IB/ipath: fix null deref during rdma ops

The problem was that node A's sending thread, which handles sending RDMA
read response data, would write the trigger word, the last packet would
be sent, node B would send a new RDMA read request, node A's interrupt
handler would initialize s_rdma_sge, then node A's sending thread would
update s_rdma_sge.  This didn't happen very often naturally but was more
frequent with 1 byte RDMA reads.  Rather than adding more locking or
increasing the QP structure size and copying sge data, I modified the
copy routine to update the pointers before writing the trigger word to
avoid the update race.

Signed-off-by: Ralph Campbell <ralphc@pathscale.com>
Signed-off-by: Bryan O'Sullivan <bos@pathscale.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/ipath/ipath_layer.c

@@ -872,12 +872,13 @@ static void copy_io(u32 __iomem *piobuf, struct ipath_sge_state *ss,
 		update_sge(ss, len);
 		length -= len;
 	}
+	/* Update address before sending packet. */
+	update_sge(ss, length);
 	/* must flush early everything before trigger word */
 	ipath_flush_wc();
 	__raw_writel(last, piobuf);
 	/* be sure trigger word is written */
 	ipath_flush_wc();
-	update_sge(ss, length);
 }
 
 /**
@@ -943,17 +944,18 @@ int ipath_verbs_send(struct ipath_devdata *dd, u32 hdrwords,
 	if (likely(ss->num_sge == 1 && len <= ss->sge.length &&
 		   !((unsigned long)ss->sge.vaddr & (sizeof(u32) - 1)))) {
 		u32 w;
+		u32 *addr = (u32 *) ss->sge.vaddr;
 
+		/* Update address before sending packet. */
+		update_sge(ss, len);
 		/* Need to round up for the last dword in the packet. */
 		w = (len + 3) >> 2;
-		__iowrite32_copy(piobuf, ss->sge.vaddr, w - 1);
+		__iowrite32_copy(piobuf, addr, w - 1);
 		/* must flush early everything before trigger word */
 		ipath_flush_wc();
-		__raw_writel(((u32 *) ss->sge.vaddr)[w - 1],
-			     piobuf + w - 1);
+		__raw_writel(addr[w - 1], piobuf + w - 1);
 		/* be sure trigger word is written */
 		ipath_flush_wc();
-		update_sge(ss, len);
 		ret = 0;
 		goto bail;
 	}