panic: Exit: Single threading fouled up

Hi,

I've seen this panic twice now, once on a heavily loaded UP machine
running gnome at the time, and once on an SMP (hyperthreaded) machine
which was mostly idle as it was shutting down. Both running with ULE.

FreeBSD buffy.york.ac.uk 5.2-CURRENT FreeBSD 5.2-CURRENT #7: Tue Apr 13
19:45:13 BST 2004     root_at_buffy.york.ac.uk:/usr/obj/usr/src/sys/GENERIC
i386

(the other machine - the idle one - is running a kernel from April 17th)

panic: Exit: Single threading fouled up
at line 157 in file /usr/src/sys/kern/kern_exit.c
cpuid = 0;
Debugger("panic")
Stopped at      Debugger+0x46:  xchgl   %ebx,in_Debugger.0
db> trace
Debugger(c07bfacf) at Debugger+0x46
__panic(c07bd400,9d,c07bd45d,c302ac08,9) at __panic+0x13d
exit1(c4005000,9,decaca10,c4005000,c2f0a000) at exit1+0x418
sigexit(c4005000,9,c2f0aaa8,0,c07bfdc9) at sigexit+0xd7
postsig(9) at postsig+0x171
thread_export_context(c4005000,1,c2d00780,0,0) at
thread_export_context+0x116
thread_userret(c4005000,decacd48) at thread_userret+0x1b9
userret(c4005000,decacd48,0,3,0) at userret+0x58
syscall(83c002f,819002f,bfa6002f,81ff200,8) at syscall+0x2d9
Xint0x80_syscall() at Xint0x80_syscall+0x1d
--- syscall (32, FreeBSD ELF32, getsockname), eip = 0x2920cd8b, esp =
0xbfacdd74, ebp = 0xbfacdda0 ---
db> sh locks
exclusive sleep mutex process lock r = 0 (0xc302ac74) locked _at_
/usr/src/sys/kern/kern_exit.c:134

Unfortunately I have been unable to get a dump on either machine.  Sadly,
it's not not reproducible.

Gavin