[Bug 228384] zfs utilities hang on [spa_namespace_lock] (transferred from 203906 as an unrelated issue)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=228384

            Bug ID: 228384
           Summary: zfs utilities hang on [spa_namespace_lock]
                    (transferred from 203906 as an unrelated issue)
           Product: Base System
           Version: 11.1-STABLE
          Hardware: i386
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: bsd.gaijin at gmail.com

Created attachment 193578
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=193578&action=edit
procstat output

zpool status hangs on [spa_namespace_lock]. Output of the procstat -a -kk and
pictures of the stack trace from thread holding the lock attached.

>From Andriy Gapon <avg at FreeBSD.org>:

So, it's:
   13 100015 geom                g_event             mi_switch+0xe6
sleepq_wait+0x2c _sx_xlock_hard+0x314 zvol_geom_access+0x148 g_access+0x1fd
g_eli_read_metadata+0x249 g_eli_config+0x3ed g_run_events+0x13e fork_exit+0x83
fork_trampoline+0xe
  930 100354 zfsd                -                   mi_switch+0xe6
sleepq_wait+0x2c _sleep+0x23e g_access+0xf7 vdev_geom_attach+0x61c
vdev_attach_ok+0x29 vdev_geom_open+0x394 vdev_open+0x115
vdev_open_children+0x30 vdev_root_open+0x3a vdev_open+0x115
spa_ld_open_vdevs+0x5e spa_ld_mos_init+0x1be
spa_ld_mos_with_trusted_config+0x19 spa_load+0x5c spa_load_best+0x6b
spa_open_common+0x11d spa_get_stats+0x4f

I think I know what's going on in your case (not sure if it's the same a as
previous reports in this bug).  It's probably a consequence of base r330977, a
fix to bug 225960.
I didn't fully realize at that time, but that commit introduced a "g_access
lock" in disguise.
So, we went from a LOR between geom topology lock and spa_namespace_lock to a
race caused by dropping the topology lock to a LOR between spa_namespace_lock
and "g_access lock".

The toughest part now is to decide how to solve the LOR without re-introducing
the race.  Or alternatively, how to solve the race without introducing a
deadlock.

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