Something holding z_teardown_inactive_lock way too long

[Garrett Wollman]

I recently upgraded all of our NFS servers to 11.1 after a year of
successful operation on 10.3 and (apparently) successful testing of a
couple of new servers.  But right now I'm seeing that there is
something on one of my server (maybe more) that is holding
z_teardown_inactive_lock for far, far, far too long, causing alerts to
be raised.  This server was first booted in 11.1 last Wednesday, and
when it's sad, I see this:

load: 0.18  cmd: ls 10408 [zfsvfs->z_teardown_inactive_lock] 28.89r 0.00u 0.00s 0% 2576k
load: 0.10  cmd: ls 10408 [zfsvfs->z_teardown_inactive_lock] 67.43r 0.00u 0.00s 0% 2576k
load: 0.06  cmd: ls 10408 [zfsvfs->z_teardown_inactive_lock] 89.75r 0.00u 0.00s 0% 2576k
load: 0.13  cmd: ls 10408 [zfsvfs->z_teardown_inactive_lock] 153.10r 0.00u 0.00s 0% 2576k
load: 0.06  cmd: ls 10408 [zfsvfs->z_teardown_inactive_lock] 196.55r 0.00u 0.00s 0% 2576k
load: 0.14  cmd: ls 10408 [zfsvfs->z_teardown_inactive_lock] 243.63r 0.00u 0.00s 0% 2576k

On the console, it seems like I can still run procstat -kk:

10408 101158 ls                  -                   mi_switch+0xe5 sleepq_wait+0x3a _sx_slock_hard+0x334 zfs_freebsd_reclaim+0x3c VOP_RECLAIM_APV+0x89 vgonel+0x21c vnlru_free_locked+0x22c getnewvnode_reserve+0x77 zfs_zget+0x24 zfs_dirent_lookup+0x162 zfs_dirlook+0x77 zfs_lookup+0x44a zfs_freebsd_lookup+0x6d VOP_CACHEDLOOKUP_APV+0x83 vfs_cache_lookup+0xd6 VOP_LOOKUP_APV+0x83 lookup+0x701 namei+0x486 

although it did previously hang when I was logging in.

Does this sound familiar to anyone?

-GAWollman