kern/53658: suspend from USB installer doesn't work if / is mounted read-write

[coypu%sdf.org@localhost]

>Number:         53658
>Category:       kern
>Synopsis:       suspend from USB installer doesn't work if / is mounted read-write
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Oct 07 19:15:00 +0000 2018
>Originator:     coypu
>Release:        NetBSD 8.99.25
>Organization:
>Environment:
NetBSD 8.99.25
>Description:
uefi booting, hand transcription:

acpi0: entering state S3
Flushing disk caches: 20 20 19 19 19 19 19 done
sd0(umass0:0:0:0): generic HBA error
sd0: cache synchronization failed
fatal page fault in supervisor mode

from ddb:
Stopped in pid 0.6 (system) at netbsd:db_disasm+0xec: movq 0(%rdx,%rcx,8),%rcx
db{0}> bt
db_disasm
db_trap
kdb_trap
trap
--- trap (number 6) ---
?() at 0
xhci_softintr
usb_soft_intr
softint_dispatch
DDB lost frame for netbsd:Xsoftintr+0x4f, trying 0xffff....
Xsoftintr()
--- interrupt ---
840eb582d69a10ab:
>How-To-Repeat:

>Fix:
No idea, but I can work around it by dropping to boot prompt and using `boot -s`, which means that / stays a read only mount.