Add some diagnostics for snapshots.

Index: current-quilt-publish/drivers/md/dm-snap.c
===================================================================
--- current-quilt-publish.orig/drivers/md/dm-snap.c	2006-01-05 14:26:06.000000000 +0000
+++ current-quilt-publish/drivers/md/dm-snap.c	2006-01-05 14:26:06.000000000 +0000
@@ -934,7 +934,7 @@ static int __origin_write(struct list_he
 	int r = 1, first = 1;
 	struct dm_snapshot *snap;
 	struct exception *e;
-	struct pending_exception *pe, *last = NULL;
+	struct pending_exception *pe, *next_pe, *last = NULL;
 	chunk_t chunk;
 
 	/* Do all the snapshots on this origin */
@@ -990,6 +990,8 @@ static int __origin_write(struct list_he
 			down_write(&pe->snap->lock);
 			if (first)
 				bio_list_add(&pe->origin_bios, bio);
+			next_pe = list_entry(pe->siblings.next,
+					struct pending_exception, siblings);
 			if (!pe->started) {
 				pe->started = 1;
 				up_write(&pe->snap->lock);
@@ -999,7 +1001,11 @@ static int __origin_write(struct list_he
 			first = 0;
 			pe = list_entry(pe->siblings.next,
 					struct pending_exception, siblings);
-
+			if (pe != next_pe) {
+				printk(KERN_ERR "%s(%s,%d): pending exception with changed sibling: next_pe=%p pe=%p last=%p\n",
+				       __FUNCTION__, current->comm, current->pid, next_pe, pe, last);
+				dump_stack();
+			}
 		} while (pe != last);
 	}