title: cryptsetup and umount fail with "target is busy." error
body: <p>I'm experimenting with cryptsetup. My target is to be able to store data into and encrypted file into a non-encrypted hard drive.</p>
  <p>I'm doing the following</p>
  <pre><code>mkdir /data/test &amp;&amp; /data/test
  
  # Create a key
  openssl genrsa -out luks_key 4096
  
  # Create file container
  dd if=/dev/zero of=device bs=1G count=50 status=progress
  
  # Create and open luks device
  cryptsetup -v -y --batch-mode luksFormat device luks_key
  sudo cryptsetup -v --batch-mode --key-file=luks_key --type luks open device device_name
  
  # Format the device
  sudo mkfs.ext4 -j /dev/mapper/device_name
  
  # Create mount point
  mkdir mount_point
  
  # Mount the device
  sudo mount -t ext4 /dev/mapper/device_name mount_point
  
  cd mount_point
  
  sudo dd if=/dev/zero of=random_data.bin bs=1G count=1 status=progress
  
  # This will fail until I close the terminal window with &quot;target is busy.&quot; error
  sudo umount /data/test/mount_point
  cryptsetup -v close device_name
  </code></pre>
  <p>All works well up to the point were I try to unmount. The error is:</p>
  <pre><code>umount: /data/test/mount_point: target is busy.
  </code></pre>
  <p>The only way to be able to unmount is to close the terminal window. Opening a second one isn't enough. I'm little bit puzzled :)</p>
  <p>I run on LinuxMint 18.04 with Linux kernel 5.15.30-25.</p>
  <p>Anyone seeing the same thing?</p>
  <p>Thanks,</p>