Store deferred ios in a queue so that we preserve ordering and hence
don't break write barriers.  dm_request is now blocked while deferred
ios are resubmitted.  [Christophe Saout]
--- diff/drivers/md/dm.c	2004-01-02 11:36:02.000000000 +0000
+++ source/drivers/md/dm.c	2004-01-02 12:04:52.000000000 +0000
@@ -63,7 +63,8 @@
 	 */
 	atomic_t pending;
 	wait_queue_head_t wait;
-	struct bio *deferred;
+	struct bio *deferred_head;
+	struct bio *deferred_tail;
 
 	/*
 	 * The current mapping.
@@ -230,8 +231,13 @@
 		return 1;
 	}
 
-	bio->bi_next = md->deferred;
-	md->deferred = bio;
+	bio->bi_next = NULL;
+
+	if (md->deferred_tail)
+		md->deferred_tail->bi_next = bio;
+	else
+		md->deferred_head = bio;
+	md->deferred_tail = bio;
 
 	up_write(&md->lock);
 	return 0;		/* deferred successfully */
@@ -511,6 +517,16 @@
  *---------------------------------------------------------------*/
 
 
+static inline void __dm_request(struct mapped_device *md, struct bio *bio)
+{
+	if (!md->map) {
+		bio_io_error(bio, bio->bi_size);
+		return;
+	}
+
+	__split_bio(md, bio);
+}
+
 /*
  * The request function that just remaps the bio built up by
  * dm_merge_bvec.
@@ -549,12 +565,7 @@
 		down_read(&md->lock);
 	}
 
-	if (!md->map) {
-		bio_io_error(bio, bio->bi_size);
-		return 0;
-	}
-
-	__split_bio(md, bio);
+	__dm_request(md, bio);
 	up_read(&md->lock);
 	return 0;
 }
@@ -787,16 +798,16 @@
 }
 
 /*
- * Requeue the deferred bios by calling generic_make_request.
+ * Process the deferred bios
  */
-static void flush_deferred_io(struct bio *c)
+static void __flush_deferred_io(struct mapped_device *md, struct bio *c)
 {
 	struct bio *n;
 
 	while (c) {
 		n = c->bi_next;
 		c->bi_next = NULL;
-		generic_make_request(c);
+		__dm_request(md, c);
 		c = n;
 	}
 }
@@ -891,11 +902,12 @@
 	dm_table_resume_targets(md->map);
 	clear_bit(DMF_SUSPENDED, &md->flags);
 	clear_bit(DMF_BLOCK_IO, &md->flags);
-	def = md->deferred;
-	md->deferred = NULL;
+
+	def = md->deferred_head;
+	md->deferred_head = md->deferred_tail = NULL;
+	__flush_deferred_io(md, def);
 	up_write(&md->lock);
 
-	flush_deferred_io(def);
 	blk_run_queues();
 
 	return 0;