One of the many changes in the 5.4 kernel is the completion (insofar as
anything in the kernel is truly complete) of the pidfd API. Getting that work done has been “a
wild ride so far”, according to its author Christian Brauner during a
session at the 2019 Kernel Recipes conference. He went on to describe
the history of this work and some lessons for others interested in adding
major new APIs to the Linux kernel.

Source: LWN.net – [$] Adding the pidfd abstraction to the kernel