panic: _mtx_lock_sleep: recursed on non-recursive mutex process lock @ /usr/src/sys/kern/kern_sig.c:981

I got this panic this morning, after a fresh upgrade to svn current
during concurrent NFS access from a couple of clients:

FreeBSD 9.0-CURRENT #7 r198588: Thu Oct 29 08:55:09 PDT 2009

Unread portion of the kernel message buffer:
panic: _mtx_lock_sleep: recursed on non-recursive mutex process lock _at_
/usr/src/sys/kern/kern_sig.c:981

cpuid = 3
KDB: enter: panic
Physical memory: 2031 MB
Dumping 196 MB: 181 165 149 133 117 101 85 69 53 37 21 5

#0  doadump () at pcpu.h:246
246             __asm __volatile("movl %%fs:0,%0" : "=r" (td));
(kgdb) where
#0  doadump () at pcpu.h:246
#1  0xc04d7ff9 in db_fncall (dummy1=1, dummy2=0, dummy3=-1056878112,
dummy4=0xe831a994 "") at /usr/src/sys/ddb/db_command.c:548
#2  0xc04d83f1 in db_command (last_cmdp=0xc0e0c29c, cmd_table=0x0,
dopager=1) at /usr/src/sys/ddb/db_command.c:445
#3  0xc04d8555 in db_command_loop () at
/usr/src/sys/ddb/db_command.c:498

#4  0xc04da49c in db_trap (type=3, code=0) at
/usr/src/sys/ddb/db_main.c:229

#5  0xc08cfbe1 in kdb_trap (type=3, code=0, tf=0xe831ab38) at
/usr/src/sys/kern/subr_kdb.c:535
#6  0xc0c0bb17 in trap (frame=0xe831ab38) at
/usr/src/sys/i386/i386/trap.c:690

#7  0xc0bedc7b in calltrap () at /usr/src/sys/i386/i386/exception.s:165

#8  0xc08cfd7a in kdb_enter (why=0xc0ce0a27 "panic", msg=0xc0ce0a27
"panic") at cpufunc.h:71
#9  0xc089e998 in panic (fmt=0xc0cdf3fe "_mtx_lock_sleep: recursed on
non-recursive mutex %s _at_ %s:%d\n") at /usr/src/sys/kern/kern_shutdown.c:562
#10 0xc088ed5a in _mtx_lock_sleep (m=0xc73065d8, tid=3341857888, opts=0,
file=0xc0ce0a96 "/usr/src/sys/kern/kern_sig.c", line=981) at
/usr/src/sys/kern/kern_mutex.c:341
#11 0xc088ef87 in _mtx_lock_flags (m=0xc73065d8, opts=0, file=0xc0ce0a96
"/usr/src/sys/kern/kern_sig.c", line=981) at
/usr/src/sys/kern/kern_mutex.c:203
#12 0xc08a107b in kern_sigprocmask (td=0xc730b460, how=1,
set=0xe831ac80, oset=0x0, flags=0) at /usr/src/sys/kern/kern_sig.c:981
#13 0xc08a4bfb in trapsignal (td=0xc730b460, ksi=0xe831accc) at
/usr/src/sys/kern/kern_sig.c:1866
#14 0xc0c0bc07 in trap (frame=0xe831ad38) at
/usr/src/sys/i386/i386/trap.c:738
#15 0xc0bedc7b in calltrap () at /usr/src/sys/i386/i386/exception.s:165
#16 <signal handler called>

(kgdb) frame 10
#10 0xc088ed5a in _mtx_lock_sleep (m=0xc73065d8, tid=3341857888, opts=0,
file=0xc0ce0a96 "/usr/src/sys/kern/kern_sig.c", line=981) at
/usr/src/sys/kern/kern_mutex.c:341
341                     KASSERT((m->lock_object.lo_flags &
LO_RECURSABLE) != 0,
(kgdb) list
336             uint64_t sleep_cnt = 0;
337             int64_t sleep_time = 0;
338     #endif
339
340             if (mtx_owned(m)) {
341                     KASSERT((m->lock_object.lo_flags &
LO_RECURSABLE) != 0,
342                 ("_mtx_lock_sleep: recursed on non-recursive mutex
%s _at_ %s:%d\n",
343                         m->lock_object.lo_name, file, line));
344                     m->mtx_recurse++;
345                     atomic_set_ptr(&m->mtx_lock, MTX_RECURSED);
(kgdb) p *m
$2 = {lock_object = {lo_name = 0xc0cdf492 "process lock", lo_flags =
21168128, lo_data = 0, lo_witness = 0xc5934340}, mtx_lock = 3341857888}