IB/hfi1: Race hazard avoidance in user SDMA driver

Set the errcode before the state and add the smb_wmb() to avoid a
potential race condition with the user.
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Michael Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/hfi1/user_sdma.c

@@ -1615,9 +1615,10 @@ static inline void set_comp_state(struct hfi1_user_sdma_pkt_q *pq,
 {
 	hfi1_cdbg(SDMA, "[%u:%u:%u:%u] Setting completion status %u %d",
 		  pq->dd->unit, pq->ctxt, pq->subctxt, idx, state, ret);
-	cq->comps[idx].status = state;
 	if (state == ERROR)
 		cq->comps[idx].errcode = -ret;
+	smp_wmb(); /* make sure errcode is visible first */
+	cq->comps[idx].status = state;
 	trace_hfi1_sdma_user_completion(pq->dd, pq->ctxt, pq->subctxt,
 					idx, state, ret);
 }