During a world update, after masking packages per portage recommendation to fix dependency issues by adding `python_targets_python2_7` to a few entries, I received the following message:

`emerge: there are no ebuilds to satisfy "dev-libs/userspace-rcu:0/6=".
(dependency required by "sys-fs/multipath-tools-0.8.1::core-kit" [installed])
(dependency required by "@selected" [set])
(dependency required by "@world" [argument])`