kernel: vop_stdlock(da92facc,c07327a0,da92facc,da92fa98,c063eaf4) at vop_stdlock+0x1e
Jun 18 19:50:13 gothmog kernel: VOP_LOCK_APV(c0732ce0,da92facc,da92faac,c06a7eb3,da92facc) at VOP_LOCK_APV+0x87
Jun 18 19:50:13 gothmog kernel: ffs_lock(da92facc,1001,c1e75bb0,da92fae8,c059a4cc) at ffs_lock+0x10
Jun 18 19:50:13 gothmog kernel: VOP_LOCK_APV(c07327a0,da92facc) at VOP_LOCK_APV+0x87
Jun 18 19:50:13 gothmog kernel: vn_lock(c1e75bb0,1001,c1bcb300) at vn_lock+0xa8
Jun 18 19:50:13 gothmog kernel: filt_vfsread(c1d2f4c8,0,0,ffffffff,0) at filt_vfsread+0x3a
Jun 18 19:50:13 gothmog kernel: kqueue_register(c1e12c00,da92fbf4,c1bcb300,1,0) at kqueue_register+0x668
Jun 18 19:50:13 gothmog kernel: kern_kevent(c1bcb300,4,1,0,da92fcc8) at kern_kevent+0xc9
Jun 18 19:50:13 gothmog kernel: kevent(c1bcb300,da92fd04,6,2,292) at kevent+0x55
Jun 18 19:50:13 gothmog kernel: syscall(3b,3b,3b,1,804e068) at syscall+0x22f
Jun 18 19:50:13 gothmog kernel: Xint0x80_syscall() at Xint0x80_syscall+0x1f
Jun 18 19:50:13 gothmog kernel: --- syscall (363, FreeBSD ELF32, kevent), eip = 0x280b7cfb, esp = 0xbfbfe67c, ebp = 0xbfbfe6c8 ---

A crash dump seems to panic at this spot:

(kgdb) bt
#0  doadump () at pcpu.h:165
#1  0xc04787e6 in db_fncall (dummy1=0, dummy2=0, dummy3=-1066924621, dummy4=0xd56b5aac "ΨZkΥΨZkΥ4ιgΐΐΑ{ΐp\f")
    at /usr/src/sys/ddb/db_command.c:531
#2  0xc04785f4 in db_command (last_cmdp=0xc0744904, cmd_table=0x0, aux_cmd_tablep=0xc06f12f0, aux_cmd_tablep_end=0xc06f12f4)
    at /usr/src/sys/ddb/db_command.c:349
#3  0xc04786bc in db_command_loop () at /usr/src/sys/ddb/db_command.c:455
#4  0xc047a241 in db_trap (type=3, code=0) at /usr/src/sys/ddb/db_main.c:221
#5  0xc0553f5f in kdb_trap (type=3, code=0, tf=0xd56b5bec) at /usr/src/sys/kern/subr_kdb.c:471
#6  0xc0695ff4 in trap (frame=
      {tf_fs = -714407928, tf_es = -1068171224, tf_ds = -1066598360, tf_edi = 1, tf_esi = -1066587760, tf_ebp = -714384340, tf_isp = -714384360, tf_ebx = -714384296, tf_edx = 0, tf_ecx = -1056878592, tf_eax = 18, tf_trapno = 3, tf_err = 0, tf_eip = -1068155677, tf_cs = 32, tf_eflags = 524418, tf_esp = -714384308, tf_ss = -1068251993}) at /usr/src/sys/i386/i386/trap.c:598
#7  0xc068900a in calltrap () at /usr/src/sys/i386/i386/exception.s:139
#8  0xd56b0008 in ?? ()
#9  0xc0550028 in device_probe_child (dev=0xc06cf8bc, child=0x1) at /usr/src/sys/kern/subr_bus.c:1800
#10 0xc053c4a7 in panic (fmt=0x20 <Address 0x20 out of bounds>) at /usr/src/sys/kern/kern_shutdown.c:537
#11 0xc055acd3 in propagate_priority (td=0xc1bcb300) at /usr/src/sys/kern/subr_turnstile.c:190
#12 0xc055b0e3 in turnstile_adjust (td=0xc1e92000, oldpri=181 '΅') at /usr/src/sys/kern/subr_turnstile.c:403
#13 0xc054c7b8 in sched_prio (td=0xc1e92000, prio=180 '΄') at /usr/src/sys/kern/sched_4bsd.c:860
#14 0xc054c3e8 in resetpriority_thread (td=0xc1e92000, kg=0x0) at /usr/src/sys/kern/sched_4bsd.c:606
#15 0xc054c27f in schedcpu () at /usr/src/sys/kern/sched_4bsd.c:526
#16 0xc054c2ed in schedcpu_thread () at /usr/src/sys/kern/sched_4bsd.c:543
#17 0xc0529798 in fork_exit (callout=0xc054c2dc <schedcpu_thread>, arg=0x0, frame=0xd56b5d38) at /usr/src/sys/kern/kern_fork.c:789
#18 0xc068906c in fork_trampoline () at /usr/src/sys/i386/i386/exception.s:208
(kgdb) up 11
#11 0xc055acd3 in propagate_priority (td=0xc1bcb300) at /usr/src/sys/kern/subr_turnstile.c:190
190			KASSERT(!TD_IS_SLEEPING(td),