When killing an entire cgroup, you might avoid the PID reuse race by getting a list of PIDs in a cgroup, then for each PID, first opening a PIDFD to that PID, c

When killing an entire cgroup, you might avoid the PID reuse race by getting a list of PIDs in a cgroup, then for each PID, first opening a PIDFD to that PID, check via that PIDFD whether it’s a member of the correct cgroup, and then killing the PIDFD using pidfd_send_signal(). This way, no process can be killed inadvertently by PID reuse. There may be a better way to do this, but one way might be to open() /proc/<PID> and use the resulting FD to both to check /proc/<PID>/cgroup (using openat(FD, "cgroup", O_RDONLY|O_NOCTTY)) and then use the same FD as a PIDFD when calling pidfd_send_signal(). ISTM that systemd should use this method if its current method is just looping over PIDs.