kern/55279: fss(4) related panic

[Manuel.Bouyer%lip6.fr@localhost]

>Number:         55279
>Category:       kern
>Synopsis:       fss(4) related panic
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue May 19 08:05:00 +0000 2020
>Originator:     Manuel BOUYER
>Release:        NetBSD 9.0_STABLE
>Organization:

>Environment:
System: NetBSD asim.lip6.fr 9.0_STABLE NetBSD 9.0_STABLE (LARGO) #0: Wed Apr 22 17:06:34 MEST 2020 bouyer%armandeche.soc.lip6.fr@localhost:/local/armandeche1/tmp/build/amd64/obj/local/armandeche2/netbsd-9/src/sys/arch/amd64/compile/LARGO amd64
Architecture: x86_64
Machine: amd64
>Description:
	While doing a nightly backup (dump -X), this server paniced with:
[ 1937611.3397989] uvm_fault(0xfffffeb5bedefbb0, 0x0, 1) -> e
[ 1937611.3788984] fatal page fault in supervisor mode
[ 1937611.4080185] trap type 6 code 0 rip 0xffffffff8075c203 cs 0x8 rflags 0x10286 cr2 0x8 ilevel 0 rsp 0xffffd40265b73d10
[ 1937611.4725002] curlwp 0xfffffeb7cf9b5060 pid 15281.1 lowest kstack 0xffffd40265b702c0
[ 1937611.5198180] panic: trap
[ 1937611.5333377] cpu0: Begin traceback...
[ 1937611.5600052] vpanic() at netbsd:vpanic+0x160
[ 1937611.5827386] snprintf() at netbsd:snprintf
[ 1937611.6118590] startlwp() at netbsd:startlwp
[ 1937611.6300705] alltraps() at netbsd:alltraps+0xc3
[ 1937611.6638584] mutex_vector_enter() at netbsd:mutex_vector_enter+0xba
[ 1937611.7028584] fss_close() at netbsd:fss_close+0xae
[ 1937611.7324989] spec_close() at netbsd:spec_close+0x199
[ 1937611.7631783] VOP_CLOSE() at netbsd:VOP_CLOSE+0x4c
[ 1937611.7933395] vn_close() at netbsd:vn_close+0x34
[ 1937611.8219379] closef() at netbsd:closef+0x6d
[ 1937611.8510584] fd_close() at netbsd:fd_close+0x1f4
[ 1937611.8812177] sys_close() at netbsd:sys_close+0x1f
[ 1937611.9108583] syscall() at netbsd:syscall+0xb4

>How-To-Repeat:
	dump -X on a a system with some activity, and be unlucky ?
>Fix: