Introduce an end_io method to the target types.
--- diff/drivers/md/dm.c	2003-01-10 10:08:09.000000000 +0000
+++ source/drivers/md/dm.c	2003-01-10 11:43:50.000000000 +0000
@@ -28,6 +28,8 @@
 struct dm_io {
 	struct mapped_device *md;
 
+	struct dm_target *ti;
+	int rw;
 	void (*end_io) (struct buffer_head * bh, int uptodate);
 	void *context;
 };
@@ -313,7 +315,19 @@
  */
 static void dec_pending(struct buffer_head *bh, int uptodate)
 {
+ 	int r;
 	struct dm_io *io = bh->b_private;
+	dm_endio_fn endio = io->ti->type->end_io;
+
+	if (endio) {
+		r = endio(io->ti, bh, io->rw, uptodate ? 0 : -EIO);
+		if (r < 0)
+			uptodate = 0;
+
+		else if (r > 0)
+			/* the target wants another shot at the io */
+			return;
+	}
 
 	if (atomic_dec_and_test(&io->md->pending))
 		/* nudge anyone waiting on suspend queue */
@@ -346,6 +360,8 @@
 		/* hook the end io request fn */
 		atomic_inc(&md->pending);
 		io->md = md;
+		io->ti = ti;
+		io->rw = rw;
 		io->end_io = bh->b_end_io;
 		io->context = bh->b_private;
 		bh->b_end_io = dec_pending;
--- diff/include/linux/device-mapper.h	2002-11-29 09:45:17.000000000 +0000
+++ source/include/linux/device-mapper.h	2003-01-10 11:36:08.000000000 +0000
@@ -34,6 +34,16 @@
  * > 0: simple remap complete
  */
 typedef int (*dm_map_fn) (struct dm_target *ti, struct buffer_head *bh, int rw);
+
+/*
+ * Returns:
+ * < 0 : error (currently ignored)
+ * 0   : ended successfully
+ * 1   : for some reason the io has still not completed (eg,
+ *       multipath target might want to requeue a failed io).
+ */
+typedef int (*dm_endio_fn) (struct dm_target *ti,
+			    struct buffer_head *bh, int rw, int error);
 typedef int (*dm_status_fn) (struct dm_target *ti, status_type_t status_type,
 			     char *result, int maxlen);
 
@@ -57,6 +67,7 @@
 	dm_ctr_fn ctr;
 	dm_dtr_fn dtr;
 	dm_map_fn map;
+	dm_endio_fn end_io;
 	dm_status_fn status;
 };