page fault in unionfs code (was: locking against myself in unionfs code)

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

> Also, it's possible that I didn't fix it, so if you see it again be
> sure to post more :-)
Not exactly this one, but a page fault (with a -10 kernel with your patch applied).

> (The thing I found was _a_ bug, but might not be _the_ bug; it's not
> like they're rare in onionfs)
Looks like it:

uvm_fault(0xffff8497fe928050, 0x0, 1) -> e
fatal page fault in supervisor mode
trap type 6 code 0 rip 0xffffffff80a165f3 cs 0x8 rflags 0x10282 cr2 0x80 ilevel 0 rsp 0xffffa384c28b4cd0
curlwp 0xffff84980a57f240 pid 11740.11740 lowest kstack 0xffffa384c28b02c0
kernel: page fault trap, code=0
Stopped in pid 11740.11740 (python3.10) at	netbsd: VOP_LOCK+0x26: movl	80(%rdi),%r13d
db{3}> bt
VOP LOCK() at netbsd:VOP_LOCK+0x26
vn_lock() at netbsd:vn_lock+0x22
ufs_link() at netbsd:ufs_link+0x3c
VOP_LINK() at netbsd:VOP_LINK+0x46
union_link() at netbsd:union_link+0x53
VOP_LINK() at netbsd:VOP_LINK+0x46
do_sys_linkat() at netbsd:do_sys_linkat+0x20a
sys_link() at netbsd:sys_link+0x28
syscall() at netbsd:syscall+0x1fc
--- syscall (number 9) ---
netbsd:syscall+0x1fc:
db{3}> show reg
ds	0
es	2
fs	2
gs	1dc0
rdi	0
rsi	2
rbp	ffffa384c28b4d30
rbx	0
rdx	ffffa384c28b4f20
rcx	ffff84991ca80000
rax	ffffffff809287b0	ufs_link+0x3c
r8	0
r9	1
r10	ffff84986446aac8
r11	0
r12	2
r13	ffff84983fa05dc0
r14	ffffa384c28b4f20
r15	0
rip	ffffffff80a165f3	VOP_LOCK+0x26
cs	8
rflags	10282
rsp	ffffa384c28b4cd0
ss	10
netbsd:VOP_LOCK+0x26:	movl	80(%rdi),%r13d
db{3)>