Geom / destroy_dev() deadlock

[Konstantin Belousov]

On Mon, Jun 11, 2012 at 10:19:07AM -0700, Steven Haber wrote:
> Hey FreeBSD devs,
> 
> I hope this is the right forum for this. I haven't used the FreeBSD
> mailing lists before. I'm a relatively new hire at EMC Isilon. We are
> using FreeBSD 7.0 as the basis for our scale-out NAS product line. We
> are frequently hitting the deadlock described here:
> 
> http://lists.freebsd.org/pipermail/freebsd-geom/2010-February/003888.htm
> l
> 
> The race is that destroy_dev() sleeps indefinitely waiting for thread
> references to drop from a dev. In the case of geom, we hold the topology
> lock the whole time we're in the dev layer. In devfs_open() and
> devfs_close(), we take a ref on the dev before calling into to geom.
> 
> The proposed solution of destroy_dev_sched() makes sense to me. I am
> trying to understand the necessity of Alexander Motin's additional
> changes. destroy_dev_tq() holds the devmtx during the destroy and devfs
> uses this lock to take refs before calling into geom. To me it seems we
> should be able to protect the cdev with just the devmtx, provided
> consumers of d_close(), d_open(), etc. ref and rel the dev
> appropriately.
devmtx is mutex. If taken, then cdevsw methods would be unable to sleep.
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