[Bug 207464] Panic when destroying ZFS snapshot on boot filesystem

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207464

--- Comment #14 from karl at denninger.net ---
Andy, is that WITH the one-line patch in?

If so, bummer.

The pattern for me here on my production machines is that during backup the
machine will panic, *always* during the root filesystem backup.

The system is all-ZFS and the backups are done using zfs send, after creating a
snapshot (which rolls so you wind up with a duplicate of the existing file
structure over time.)  The system also uses timed snapshots that roll off so
recovery of files is simple if they're accidentally deleted or modified without
resorting to the backups.

The panic is usually (but not always) preceded by a snapshot that is present,
cannot be destroyed, but *also* cannot be CD'd into.  Once that condition
exists the following is also true:

1. If you *reboot* the box the condition is cleared and all is well; the system
will *not* panic in that instance.

2. If you attempt to zfs send a stream that *includes* the "orphan" snapshot
the machine will panic in dounmount (identical behavior to the traces posted
here) every time.

3. You cannot zfs destroy the "rogue" snapshot that is present in the directory
but cannot be cd'd into.

These three things make it possible for my backup script to, most of the time,
detect the panic condition on the backup run *before* the one that blows up.

What I've been unable to do, however, is reproduce a test case that reliably
leaves the system in the pre-panic state.

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