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>

1 files changed, 13 insertions, 4 deletions

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index c6727b5cefd..6d912ab4729 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -420,9 +420,19 @@ 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);
+        /*
+         * 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 @@ 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);