IB/rxe: Advance the consumer pointer before posting the CQE

A simple userspace application might poll the CQ, find a completion,
and then attempt to post a new WQE to the SQ. A spurious error can
occur if the userspace application detects a full SQ in the instant
before the kernel is able to advance the SQ consumer pointer.

This is noticeable when using single-entry SQs with ibv_rc_pingpong
if lots of kernel and userspace library debugging is enabled.

Signed-off-by: Andrew Boyer <andrew.boyer@dell.com>
Reviewed-by: Yonatan Cohen <yonatanc@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/sw/rxe/rxe_comp.c

@@ -420,11 +420,12 @@ static void do_complete(struct rxe_qp *qp, struct rxe_send_wqe *wqe)
 	    (wqe->wr.send_flags & IB_SEND_SIGNALED) ||
 	    (qp->req.state == QP_STATE_ERROR)) {
 		make_send_cqe(qp, wqe, &cqe);
+		advance_consumer(qp->sq.queue);
 		rxe_cq_post(qp->scq, &cqe, 0);
+	} else {
+		advance_consumer(qp->sq.queue);
 	}
 
-	advance_consumer(qp->sq.queue);
-
 	/*
 	 * we completed something so let req run again
 	 * if it is trying to fence