Don't hook the io until after the target->map function has been
called.
--- diff/drivers/md/dm.c	2002-12-04 16:00:03.000000000 +0000
+++ source/drivers/md/dm.c	2003-01-10 10:07:05.000000000 +0000
@@ -344,15 +344,14 @@
 	if (!io)
 		return -ENOMEM;
 
-	io->md = md;
-	io->end_io = bh->b_end_io;
-	io->context = bh->b_private;
-
 	r = ti->type->map(ti, bh, rw);
 
 	if (r > 0) {
 		/* hook the end io request fn */
 		atomic_inc(&md->pending);
+		io->md = md;
+		io->end_io = bh->b_end_io;
+		io->context = bh->b_private;
 		bh->b_end_io = dec_pending;
 		bh->b_private = io;