Drop of spa_namespace lock in vdev_geom.c

Pawel Jakub Dawidek pjd at FreeBSD.org
Mon Jun 13 20:15:53 UTC 2011

On Fri, Jun 10, 2011 at 11:32:52AM -0600, Justin T. Gibbs wrote:
> 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?

I did that as part of @182208 to fix another LOR. Full commit log:

Change 182208 on 2010/08/10 by pjd at pjd_zoo

	OpenSolaris switched to lazy creation of /dev/ entires for ZVOLs.
	It creates /dev/ entries on VOP_LOOKUP() or VOP_READDIR().

	This of course can't work this way on FreeBSD with GEOM, so we need
	to create ZVOL providers where appropriate. I found the following
	1. Pool first open (pool is loaded based on zpool/cache configuration
	   and is then opened for a first time on eg. zfs mount).
	2. Pool import. It's not the same as 1.
	3. ZVOL creation: zfs create -V<size> <zvol>.
	4. Creation of ZVOL snapshot, this includes recursive snapshot

	To make it work I had to fix LOR between the zfsdev_state_lock, the
	GEOM topology lock and the spa_namespace_lock. They are now always
	obtained in the following order:
	1. zfsdev_state_lock
	2. g_topology_lock
	3. spa_namespace_lock
	Also, we can't use taskqueue to scan for VDEVs as this introduces
	deadlock (because there is no way to honour the order above).

	This also allows to simplify vdev_geom.c quite a bit as it is no
	longer a problem to taste ZVOL or ZVOL-based provider.

	Update /etc/rc.d/zvol as there are no longer volinit and volfini
	subcommands to zfs(8).

Pawel Jakub Dawidek                       http://www.wheelsystems.com
FreeBSD committer                         http://www.FreeBSD.org
Am I Evil? Yes, I Am!                     http://yomoli.com
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