kern/53506: Easy trigger of "panic: biodone2 already" on NetBSD 8.0 Xen domU

[manu%netbsd.org@localhost]

>Number:         53506
>Category:       kern
>Synopsis:       Easy trigger of "panic: biodone2 already" on NetBSD 8.0 Xen domU
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Aug 07 16:40:00 +0000 2018
>Originator:     Emmanuel Dreyfus
>Release:        NetBSD 8.0
>Organization:
NetBSD
>Environment:
NetBSD bacasable 8.0 NetBSD 8.0 (XEN3PAE_DOMU) #0: Tue Jul 17 14:59:51 UTC 2018  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/xen/compile/XEN3PAE_DOMU i386
>Description:
On NetBSD 8.0 domU (either i386 and amd64), running dump and a concurent filesystem access crashes the kernel with "panic: biodone2 already". Stack trace is below.


panic: biodone2 already
fatal breakpoint trap in supervisor mode
trap type 1 code 0 eip 0xc0105334 cs 0x9 eflags 0x246 cr2 0xbb56d000 ilevel 0 esp 0xda67ff2c
curlwp 0xc2d4e7e0 pid 0 lid 4 lowest kstack 0xda67e2c0
Stopped in pid 0.4 (system) at  netbsd:breakpoint+0x4:  popl    %ebp
breakpoint(c04da2c8,c06d84e0,c04f9e2c,da67ff48,c42a1718,c057fe00,da6b82cc,da67ff3c,c03c4e48,c04f9e2c) at netbsd:breakpoint+0x4
vpanic(c04f9e2c,da67ff48,da67ff54,c03fdd2f,c04f9e2c,c0114934,c42a1718,c057fe00,da67ff68,c03fdd5d) at netbsd:vpanic+0x12d
panic(c04f9e2c,c0114934,c42a1718,c057fe00,da67ff68,c03fdd5d,0,0,da6b8004,da67ff8c) at netbsd:panic+0x18
biodone2(0,0,da6b8004,da67ff8c,c039e66d,0,da67ff88,c2d4e7e0,0,da6b8004) at netbsd:biodone2+0xff
biointr(0,da67ff88,c2d4e7e0,0,da6b8004,c039e610,c2d4e7e0,0,c0102031,da6b8004) at netbsd:biointr+0x2d
softint_thread(da6b8004,96e000,c0595200,0,c0100075,0,0,0,0,0) at netbsd:softint_thread+0x5d


Relevant processes:
PID    LID S CPU     FLAGS       STRUCT LWP *               NAME WAIT
446      1 2   0         0           c3eef2a0               grep
562      1 3   0        80           c33cfaa0               dump lockf
169      1 3   0        80           c345d800               dump lockf
1487     1 3   0         0           c3f9cd40               dump physio
145      1 3   0        80           c3eef7e0               dump netio
1351     1 3   0        80           c33f27e0               dump wait


>How-To-Repeat:
Run as root in two different terminals:

/sbin/dump -a0f /dev/null /
grep -r something /var
>Fix: