EPOLLEXCLUSIVE doc from Linux manual page:

"""
EPOLLEXCLUSIVE (since Linux 4.5)

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 EPOLLEXCLU‐
SIVE 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.

If the same file descriptor is in multiple epoll instances, some
with  the  EPOLLEXCLUSIVE  flag, and others without, then events
will be provided to all epoll instances  that  did  not  specify
EPOLLEXCLUSIVE, and at least one of the epoll instances that did
specify EPOLLEXCLUSIVE.

The following  values  may  be  specified  in  conjunction  with
EPOLLEXCLUSIVE:  EPOLLIN,  EPOLLOUT,  EPOLLWAKEUP,  and EPOLLET.
EPOLLHUP and EPOLLERR can also be specified,  but  this  is  not
required:  as  usual,  these  events are always reported if they
occur, regardless of  whether  they  are  specified  in  events.
Attempts  to  specify  other  values  in  events yield an error.
EPOLLEXCLUSIVE may be used only in an  EPOLL_CTL_ADD  operation;
attempts  to  employ  it  with EPOLL_CTL_MOD yield an error.  If
EPOLLEXCLUSIVE has been set using epoll_ctl(), then a subsequent
EPOLL_CTL_MOD on the same epfd, fd pair yields an error.  A call
to epoll_ctl() that specifies EPOLLEXCLUSIVE in events and spec‐
ifies  the  target  file descriptor fd as an epoll instance will
likewise fail.  The error in all of these cases is EINVAL.
"""

See also https://lwn.net/Articles/633422/