Another kassert in xbdback_xenbus.c

[Jeff Rizzo <riz%tastylime.net@localhost>]

I updated my dom0 last night, and shortly after the domUs started up, it 
triggered another KASSERT.  It's been running OK since a reboot, though, 
so it looks like it's not quite as easy to trigger as the earlier ones 
(and may or may not be a new problem).

panic: kernel diagnostic assertion "xbdi->xbdi_pendingreqs == 0" failed: 
file 
"/Users/riz/Documents/code/netbsd/src/sys/arch/xen/xen/xbdback_xenbus.c", line 
1017
cpu0: Begin traceback...
kern_assert() at netbsd:kern_assert+0x2d
xbdback_co_cache_flush() at netbsd:xbdback_co_cache_flush+0xae
xbdback_trampoline() at netbsd:xbdback_trampoline+0x5b
xbdback_evthandler() at netbsd:xbdback_evthandler+0x28
Xresume_xenev6() at netbsd:Xresume_xenev6+0x47
--- interrupt ---
Xspllower() at netbsd:Xspllower+0xe
idle_loop() at netbsd:idle_loop+0xc3
cpu0: End traceback...
fatal breakpoint trap in supervisor mode
trap type 1 code 0 rip ffffffff802229e5 cs e030 rflags 246 cr2  
7f7ff7fd3000 cpl 6 rsp ffffa000122a4948
Stopped in pid 0.2 (system) at  netbsd:breakpoint+0x5:  leave
breakpoint() at netbsd:breakpoint+0x5
panic() at netbsd:panic+0x28d
kern_assert() at netbsd:kern_assert+0x2d
xbdback_co_cache_flush() at netbsd:xbdback_co_cache_flush+0xae
xbdback_trampoline() at netbsd:xbdback_trampoline+0x5b
xbdback_evthandler() at netbsd:xbdback_evthandler+0x28
Xresume_xenev6() at netbsd:Xresume_xenev6+0x47
--- interrupt ---
Xspllower() at netbsd:Xspllower+0xe
idle_loop() at netbsd:idle_loop+0xc3
ds          b776
es          4928
fs          97fa
gs          4928
rdi         0
rsi         0
rbp         ffffa000122a4948
rbx         104
rdx         0
rcx         8
rax         0
r8          ffffffff80c1c740    cpu_info_primary
r9          1
r10         0
r11         ffffffff80e39445
r12         ffffffff80b02bf0    
__link_set_malloc_types_sym_M_PROP_ARRAY+0x5f800

r13         deadbeef
r14         ffffa000007c5000
r15         0
rip         ffffffff802229e5    breakpoint+0x5
cs          e030
rflags      246
rsp         ffffa000122a4948
ss          e02b
netbsd:breakpoint+0x5:  leave