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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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@xxxxxxxxxx>

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);



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