ibmvscsi: Add memory barriers for send / receive

commit 7114aae0 upstream.

Add a memory barrier prior to sending a new command to the VIOS
to ensure the VIOS does not receive stale data in the command buffer.
Also add a memory barrier when processing the CRQ for completed commands.
Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Acked-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
[bwh: Backported to 3.2: as the iSeries code is still present, these
 functions have different names and live in rpa_vscsi.c.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/scsi/ibmvscsi/rpa_vscsi.c

@@ -104,6 +104,11 @@ static struct viosrp_crq *crq_queue_next_crq(struct crq_queue *queue)
 	if (crq->valid & 0x80) {
 		if (++queue->cur == queue->size)
 			queue->cur = 0;
+
+		/* Ensure the read of the valid bit occurs before reading any
+		 * other bits of the CRQ entry
+		 */
+		rmb();
 	} else
 		crq = NULL;
 	spin_unlock_irqrestore(&queue->lock, flags);
@@ -122,6 +127,11 @@ static int rpavscsi_send_crq(struct ibmvscsi_host_data *hostdata,
 {
 	struct vio_dev *vdev = to_vio_dev(hostdata->dev);
 
+	/*
+	 * Ensure the command buffer is flushed to memory before handing it
+	 * over to the VIOS to prevent it from fetching any stale data.
+	 */
+	mb();
 	return plpar_hcall_norets(H_SEND_CRQ, vdev->unit_address, word1, word2);
 }