[Bug 226499] Partial system freeze via ZFS rename and possible other operations

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=226499

            Bug ID: 226499
           Summary: Partial system freeze via ZFS rename and possible
                    other operations
           Product: Base System
           Version: 11.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: paul.g.webster at googlemail.com

It is possible to create a partial system freeze with zfs by renaming an active
mountpoint; in my case I had a vdev mounted within bhyve.

When I accidentally did a:

665  19:39   zfs rename zroot/venv/store/paul.daemon.space-extended
zroot/venv/store/services.daemon.space-extended

The zfs command froze and refuse to return (even via kill -9) the bhyve vm will
refuse to exit, and the system will not reboot; it will get stuck trying to
unmount filesystems.

Likewise if you open up another terminal and try any command via 'zfs' it will
end up in the frozen state, (actual state reported in top is: tx->tx)

This is a partial freeze because all other system operations will continue
normally.

It appears though have not investigated earlier, that rename does not check a
dataset is bust like destroy does, so then gets stuck in some sort of lock
waiting for it to become available in the meantime nothing else can do
anything, including any process that is actually locking whatever you are
trying to rename.

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