IB/ipath: Add barrier before updating WC head in shared memory

Add a barrier to make sure the CPU doesn't reorder writes to memory,
since user programs can be polling on the head index update and the
entry should be written before that.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/ipath/ipath_ud.c

@@ -176,6 +176,8 @@ static void ipath_ud_loopback(struct ipath_qp *sqp,
 			dev->n_pkt_drops++;
 			goto bail_sge;
 		}
+		/* Make sure entry is read after head index is read. */
+		smp_rmb();
 		wqe = get_rwqe_ptr(rq, tail);
 		if (++tail >= rq->size)
 			tail = 0;