As far as I can tell, a (countdown) latch is different from a barrier.
In a barrier signaling and waiting is a single atomic operation. Each thread (of a group of N) reaching a barrier will wait until all N threads have reached an waited (and implicitly signaled) it.
A countdown latch is an event that will release one (or more) waiter only after has been signaled N times. Signaling and waiting threads are not necessarily the same and often are distinct sets.
I guess you could build a barrier from a countdown latch, but I suspect that a trivial mapping of barrier::wait to latch::signal+wait is going to racey and you need an additional sinchronization primitive. For example pthread_barrier_wait requires an additional mutex (similar to condition variables).
edit: std::barrier has (optionally) separate singal+wait and uses an explicit arrival token instead of a mutex to tie the signaling with waiting.
In practice a barrier and a countdown latch are used for different purposes (the former to coordinate multiple symmetric threads across phases of a distributed computation, the latter to wait for completion of N events).
> In a barrier signaling and waiting is a single atomic operation. Each thread (of a group of N) reaching a barrier will wait until all N threads have reached an waited (and implicitly signaled) it.
Yeah that's exactly what std::latch::arrive_and_wait does.
I don't think atomicity is a thing here though... arrive_and_wait() is just count_down() followed by wait().
> A countdown latch is an event that will release one (or more) waiter only after has been signaled N times. Signaling and waiting threads are not necessarily the same and often are distinct sets.
This is a little bit like a mutex that has a try_lock. It's not strictly the vanilla Computer Science "mutex" with lock/unlock per se, but it's not a fundamentally different concept; it's just a handy yet pretty close generalization.
I guess if you really want to give this a new name then maybe it's not a terrible idea given it's a slight generalization of a barrier, but latch is certainly not going to be any more accurate (or less confusing) than barrier.
In a barrier signaling and waiting is a single atomic operation. Each thread (of a group of N) reaching a barrier will wait until all N threads have reached an waited (and implicitly signaled) it.
A countdown latch is an event that will release one (or more) waiter only after has been signaled N times. Signaling and waiting threads are not necessarily the same and often are distinct sets.
I guess you could build a barrier from a countdown latch, but I suspect that a trivial mapping of barrier::wait to latch::signal+wait is going to racey and you need an additional sinchronization primitive. For example pthread_barrier_wait requires an additional mutex (similar to condition variables).
edit: std::barrier has (optionally) separate singal+wait and uses an explicit arrival token instead of a mutex to tie the signaling with waiting.
In practice a barrier and a countdown latch are used for different purposes (the former to coordinate multiple symmetric threads across phases of a distributed computation, the latter to wait for completion of N events).