Barrier

public sealed class BarrierSlim
{
    private readonly int _threshold;
    private int _arrived;

    private readonly SemaphoreSlim _gate;
    private readonly SemaphoreSlim _mutex;

    public BarrierSlim(int count)
    {
        _threshold = count;
        _gate = new SemaphoreSlim(initialCount: 0, maxCount: count);
        _mutex = new SemaphoreSlim(initialCount: 1, maxCount: 1);
    }

    public async Task ArriveAsync()
    {
        await _mutex.WaitAsync();

        _arrived++;

        if (_threshold == _arrived)
        {
            for (int i = 0; i < _threshold; i++)
            {
                _gate.Release();
            }
            _mutex.Release();
        }
        else
        {
            _mutex.Release();
            await _gate.WaitAsync();
        }
    }
}