[OpenAFS] Panic on FreeBSD 10.3

[Michael H Lambert]

We've been seeing kernel panics like the following on FreeBSD:

----------
vputx: negative ref count
0xfffff800aef07588: tag afs, type VDIR
    usecount 0, writecount 0, refcount 11 mountedhere 0
    flags (VV_ROOT|VI_ACTIVE)
 VI_LOCKed    v_object 0xfffff8006e4b3100 ref 0 pages 0 cleanbuf 0 =
dirtybuf 0
    lock type afs: EXCL by thread 0xfffff800ae706960 (pid 43045, httpd, =
tid 100136)
 with exclusive waiters pending
vc 0xfffffe0001fe9000 vp 0xfffff800aef07588 tag afs, fid: 0.1.1.1, opens =
0, writers 0
  states readonly
panic: vputx: negative ref cnt
cpuid =3D 1
KDB: stack backtrace:
#0 0xffffffff8098ead0 at kdb_backtrace+0x60
#1 0xffffffff809517a6 at vpanic+0x126
#2 0xffffffff80951673 at panic+0x43
#3 0xffffffff809f80d5 at vputx+0x2d5
#4 0xfffffe0001ca6a0a at afs_PutVCache+0x8a
#5 0xfffffe0001cf1a80 at afs_root+0xc0
#6 0xffffffff809ed663 at lookup+0x823
#7 0xffffffff809ecb44 at namei+0x4d4
#8 0xffffffff80a0625d at vn_open_cred+0x24d
#9 0xffffffff809ff53f at kern_openat+0x26f
#10 0xffffffff80d5722f at amd64_syscall+0x40f
#11 0xffffffff80d3c48b at Xfast_syscall+0xfb
----------

% uname -a
FreeBSD www.psc.edu 10.3-RELEASE-p24 FreeBSD 10.3-RELEASE-p24 #0: Wed =
Nov 15 04:57:40 UTC 2017     =
root@amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64

The OpenAFS client was built from source (/usr/ports/net/openafs; =
openafs-1.6.20.1-src.tar.bz2) using system sources updated for the =
running kernel.

I don't know exactly what was going on at the time of the panic(s), but =
it's somewhat likely that access to one or more very large files was =
involved.  Any thoughts on debugging this problem based on the kernel =
stack trace?

Thanks,

Michael

-----
Michael H Lambert, GigaPoP Manager             Phone: +1 412 268-4960
Pittsburgh Supercomputing Center/3ROX          FAX:   +1 412 268-5832
300 S Craig St, Pittsburgh, PA  15213 USA      lambert@psc.edu