kern/55256: lockup on process exit

[martin%NetBSD.org@localhost]

>Number:         55256
>Category:       kern
>Synopsis:       lockup on process exit
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue May 12 11:15:00 +0000 2020
>Originator:     Martin Husemann
>Release:        NetBSD 9.99.60
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD last-of-the-heroes.aprisoft.de 9.99.60 NetBSD 9.99.60 (GENERIC) #55: Tue May 12 11:36:42 CEST 2020 martin%seven-days-to-the-wolves.aprisoft.de@localhost:/work/src/sys/arch/landisk/compile/GENERIC landisk
Architecture: sh3el
Machine: landisk
>Description:

Not yet sure if this is reproducable, but a -curren landisk machine locked
up for me while running ATF tests.

ddb worked, no progress anywhere:

db> bt
cpu_Debugger() at netbsd:scifintr+0x56
scifintr() at netbsd:intc_intr+0x5a
intc_intr() at 8c000676
<EXPEVT 000; SSR=400001c0> at netbsd:__pmap_pte_lookup+0xe
[trapframe 0x6fa02000]
   sr=400001c0   gbr=700f6000    pc=8c00a026     pr=8c00a08a
   r0=00000000    r1=ffffffe9    r2=40000101     r3=00000000
   r4=8fe51f34    r6=6fa01000    r7=6fa02000     r8=00000001
   r5=6fa01000    r9=8fe51f34   r10=8c00a050    r11=8c1cb330
  r12=8eb90bb4   r13=1ffff000   r14=6fa02000 sp=r15=c30a6dd0
__pmap_pte_lookup() at netbsd:pmap_remove+0x3a
pmap_remove() at netbsd:pmap_page_protect+0x78
pmap_page_protect() at netbsd:uvm_anfree+0x70
uvm_anfree() at netbsd:amap_wipeout+0x64
amap_wipeout() at netbsd:uvm_unmap_detach+0x44
uvm_unmap_detach() at netbsd:uvmspace_free+0x58
uvmspace_free() at netbsd:exit1+0x1d2
exit1() at netbsd:sys_exit+0x3a
sys_exit() at netbsd:syscall+0xae
syscall() at netbsd:general_exception+0x184

Full dmesg for this machine at:
	https://www.NetBSD.org/~martin/landisk-atf/dmesg.txt

>How-To-Repeat:
cd /usr/tests && atf-run

>Fix:
n/a