wait

            fn wait(ptr: *Run, val: Run, io: std.Io) error{Canceled}!void {
                assert(val.pad == 0);
                while (true) {
                    // This cannot load a previous value since this thread previously loaded the
                    // latest value.
                    const prev = @atomicLoad(Run, ptr, .acquire);
                    assert(prev.pad == 0);
                    if (prev == val) break;

                    try io.futexWait(Run, ptr, prev);
                }
            }