[PATCH RFC] xen/blkfront: use tagged queuing for barriers

[Jeremy Fitzhardinge <jeremy@goop.org>]

When barriers are supported, then use QUEUE_ORDERED_TAG to tell the block
subsystem that it doesn't need to do anything else with the barriers.
Previously we used ORDERED_DRAIN which caused the block subsystem to
drain all pending IO before submitting the barrier, which would be
very expensive.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 92091a7..b61a021 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -418,10 +418,20 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
 static int xlvbd_barrier(struct blkfront_info *info)
 {
 	int err;
+	unsigned ordered = QUEUE_ORDERED_NONE;
 
-	err = blk_queue_ordered(info->rq,
-				info->feature_barrier ? QUEUE_ORDERED_DRAIN : QUEUE_ORDERED_NONE,
-				NULL);
+	/*
+	 * If we don't have barrier support, then there's really no
+	 * way to guarantee write ordering, so we really just have to
+	 * send writes to the backend and hope for the best.  If
+	 * barriers are supported, we don't really know what the
+	 * flushing semantics of the barrier are, so again, tag the
+	 * requests in order and hope for the best.
+	 */
+	if (info->feature_barrier)
+		ordered = QUEUE_ORDERED_TAG;
+
+	err = blk_queue_ordered(info->rq, ordered, NULL);
 
 	if (err)
 		return err;
@@ -508,8 +518,7 @@ static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
 	info->rq = gd->queue;
 	info->gd = gd;
 
-	if (info->feature_barrier)
-		xlvbd_barrier(info);
+	xlvbd_barrier(info);
 
 	if (vdisk_info & VDISK_READONLY)
 		set_disk_ro(gd, 1);