[$] Rethinking race-free process signaling

One of the new features in the 5.1 kernel is the
pidfd_send_signal() system call. Combined with the (also new)
ability to create a file descriptor referring
to a process (a 'pidfd') by
opening its directory in /proc, this system call allows for
the sending of signals to processes in a race-free manner. An extension to
this feature proposed for 5.2 has, however, sparked a discussion that has
brought the whole concept into question. It may yet be that the pidfd
feature will be put on hold before the final 5.1 release while the API around
it is rethought.