xen/blkfront: use tagged queuing for barriers
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 c6727b5..6d912ab 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -420,9 +420,19 @@
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);
+ /*
+ * 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 then we can treat them as proper
+ * ordering tags.
+ */
+ if (info->feature_barrier)
+ ordered = QUEUE_ORDERED_TAG;
+
+ err = blk_queue_ordered(info->rq, ordered);
if (err)
return err;
@@ -509,8 +519,7 @@
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);