Add the ability to track writes to the snapshot-merged target which
have been mapped to the COW device.

Rationale is that we won't want to start merging a chunk until all
writes to that chunk have completed. Reads don't matter because the
contents of the chunk on the COW device is not affected by the merging
process.

Index: linux-2.6.16.i686/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.16.i686.orig/drivers/md/dm-snap.c	2006-05-05 14:45:22.000000000 +0100
+++ linux-2.6.16.i686/drivers/md/dm-snap.c	2006-05-05 14:45:27.000000000 +0100
@@ -1322,6 +1322,7 @@ static int merged_map(struct dm_target *
 	struct dm_merged *merged = (struct dm_merged *) ti->private;
 	struct dm_snapshot *s = &merged->snap;
 	struct exception *e;
+	struct pending_exception *pe;
 	chunk_t chunk;
 	int r = 1;
 
@@ -1330,28 +1331,53 @@ static int merged_map(struct dm_target *
 
 	chunk = sector_to_chunk(s, bio->bi_sector);
 
-	down_read(&s->lock);
+	down_write(&s->lock);
 
 	if (!s->valid) {
-		up_read(&s->lock);
+		up_write(&s->lock);
 		return -EIO;
 	}
 
 	e = lookup_exception(&s->complete, chunk);
-	if (e)
+	if (e) {
+		if (bio_rw(bio) == WRITE) {
+			pe = __find_pending_exception(s, chunk, 0);
+			if (!pe) {
+				__invalidate_snapshot(s, pe, -ENOMEM);
+				r = -EIO;
+				goto out_unlock;
+			}
+                       map_context->ptr = pe;
+		}
+
 		remap_exception(s, e, bio);
-	else {
+	} else {
 		bio->bi_bdev = s->origin->bdev;
 
 		if (bio_rw(bio) == WRITE)
 			r = do_origin(s->origin, bio->bi_sector, bio);
 	}
 
-	up_read(&s->lock);
+ out_unlock:
+	up_write(&s->lock);
 
 	return r;
 }
 
+static int merged_end_io(struct dm_target *ti, struct bio *bio,
+			 int error, union map_info *map_context)
+{
+	struct pending_exception *pe =
+		(struct pending_exception *) map_context->ptr;
+
+	if (bio_rw(bio) != WRITE || pe == NULL)
+		return error;
+
+	unref_pending_exception(pe);
+
+	return error;
+}
+
 static void merged_resume(struct dm_target *ti)
 {
 	struct dm_merged *merged = (struct dm_merged *) ti->private;
@@ -1419,6 +1445,7 @@ static struct target_type merged_target 
 	.ctr     = merged_ctr,
 	.dtr     = merged_dtr,
 	.map     = merged_map,
+	.end_io  = merged_end_io,
 	.resume  = merged_resume,
 	.status  = merged_status,
 };