Reading the link provided by https://news.ycombinator.com/item?id=47511778, I believe "atomically acquire multiple objects". The link states they try to emulate it by performing a poll then a read, but the gap between those results in a race, which is a terrible thing to have in a synchronisation primitive.
There was also something about needing to back out if any of the reads fails to acquire, which also sounds nasty.
Ah, interesting, so wfm does both the wait and the acquire!
When using eventfd it is indeed annoying having to both poll and later read to disarm the object (there are epoll tricks that can be used but are not generalizable).
The signal+wait is also a primitive that it is hard to implement atomically on posix.
