inotify?

Actually it's not so far away from inotify.

We have leases in fcntl() F_SETLEASE, they go alongside dnotify in fcntl() F_NOTIFY.

inotify could similarly benefit from an an ilease extension, as well as further scalability improvements. (inotify was designed to improve on dnotify in many ways including scalability; it's a shame it stopped half way).

That's blocking readers right there.