Congestion Control (CC) plays a vital role in deploying lossless datacenter networks based on Remote Direct Memory Access (RDMA). A high-performance CC scheme should provide low-latency and precise feedback to congestion events. However, no existing CC schemes achieved both features simultaneously. In this paper, we propose LHCC, a Low-latency and Hi-precision Congestion Control scheme for RDMA datacenter networks. LHCC uses out-band signaling to notify the network status and hence a packet sender can detect congestion events within an RTT. In addition, LHCC adjusts packet sending rate by taking into consideration all queues along the entire path that a packet has gone through. Accordingly, it provides a more precise CC compared with existing schemes especially when there are multiple bottlenecks in the networks. We build the LHCC prototype on a real testbed carrying NVIDIA BlueField-3 NICs and AGM39D FPGAs. Both testbed experiments and extensive simulations show that LHCC can reduce the Flow Completion Time (FCT) slow down and reduce the buffer usage (i.e., reduce the queue lengths) by up to 62.5% and 58%, respectively, compared with the state-of-the-art high-precision CC scheme, HPCC.