commit 1327d9674e235b4c3004034298ba4863d038ee88
Author: S.Çağlar Onur <caglar@cs.princeton.edu>
Date:   Fri Mar 19 15:12:59 2010 -0400

    support_barriers_on_single_device_dm_devices.patch

diff --git a/drivers/md/dm-linear.c b/drivers/md/dm-linear.c
index 17753d8..cda44f6 100644
--- a/drivers/md/dm-linear.c
+++ b/drivers/md/dm-linear.c
@@ -123,6 +123,7 @@ static struct target_type linear_target = {
 	.map    = linear_map,
 	.status = linear_status,
 	.ioctl  = linear_ioctl,
+	.features = DM_TARGET_SUPPORTS_BARRIERS,
 };
 
 int __init dm_linear_init(void)
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 2fc199b..e540f57 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -38,6 +38,8 @@ struct dm_table {
 	sector_t *highs;
 	struct dm_target *targets;
 
+	unsigned barriers_supported:1;
+
 	/*
 	 * Indicates the rw permissions for the new logical
 	 * device.  This should be a combination of FMODE_READ
@@ -221,6 +223,7 @@ int dm_table_create(struct dm_table **result, int mode,
 	memset(t, 0, sizeof(*t));
 	INIT_LIST_HEAD(&t->devices);
 	atomic_set(&t->holders, 1);
+	t->barriers_supported = 1;
 
 	if (!num_targets)
 		num_targets = KEYS_PER_NODE;
@@ -765,6 +768,10 @@ int dm_table_add_target(struct dm_table *t, const char *type,
 	/* FIXME: the plan is to combine high here and then have
 	 * the merge fn apply the target level restrictions. */
 	combine_restrictions_low(&t->limits, &tgt->limits);
+
+	if (!(tgt->type->features & DM_TARGET_SUPPORTS_BARRIERS))
+		t->barriers_supported = 0;
+
 	return 0;
 
  bad:
@@ -809,6 +816,12 @@ int dm_table_complete(struct dm_table *t)
 
 	check_for_valid_limits(&t->limits);
 
+	/*
+	 * We only support barriers if there is exactly one underlying device.
+	 */
+	if (!list_is_singular(&t->devices))
+		t->barriers_supported = 0;
+
 	/* how many indexes will the btree have ? */
 	leaf_nodes = dm_div_up(t->num_targets, KEYS_PER_NODE);
 	t->depth = 1 + int_log(leaf_nodes, CHILDREN_PER_NODE);
@@ -1033,6 +1046,11 @@ struct mapped_device *dm_table_get_md(struct dm_table *t)
 	return t->md;
 }
 
+int dm_table_barrier_ok(struct dm_table *t)
+{
+	return t->barriers_supported;
+}
+EXPORT_SYMBOL(dm_table_barrier_ok);
 EXPORT_SYMBOL(dm_vcalloc);
 EXPORT_SYMBOL(dm_get_device);
 EXPORT_SYMBOL(dm_put_device);
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 12ecab3..bdf5107 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -781,6 +781,12 @@ static void __split_bio(struct mapped_device *md, struct bio *bio)
 		return;
 	}
 
+	if (unlikely(bio_barrier(bio) && !dm_table_barrier_ok(ci.map))) {
+		dm_table_put(ci.map);
+		bio_endio(bio, bio->bi_size, -EOPNOTSUPP);
+		return;
+	}
+
 	ci.md = md;
 	ci.bio = bio;
 	ci.io = alloc_io(md);
@@ -814,15 +820,6 @@ static int dm_request(request_queue_t *q, struct bio *bio)
 	int rw = bio_data_dir(bio);
 	struct mapped_device *md = q->queuedata;
 
-	/*
-	 * There is no use in forwarding any barrier request since we can't
-	 * guarantee it is (or can be) handled by the targets correctly.
-	 */
-	if (unlikely(bio_barrier(bio))) {
-		bio_endio(bio, bio->bi_size, -EOPNOTSUPP);
-		return 0;
-	}
-
 	down_read(&md->io_lock);
 
 	disk_stat_inc(dm_disk(md), ios[rw]);
diff --git a/drivers/md/dm.h b/drivers/md/dm.h
index e71b22d..a67e8b7 100644
--- a/drivers/md/dm.h
+++ b/drivers/md/dm.h
@@ -80,6 +80,7 @@ int dm_table_resume_targets(struct dm_table *t);
 int dm_table_any_congested(struct dm_table *t, int bdi_bits);
 void dm_table_unplug_all(struct dm_table *t);
 int dm_table_flush_all(struct dm_table *t);
+int dm_table_barrier_ok(struct dm_table *t);
 
 /*-----------------------------------------------------------------
  * A registry of target types.
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 499f537..6bc9120 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -89,9 +89,15 @@ int dm_get_device(struct dm_target *ti, const char *path, sector_t start,
 void dm_put_device(struct dm_target *ti, struct dm_dev *d);
 
 /*
+ * Target features
+ */
+#define DM_TARGET_SUPPORTS_BARRIERS 0x00000001
+
+/*
  * Information about a target type
  */
 struct target_type {
+	uint64_t features;
 	const char *name;
 	struct module *module;
 	unsigned version[3];
diff --git a/include/linux/list.h b/include/linux/list.h
index f29fc9c..55051eb 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -319,6 +319,15 @@ static inline int list_empty_careful(const struct list_head *head)
 	return (next == head) && (next == head->prev);
 }
 
+/**
+ * list_is_singular - tests whether a list has just one entry.
+ * @head: the list to test.
+ */
+static inline int list_is_singular(const struct list_head *head)
+{
+        return !list_empty(head) && (head->next == head->prev);
+}
+
 static inline void __list_splice(struct list_head *list,
 				 struct list_head *head)
 {