That affects a few video games from Valve, with Dota 2 and Counter-Strike 2 being the most notable.
From Linux's epoll_ctl man page: Sets an exclusive wakeup mode for the epoll file descriptor that is being attached to the target file descriptor, fd. When a wakeup event occurs and multiple epoll file descriptors are attached to the same target file using EPOLLEXCLUSIVE, one or more of the epoll file descriptors will receive an event with epoll_wait(2). The default in this scenario (when EPOLLEXCLUSIVE is not set) is for all epoll file descriptors to receive an event. EPOLLEXCLUSIVE is thus useful for avoiding thundering herd problems in certain scenarios. "one or more of the epoll file descriptors will receive an event" implies that we may be able to provide a naive but conforming implementation that just ignores the flag (at least initially). Are you able to apply a patch, build a kernel, and test?
(In reply to Ed Maste from comment #1) > we may be able to provide a naive but conforming implementation that just ignores the flag (at least initially). That should be enough for the aforementioned apps. I'm already doing just that, in fact. > Are you able to apply a patch, build a kernel, and test? Yes, I'm able to do that.
(In reply to Alex S from comment #2) It turns out there is an old patch from trasz@ in https://reviews.freebsd.org/D23172. I've rebased it now.