Recheck that the exception hasn't entered the pending table by another
thread.
--- diff/drivers/md/dm-snapshot.c	2003-11-26 10:19:17.000000000 +0000
+++ source/drivers/md/dm-snapshot.c	2003-11-26 10:19:22.000000000 +0000
@@ -808,22 +808,28 @@
 		 * to hold the lock while we do this.
 		 */
 		up_write(&s->lock);
-
 		pe = alloc_pending_exception();
-		pe->e.old_chunk = chunk;
-		pe->origin_bios = pe->snapshot_bios = NULL;
-		INIT_LIST_HEAD(&pe->siblings);
-		pe->snap = s;
-		pe->started = 0;
-
 		down_write(&s->lock);
-		if (s->store.prepare_exception(&s->store, &pe->e)) {
+
+		e = lookup_exception(&s->pending, chunk);
+		if (e) {
 			free_pending_exception(pe);
-			s->valid = 0;
-			return NULL;
-		}
+			pe = list_entry(e, struct pending_exception, e);
+		} else {
+			pe->e.old_chunk = chunk;
+			pe->origin_bios = pe->snapshot_bios = NULL;
+			INIT_LIST_HEAD(&pe->siblings);
+			pe->snap = s;
+			pe->started = 0;
 
-		insert_exception(&s->pending, &pe->e);
+			if (s->store.prepare_exception(&s->store, &pe->e)) {
+				free_pending_exception(pe);
+				s->valid = 0;
+				return NULL;
+			}
+
+			insert_exception(&s->pending, &pe->e);
+		}
 	}
 
 	return pe;