[$] Rethinking race-free process signaling

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