[Bug 203906] ZFS lockup, spa_namespace_lock

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=203906

--- Comment #15 from Andriy Gapon <avg at FreeBSD.org> ---
(In reply to Alexandre Kovalenko from comment #11)
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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