See: https://github.com/lxc/lxd/issues/4656#issuecomment-535531229

In https://github.com/lxc/lxd/blob/master/lxd/storage_zfs_utils.go#L255
the umount is done by

    err := unix.Unmount(mountpoint, unix.MNT_DETACH)

The umount2(2) manpage writes about MNT_DETACH:

    Perform a lazy unmount: make the mount point unavailable for new
accesses, immediately disconnect the filesystem and all filesystems
mounted below it from each other and from the mount table, and actually
perform the unmount when the mount point ceases to be busy.

Could this be it? The MNT_DETACH umount looks partially asynchronous.
All the subsequent destroy commands may fail because they keep the mount
point busy. Finally the retry loop ends, the umount happens for real and
the following destroy succeeds.

—

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https://bugs.launchpad.net/bugs/1779156

Title:
  lxc 'delete' fails to destroy ZFS filesystem 'dataset is busy'

Status in linux package in Ubuntu:
  Triaged
Status in lxc package in Ubuntu:
  Confirmed
Status in linux source package in Cosmic:
  Triaged
Status in lxc source package in Cosmic:
  Confirmed
Status in linux source package in Disco:
  New
Status in lxc source package in Disco:
  New
Status in linux source package in Eoan:
  Triaged
Status in lxc source package in Eoan:
  Confirmed

Bug description:
  I'm not sure exactly what got me into this state, but I have several
  lxc containers that cannot be deleted.

  $ lxc info
  <snip>
  api_status: stable
  api_version: "1.0"
  auth: trusted
  public: false
  auth_methods:
  - tls
  environment:
    addresses: []
    architectures:
    - x86_64
    - i686
    certificate: |
      -----BEGIN CERTIFICATE-----
      <snip>
      -----END CERTIFICATE-----
    certificate_fingerprint: 
3af6f8b8233c5d9e898590a9486ded5c0bec045488384f30ea921afce51f75cb
    driver: lxc
    driver_version: 3.0.1
    kernel: Linux
    kernel_architecture: x86_64
    kernel_version: 4.15.0-23-generic
    server: lxd
    server_pid: 15123
    server_version: "3.2"
    storage: zfs
    storage_version: 0.7.5-1ubuntu15
    server_clustered: false
    server_name: milhouse

  $ lxc delete --force b1
  Error: Failed to destroy ZFS filesystem: cannot destroy 
'default/containers/b1': dataset is busy

  Talking in #lxc-dev, stgraber and sforeshee provided diagnosis:

   | short version is that something unshared a mount namespace causing
   | them to get a copy of the mount table at the time that dataset was
   | mounted, which then prevents zfs from being able to destroy it)

  The work around provided was

   | you can unstick this particular issue by doing:
   |  grep default/containers/b1 /proc/*/mountinfo
   | then for any of the hits, do:
   |   nsenter -t PID -m -- umount 
/var/snap/lxd/common/lxd/storage-pools/default/containers/b1
   | then try the delete again

  ProblemType: Bug
  DistroRelease: Ubuntu 18.10
  Package: linux-image-4.15.0-23-generic 4.15.0-23.25
  ProcVersionSignature: Ubuntu 4.15.0-23.25-generic 4.15.18
  Uname: Linux 4.15.0-23-generic x86_64
  NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
  ApportVersion: 2.20.10-0ubuntu3
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC1:  smoser    31412 F.... pulseaudio
   /dev/snd/controlC2:  smoser    31412 F.... pulseaudio
   /dev/snd/controlC0:  smoser    31412 F.... pulseaudio
  CurrentDesktop: ubuntu:GNOME
  Date: Thu Jun 28 10:42:45 2018
  EcryptfsInUse: Yes
  InstallationDate: Installed on 2015-07-23 (1071 days ago)
  InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Alpha amd64 (20150722.1)
  MachineType: 
b'\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff'
 
b'\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff'
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-23-generic 
root=UUID=f897b32a-eacf-4191-9717-844918947069 ro quiet splash vt.handoff=1
  RelatedPackageVersions:
   linux-restricted-modules-4.15.0-23-generic N/A
   linux-backports-modules-4.15.0-23-generic  N/A
   linux-firmware                             1.174
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 03/09/2015
  dmi.bios.vendor: Intel Corporation
  dmi.bios.version: RYBDWi35.86A.0246.2015.0309.1355
  dmi.board.asset.tag: ���������������������������������
  dmi.board.name: NUC5i5RYB
  dmi.board.vendor: Intel Corporation
  dmi.board.version: H40999-503
  dmi.chassis.asset.tag: ���������������������������������
  dmi.chassis.type: 3
  dmi.chassis.vendor: ���������������������������������
  dmi.chassis.version: ���������������������������������
  dmi.modalias: 
dmi:bvnIntelCorporation:bvrRYBDWi35.86A.0246.2015.0309.1355:bd03/09/2015:svn:pn:pvr:rvnIntelCorporation:rnNUC5i5RYB:rvrH40999-503:cvn:ct3:cvr:
  dmi.product.family: ���������������������������������
  dmi.product.name: ���������������������������������
  dmi.product.version: ���������������������������������
  dmi.sys.vendor: ���������������������������������

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