Is epoll thread-safe?
Note: The existing accepted answer is correct, but it mentions "there isn't much documentation that explicitly states that", while epoll documentation does state it.
The manual page for epoll_wait explicitly allows adding a file descriptor to an epoll set while it is being waited for in another thread:
Section "Notes":
While one thread is blocked in a call to epoll_wait(), it is possiblefor another thread to add a file descriptor to the waited-upon epollinstance. If the new file descriptor becomes ready, it will cause theepoll_wait() call to unblock.