From owner-freebsd-fs@FreeBSD.ORG  Fri Jun 10 18:07:48 2011
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Date: Fri, 10 Jun 2011 11:32:52 -0600
From: "Justin T. Gibbs" <gibbs@FreeBSD.org>
Organization: The FreeBSD Project
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	Fri, 10 Jun 2011 11:32:59 -0600 (MDT)
Cc: 
Subject: Drop of spa_namespace lock in vdev_geom.c
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Dropping and reacquiring the spa_namespace lock in vdev_geom_open()
creates a lock order reversal with the spa_config locks.  As the
spa_config locks are not standard mutexes, witness will not warn
about this issue.  I only noticed this problem when debugging a ZFS
deadlock.  The deadlock can be triggered anytime that there are
multiple insert/remove processes going on (e.g. vdev orphan processing
while a fault management daemon is onlining a replacement device for
some other vdev).

I haven't noticed any issues with just holding the namespace lock
for the duration of the open.  Does anyone know why this lock drop
was added in v28?

Thanks,
Justin