kern/53815: vnd+wapbl+snapshots panic

[bouyer%antioche.eu.org@localhost]

>Number:         53815
>Category:       kern
>Synopsis:       vnd+wapbl+snapshots panic
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Dec 27 08:30:00 +0000 2018
>Originator:     Manuel Bouyer
>Release:        HEAD as of yesterday
>Organization:
>Environment:
NetBSD/amd64 XEN3_DOM0, built from yesterday's sources
>Description:
	While running a build.sh release in a domU, the dom0 paniced:
login: [ 34672.3034884] panic: kernel diagnostic assertion "rw_lock_held(&wl->wl
_rwlock)" failed: file "/dsk/l1/misc/bouyer/HEAD/clean/src/sys/kern/vfs_wapbl.c$
, line 2027                                                                     
[ 34672.3034884] cpu0: Begin traceback...                                       
[ 34672.3134769] vpanic() at netbsd:vpanic+0x152                                
[ 34672.3134769] kern_assert() at netbsd:kern_assert+0x48                       
[ 34672.3134769] wapbl_add_buf() at netbsd:wapbl_add_buf+0x147                  
[ 34672.3134769] bdwrite() at netbsd:bdwrite+0xbd                               
[ 34672.3134769] ffs_alloccg() at netbsd:ffs_alloccg+0x4cc
[ 34672.3134769] ffs_hashalloc() at netbsd:ffs_hashalloc+0x2b
[ 34672.3134769] ffs_alloc() at netbsd:ffs_alloc+0x164
[ 34672.3134769] ffs_balloc() at netbsd:ffs_balloc+0x172b
[ 34672.3134769] wrsnapblk() at netbsd:wrsnapblk+0x4f
[ 34672.3134769] ffs_copyonwrite() at netbsd:ffs_copyonwrite+0x1f0
[ 34672.3134769] fscow_run() at netbsd:fscow_run+0xda
[ 34672.3134769] spec_strategy() at netbsd:spec_strategy+0x8a
[ 34672.3234728] VOP_STRATEGY() at netbsd:VOP_STRATEGY+0x5c
[ 34672.3234728] vndthread() at netbsd:vndthread+0x3a5
[ 34672.3234728] cpu0: End traceback...
[ 34672.3234728] fatal breakpoint trap in supervisor mode

I guess it hapenned while the nightly daily run in the dom0, which runs a
fsck_ffs -X.

>How-To-Repeat:
	write to a vnd device while running fsck_ffs -X on the underlying
	filesystem ? 
>Fix:
	unkown