repeated panics in mutex_vector_enter (from unp_thread)

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

Since a few days, I'm experiencing repeated panics in mutex_vector_enter.
Nothing was changed to the server in question, probably, it's experiencing more 
load/forks than before. The machine is still on 6.1, but I can't tell whether 
the problem is version specific.

The tracebacks look similar (third and fourth coulumn in "show reg" output are 
from subsequend panics, missing values mean same as first one):

uvm_fault(0xffffffff8076d460, 0x0, 1) -> e
fatal page fault in supervisor mode
trap type 6 code 0 rip ffffffff8027663b cs 8 rflags 10286 cr2  8 cpl 0 rsp fffffe811d83bbc0
kernel: page fault trap, code=0
Stopped in pid 0.67 (system) at netbsd:mutex_vector_enter+0x32c:	movq	1
8(%rdx),%rax
db{5}> bt
mutex_vector_enter() at netbsd:mutex_vector_enter+0x32c
unp_thread() at netbsd:unp_thread+0x2eb
db{5}> show reg
ds	64Y						?
es	d2a0			6405			?
fs	269d			563			563
gs	0
rdi	fffffe834689f040	fffffe83471f1700	fffffe83481f1700
rsi	1000
rbp	fffffe811d83bc20	fffffe811d811c20	fffffe811d811c20
rbx	fffffe834689f040	fffffe83481f1700	fffffe83481f1700
rdx	fffffffffffffff0
rcx	fffffffffffffff0
rax	fffffe811d7ed2a0
r8	fffffe811d7ed2a0
r9	0
r10	0
r11	2			1			1
r12	0
r13	0
r14	fffffe811d7ed2a0
r15	0
rip	fffffff8027663b	mutex_vector_enter+0x32c
cs	8
rflags	10286
rsp	fffffe811d83bbc0	fffffe811d811bc0	fffffe811d811bc0
ss	0
netbsd:mutex_vector_enter+0x32c:	movq	18(%rdx),%rax
db{5}> reboot 4

The machine than hangs hard, I need to press reset.