Bug in latest rev kern_acct.c: panic: Trying sleep, but thread marked as sleeping prohibited

I've been getting this panic a good bit recently after updating -current
sources a couple days ago.  The panic occurs in the acctwatch() callout 
routine run from the clock ithread, apparently when it tries to sleep 
on a lock (which is, I gather, verboten in ithreads.)   The panics seemed
to occur randomly but, on reflection, always occured when 
  1) there was a good bit of system activity and
  2) I had just done something to allocate or free enough space on /usr to
     cause accounting to be either suspended or resumed.  
and thus would cause the acctwatch() routine to have to do something.  
"make clean; make" in a big port seemed to be fairly effective in triggering
the bug. :-)  

My previous kernel, which dated from Oct 24, didn't have this
problem, which helps point suspicion at the latest rev of kern_acct.c 
(rev 1.76, date: 2005/11/12 10:45:13) which involved a bunch of changes 
in the lock handling of kernel accounting.   

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Unread portion of the kernel message buffer:
panic: Trying sleep, but thread marked as sleeping prohibited
cpuid = 1
KDB: stack backtrace:
kdb_backtrace(100,c3b7f900,c3b7f900,c0976100,c096f6a8) at kdb_backtrace+0x29
panic(c08a19c6,0,c096f6a8,c0970598,d85f0a14) at panic+0x114
sleepq_add(c096f6a8,c0970598,c089a4ab,1,c0970598,0,c089a875,96) at sleepq_add+0x8c
cv_wait_unlock(c096f6a8,c0970598,180,d85f0a4c,c096f680) at cv_wait_unlock+0x171
cv_wait(c096f6a8,c0970598,0,c3b7c480,c3ddb1c0) at cv_wait+0x36
_sx_xlock(c096f680,c089a5a2,180) at _sx_xlock+0x68
acctwatch(0) at acctwatch+0x20
softclock(0) at softclock+0x22b
ithread_execute_handlers(c3b7e8b0,c3b7c480) at ithread_execute_handlers+0xe6
ithread_loop(c3b488a0,d85f0d38,c3b488a0,c065098c,0) at ithread_loop+0x67
fork_exit(c065098c,c3b488a0,d85f0d38) at fork_exit+0xa4
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xd85f0d6c, ebp = 0 ---
Uptime: 49m58s
Dumping 637 MB (2 chunks)
  chunk 0: 1MB (159 pages) ... ok
  chunk 1: 637MB (163072 pages) 622 606 590 574 558 542 526 510 494 478 462 446 430 414 398 382 366 350 334 318 302 286 270 254 238 222 206 190 174 158 142 126 110 94 78 62 46 30 14

#0  doadump () at pcpu.h:165
165		__asm __volatile("movl %%fs:0,%0" : "=r" (td));
(kgdb) bt
#0  doadump () at pcpu.h:165
#1  0xc0663880 in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:399
#2  0xc0663b95 in panic (
    fmt=0xc08a19c6 "Trying sleep, but thread marked as sleeping prohibited")
    at /usr/src/sys/kern/kern_shutdown.c:555
#3  0xc06830fc in sleepq_add (wchan=0xc096f6a8, lock=0xc0970598, wmesg=0x0, 
    flags=1) at /usr/src/sys/kern/subr_sleepqueue.c:273
#4  0xc063dd31 in cv_wait_unlock (cvp=0xc096f6a8, mp=0xc0970598)
    at /usr/src/sys/kern/kern_condvar.c:152
#5  0xc063db9a in cv_wait (cvp=0xc096f6a8, mp=0xc0970598)
    at /usr/src/sys/kern/kern_condvar.c:112
#6  0xc06697cc in _sx_xlock (sx=0xc096f680, 
    file=0xc089a5a2 "/usr/src/sys/kern/kern_acct.c", line=384)
    at /usr/src/sys/kern/kern_sx.c:188
#7  0xc063bcdc in acctwatch (a=0x0) at /usr/src/sys/kern/kern_acct.c:384
#8  0xc0670bf7 in softclock (dummy=0x0) at /usr/src/sys/kern/kern_timeout.c:290
#9  0xc06508c2 in ithread_execute_handlers (p=0xc3b7e8b0, ie=0xc3b7c480)
    at /usr/src/sys/kern/kern_intr.c:662
#10 0xc06509f3 in ithread_loop (arg=0xc3b488a0)
    at /usr/src/sys/kern/kern_intr.c:745
#11 0xc064fb54 in fork_exit (callout=0xc065098c <ithread_loop>, 
    arg=0xc3b488a0, frame=0xd85f0d38) at /usr/src/sys/kern/kern_fork.c:790
#12 0xc081ac7c in fork_trampoline () at /usr/src/sys/i386/i386/exception.s:198
(kgdb) fr 9
#9  0xc06508c2 in ithread_execute_handlers (p=0xc3b7e8b0, ie=0xc3b7c480)
    at /usr/src/sys/kern/kern_intr.c:662
662			ih->ih_handler(ih->ih_argument);
(kgdb) p *p
$1 = {p_list = {le_next = 0xc3b7eadc, le_prev = 0xc3b7e684}, p_ksegrps = {
    tqh_first = 0xc3b81d80, tqh_last = 0xc3b81d84}, p_threads = {
    tqh_first = 0xc3b7f900, tqh_last = 0xc3b7f908}, p_suspended = {
    tqh_first = 0x0, tqh_last = 0xc3b7e8c8}, p_ucred = 0xc3b79100, 
  p_fd = 0xc3b85200, p_fdtol = 0x0, p_stats = 0xc3b67a00, 
  p_limit = 0xc3b7a100, p_sigacts = 0xc3b8c000, p_flag = 524, p_sflag = 1, 
  p_state = PRS_NORMAL, p_pid = 12, p_hash = {le_next = 0x0, 
    le_prev = 0xc3b73030}, p_pglist = {le_next = 0xc3b7eadc, 
    le_prev = 0xc3b7e6d4}, p_pptr = 0xc096f0c0, p_sibling = {
    le_next = 0xc3b7eadc, le_prev = 0xc3b7e6e0}, p_children = {
    lh_first = 0x0}, p_mtx = {mtx_object = {lo_class = 0xc0931b24, 
      lo_name = 0xc089df1e "process lock", 
      lo_type = 0xc089df1e "process lock", lo_flags = 4390912, 
      lo_witness_data = {lod_list = {stqe_next = 0xc0981260}, 
        lod_witness = 0xc0981260}}, mtx_lock = 4, mtx_recurse = 0}, 
  p_ksi = 0x0, p_sigqueue = {sq_signals = {__bits = {0, 0, 0, 0}}, sq_list = {
      tqh_first = 0x0, tqh_last = 0xc3b7e948}, sq_proc = 0xc3b7e8b0, 
    sq_flags = 1}, p_oppid = 0, p_vmspace = 0xc096f460, p_swtime = 2986, 
  p_realtimer = {it_interval = {tv_sec = 0, tv_usec = 0}, it_value = {
      tv_sec = 0, tv_usec = 0}}, p_rux = {rux_runtime = {sec = 59, 
      frac = 4151612463131724764}, rux_uticks = 0, rux_sticks = 0, 
    rux_iticks = 157, rux_uu = 0, rux_su = 191765, rux_iu = 58831433}, 
  p_crux = {rux_runtime = {sec = 0, frac = 0}, rux_uticks = 0, rux_sticks = 0, 
    rux_iticks = 0, rux_uu = 0, rux_su = 0, rux_iu = 0}, p_profthreads = 0, 
  p_maxthrwaits = 0, p_traceflag = 0, p_tracevp = 0x0, p_tracecred = 0x0, 
  p_textvp = 0x0, p_lock = 1 '\001', p_sigiolst = {slh_first = 0x0}, 
  p_sigparent = 20, p_sig = 0, p_code = 0, p_stops = 0, p_stype = 0, 
  p_step = 0 '\0', p_pfsflags = 0 '\0', p_nlminfo = 0x0, p_aioinfo = 0x0, 
  p_singlethread = 0x0, p_suspcount = 0, p_xthread = 0x0, 
  p_boundary_count = 0, p_procscopegrp = 0x0, p_pendingcnt = 0, 
  p_itimers = 0x0, p_magic = 3203398350, 
  p_comm = "swi4: clock sio\000\000\000\000", p_pgrp = 0xc096f600, 
  p_sysent = 0xc092cb80, p_args = 0x0, p_cpulimit = 9223372036854775807, 
  p_nice = 0 '\0', p_xstat = 0, p_klist = {kl_list = {slh_first = 0x0}, 
    kl_lock = 0xc064b31c <knlist_mtx_lock>, 
    kl_unlock = 0xc064b338 <knlist_mtx_unlock>, 
    kl_locked = 0xc064b354 <knlist_mtx_locked>, kl_lockarg = 0xc3b7e918}, 
  p_numthreads = 1, p_numksegrps = 1, p_md = {md_ldt = 0x0}, p_itcallout = {
    c_links = {sle = {sle_next = 0x0}, tqe = {tqe_next = 0x0, 
        tqe_prev = 0x0}}, c_time = 0, c_arg = 0x0, c_func = 0, c_mtx = 0x0, 
    c_flags = 16}, p_acflag = 1, p_ru = 0x0, p_peers = 0x0, 
  p_leader = 0xc3b7e8b0, p_emuldata = 0x0, p_label = 0x0, 
  p_sched = 0xc3b7eadc, p_ktr = {stqh_first = 0x0, stqh_last = 0xc3b7ead0}, 
  p_mqnotifier = {lh_first = 0x0}}
(kgdb) fr 7
#7  0xc063bcdc in acctwatch (a=0x0) at /usr/src/sys/kern/kern_acct.c:384
384		sx_xlock(&acct_sx);
(kgdb) q