RDMA/mlx5: Fix potential race between destroy and CQE poll

The SRQ can be destroyed right before mlx5_cmd_get_srq is called.
In such case the latter will return NULL instead of expected SRQ.

Fixes: e126ba97 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Link: https://lore.kernel.org/r/20200830084010.102381-5-leon@kernel.orgSigned-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mlx5/cq.c

@@ -168,7 +168,7 @@ static void handle_responder(struct ib_wc *wc, struct mlx5_cqe64 *cqe,
 {
 	enum rdma_link_layer ll = rdma_port_get_link_layer(qp->ibqp.device, 1);
 	struct mlx5_ib_dev *dev = to_mdev(qp->ibqp.device);
-	struct mlx5_ib_srq *srq;
+	struct mlx5_ib_srq *srq = NULL;
 	struct mlx5_ib_wq *wq;
 	u16 wqe_ctr;
 	u8  roce_packet_type;
@@ -180,7 +180,8 @@ static void handle_responder(struct ib_wc *wc, struct mlx5_cqe64 *cqe,
 
 		if (qp->ibqp.xrcd) {
 			msrq = mlx5_cmd_get_srq(dev, be32_to_cpu(cqe->srqn));
-			srq = to_mibsrq(msrq);
+			if (msrq)
+				srq = to_mibsrq(msrq);
 		} else {
 			srq = to_msrq(qp->ibqp.srq);
 		}