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.

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