RDMA/rxe: Fix incomplete state save in rxe_requester

If a send packet is dropped by the IP layer in rxe_requester()
the call to rxe_xmit_packet() can fail with err == -EAGAIN.
To recover, the state of the wqe is restored to the state before
the packet was sent so it can be resent. However, the routines
that save and restore the state miss a significnt part of the
variable state in the wqe, the dma struct which is used to process
through the sge table. And, the state is not saved before the packet
is built which modifies the dma struct.

Under heavy stress testing with many QPs on a fast node sending
large messages to a slow node dropped packets are observed and
the resent packets are corrupted because the dma struct was not
restored. This patch fixes this behavior and allows the test cases
to succeed.

Fixes: 3050b998 ("IB/rxe: Fix race condition between requester and completer")
Link: https://lore.kernel.org/r/20230721200748.4604-1-rpearsonhpe@gmail.comSigned-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/sw/rxe/rxe_req.c

@@ -581,6 +581,7 @@ static void save_state(struct rxe_send_wqe *wqe,
 	rollback_wqe->state = wqe->state;
 	rollback_wqe->first_psn = wqe->first_psn;
 	rollback_wqe->last_psn = wqe->last_psn;
+	rollback_wqe->dma = wqe->dma;
 	*rollback_psn = qp->req.psn;
 }
 
@@ -592,6 +593,7 @@ static void rollback_state(struct rxe_send_wqe *wqe,
 	wqe->state = rollback_wqe->state;
 	wqe->first_psn = rollback_wqe->first_psn;
 	wqe->last_psn = rollback_wqe->last_psn;
+	wqe->dma = rollback_wqe->dma;
 	qp->req.psn = rollback_psn;
 }
 
@@ -797,6 +799,9 @@ int rxe_requester(struct rxe_qp *qp)
 	pkt.mask = rxe_opcode[opcode].mask;
 	pkt.wqe = wqe;
 
+	/* save wqe state before we build and send packet */
+	save_state(wqe, qp, &rollback_wqe, &rollback_psn);
+
 	av = rxe_get_av(&pkt, &ah);
 	if (unlikely(!av)) {
 		rxe_dbg_qp(qp, "Failed no address vector\n");
@@ -829,31 +834,31 @@ int rxe_requester(struct rxe_qp *qp)
 	if (ah)
 		rxe_put(ah);
 
-	/*
-	 * To prevent a race on wqe access between requester and completer,
-	 * wqe members state and psn need to be set before calling
-	 * rxe_xmit_packet().
-	 * Otherwise, completer might initiate an unjustified retry flow.
-	 */
-	save_state(wqe, qp, &rollback_wqe, &rollback_psn);
+	/* update wqe state as though we had sent it */
 	update_wqe_state(qp, wqe, &pkt);
 	update_wqe_psn(qp, wqe, &pkt, payload);
 
 	err = rxe_xmit_packet(qp, &pkt, skb);
 	if (err) {
-		qp->need_req_skb = 1;
+		if (err != -EAGAIN) {
+			wqe->status = IB_WC_LOC_QP_OP_ERR;
+			goto err;
+		}
 
+		/* the packet was dropped so reset wqe to the state
+		 * before we sent it so we can try to resend
+		 */
 		rollback_state(wqe, qp, &rollback_wqe, rollback_psn);
 
-		if (err == -EAGAIN) {
+		/* force a delay until the dropped packet is freed and
+		 * the send queue is drained below the low water mark
+		 */
+		qp->need_req_skb = 1;
+
 		rxe_sched_task(&qp->req.task);
 		goto exit;
 	}
 
-		wqe->status = IB_WC_LOC_QP_OP_ERR;
-		goto err;
-	}
-
 	update_state(qp, &pkt);
 
 	/* A non-zero return value will cause rxe_do_task to