IB/mlx5: Make sure doorbell record is visible before doorbell

Put a wmb() to make sure the doorbell record is visible to the HCA before we
hit doorbell.
Signed-off-by: NEli Cohen <eli@mellanox.com>
Signed-off-by: NRoland Dreier <roland@purestorage.com>

drivers/infiniband/hw/mlx5/qp.c

@@ -2251,6 +2251,10 @@ int mlx5_ib_post_send(struct ib_qp *ibqp, struct ib_send_wr *wr,
 
 		qp->db.db[MLX5_SND_DBR] = cpu_to_be32(qp->sq.cur_post);
 
+		/* Make sure doorbell record is visible to the HCA before
+		 * we hit doorbell */
+		wmb();
+
 		if (bf->need_lock)
 			spin_lock(&bf->lock);