To quote Illumos #3875:
The problem here is that if we ever end up in the error
path, we drop the locks protecting access to the zfsvfs_t
prior to forcibly unmounting the filesystem. Because z_os
is NULL, any thread that had already picked up the zfsvfs_t
and was sitting in ZFS_ENTER() when we dropped our locks
in zfs_resume_fs() will now acquire the lock, attempt to
use z_os, and panic.
Illumos ZFS issues:
3875 panic in zfs_root() after failed rollback
MFC after: 2 weeks
44e362e207