Evironment: system with at least one filesystem on an LVM Thin Pool (as
recommended by snapper for ext4 snapshotting). It's not the root filesystem
(fortunately).

Situation: when booting, the system halts for 90 seconds waiting for
lvm2-activation-early.service and lvm2-activation.service to finish, which
failed. After that, systemd drops me to an emergency shell because it couldn't
mount all filesystems.

The reason is that dm-thin-pool.ko was not loaded, so vgchange could not
activate the Thin Pool LVs. In that emergency shell, if I run
  modprobe dm-thin-pool && vgchange -ay

Then the boot proceeds normally.

This began happening around the time kernel 4.6.0-1-default was released. It
did not happen on May 24 and it definitely did not on May 17.

Workaround: edit /etc/default/grub and add
 rdloaddriver=dm-thin-pool
to GRUB_CMDLINE_LINUX_DEFAULT.