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_srq.c

@@ -80,6 +80,8 @@ int ipath_post_srq_receive(struct ib_srq *ibsrq, struct ib_recv_wr *wr,
 		wqe->num_sge = wr->num_sge;
 		for (i = 0; i < wr->num_sge; i++)
 			wqe->sg_list[i] = wr->sg_list[i];
+		/* Make sure queue entry is written before the head index. */
+		smp_wmb();
 		wq->head = next;
 		spin_unlock_irqrestore(&srq->rq.lock, flags);
 	}