kern/54498: instant + total VFS deadlock

[martin%NetBSD.org@localhost]

>Number:         54498
>Category:       kern
>Synopsis:       instant + total VFS deadlock
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Aug 28 11:25:00 +0000 2019
>Originator:     Martin Husemann
>Release:        NetBSD 9.99.10
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD seven-days-to-the-wolves.aprisoft.de 9.99.10 NetBSD 9.99.10 (GENERIC) #302: Mon Aug 26 09:04:19 CEST 2019 martin%seven-days-to-the-wolves.aprisoft.de@localhost:/work/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:

Stupid mistake, don't try this at home kits:

 1) create a new GPT partition somewhere (type ffs), let's say the wedge gets
    called dk2
 2) newfs -O2 /dev/rdk2
 2) mount /dev/dk2 /mnt
    (first mistake here: forgot to specify async or log)
 3) cd $somewhere; pax -rw -pe . /mnt
 4) while pax is still running, notice the slow transfer and try to add async:
    mount -u -o async /dev/dk2 /mnt
    (in another terminal)

Watch all disk activity die, mount -u never proceeding (unstoppable), system
still reacts to ping and existing process carry on, but all disk access seems
to block.

>How-To-Repeat:
s/a

>Fix:
n/a