diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 874f145431d87a51b957916bf2a7213ee1dad3c9..475514bda9d0f7c87c131e7f361e2b35a8090030 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -54,11 +54,21 @@ struct pending_exception {
 	struct list_head list;
 
 	/*
-	 * Other pending_exceptions that are processing this
-	 * chunk.  When this list is empty, we know we can
-	 * complete the origins.
+	 * The primary pending_exception is the one that holds
+	 * the sibling_count and the list of origin_bios for a
+	 * group of pending_exceptions.  It is always last to get freed.
+	 * These fields get set up when writing to the origin.
 	 */
-	struct list_head siblings;
+	struct pending_exception *primary_pe;
+
+	/*
+	 * Number of pending_exceptions processing this chunk.
+	 * When this drops to zero we must complete the origin bios.
+	 * If incrementing or decrementing this, hold pe->snap->lock for
+	 * the sibling concerned and not pe->primary_pe->snap->lock unless
+	 * they are the same.
+	 */
+	atomic_t sibling_count;
 
 	/* Pointer back to snapshot context */
 	struct dm_snapshot *snap;
@@ -593,20 +603,15 @@ static void error_bios(struct bio *bio)
 
 static struct bio *__flush_bios(struct pending_exception *pe)
 {
-	struct pending_exception *sibling;
-
-	if (list_empty(&pe->siblings))
-		return bio_list_get(&pe->origin_bios);
-
-	sibling = list_entry(pe->siblings.next,
-			     struct pending_exception, siblings);
-
-	list_del(&pe->siblings);
-
-	/* This is fine as long as kcopyd is single-threaded. If kcopyd
-	 * becomes multi-threaded, we'll need some locking here.
+	/*
+	 * If this pe is involved in a write to the origin and
+	 * it is the last sibling to complete then release
+	 * the bios for the original write to the origin.
 	 */
-	bio_list_merge(&sibling->origin_bios, &pe->origin_bios);
+
+	if (pe->primary_pe &&
+	    atomic_dec_and_test(&pe->primary_pe->sibling_count))
+		return bio_list_get(&pe->primary_pe->origin_bios);
 
 	return NULL;
 }
@@ -614,6 +619,7 @@ static struct bio *__flush_bios(struct pending_exception *pe)
 static void pending_complete(struct pending_exception *pe, int success)
 {
 	struct exception *e;
+	struct pending_exception *primary_pe;
 	struct dm_snapshot *s = pe->snap;
 	struct bio *flush = NULL;
 
@@ -662,7 +668,20 @@ static void pending_complete(struct pending_exception *pe, int success)
 	}
 
  out:
-	free_pending_exception(pe);
+	primary_pe = pe->primary_pe;
+
+	/*
+	 * Free the pe if it's not linked to an origin write or if
+	 * it's not itself a primary pe.
+	 */
+	if (!primary_pe || primary_pe != pe)
+		free_pending_exception(pe);
+
+	/*
+	 * Free the primary pe if nothing references it.
+	 */
+	if (primary_pe && !atomic_read(&primary_pe->sibling_count))
+		free_pending_exception(primary_pe);
 
 	if (flush)
 		flush_bios(flush);
@@ -757,7 +776,8 @@ __find_pending_exception(struct dm_snapshot *s, struct bio *bio)
 			pe->e.old_chunk = chunk;
 			bio_list_init(&pe->origin_bios);
 			bio_list_init(&pe->snapshot_bios);
-			INIT_LIST_HEAD(&pe->siblings);
+			pe->primary_pe = NULL;
+			atomic_set(&pe->sibling_count, 1);
 			pe->snap = s;
 			pe->started = 0;
 
@@ -916,26 +936,12 @@ static int snapshot_status(struct dm_target *ti, status_type_t type,
 /*-----------------------------------------------------------------
  * Origin methods
  *---------------------------------------------------------------*/
-static void list_merge(struct list_head *l1, struct list_head *l2)
-{
-	struct list_head *l1_n, *l2_p;
-
-	l1_n = l1->next;
-	l2_p = l2->prev;
-
-	l1->next = l2;
-	l2->prev = l1;
-
-	l2_p->next = l1_n;
-	l1_n->prev = l2_p;
-}
-
 static int __origin_write(struct list_head *snapshots, struct bio *bio)
 {
-	int r = 1, first = 1;
+	int r = 1, first = 0;
 	struct dm_snapshot *snap;
 	struct exception *e;
-	struct pending_exception *pe, *next_pe, *last = NULL;
+	struct pending_exception *pe, *next_pe, *primary_pe = NULL;
 	chunk_t chunk;
 	LIST_HEAD(pe_queue);
 
@@ -962,6 +968,9 @@ static int __origin_write(struct list_head *snapshots, struct bio *bio)
 		 * Check exception table to see if block
 		 * is already remapped in this snapshot
 		 * and trigger an exception if not.
+		 *
+		 * sibling_count is initialised to 1 so pending_complete()
+		 * won't destroy the primary_pe while we're inside this loop.
 		 */
 		e = lookup_exception(&snap->complete, chunk);
 		if (!e) {
@@ -971,31 +980,60 @@ static int __origin_write(struct list_head *snapshots, struct bio *bio)
 				snap->valid = 0;
 
 			} else {
-				if (first) {
-					bio_list_add(&pe->origin_bios, bio);
+				if (!primary_pe) {
+					/*
+					 * Either every pe here has same
+					 * primary_pe or none has one yet.
+					 */
+					if (pe->primary_pe)
+						primary_pe = pe->primary_pe;
+					else {
+						primary_pe = pe;
+						first = 1;
+					}
+
+					bio_list_add(&primary_pe->origin_bios,
+						     bio);
 					r = 0;
-					first = 0;
 				}
-				if (last && list_empty(&pe->siblings))
-					list_merge(&pe->siblings,
-						   &last->siblings);
+				if (!pe->primary_pe) {
+					atomic_inc(&primary_pe->sibling_count);
+					pe->primary_pe = primary_pe;
+				}
 				if (!pe->started) {
 					pe->started = 1;
 					list_add_tail(&pe->list, &pe_queue);
 				}
-				last = pe;
 			}
 		}
 
 		up_write(&snap->lock);
 	}
 
+	if (!primary_pe)
+		goto out;
+
+	/*
+	 * If this is the first time we're processing this chunk and
+	 * sibling_count is now 1 it means all the pending exceptions
+	 * got completed while we were in the loop above, so it falls to
+	 * us here to remove the primary_pe and submit any origin_bios.
+	 */
+
+	if (first && atomic_dec_and_test(&primary_pe->sibling_count)) {
+		flush_bios(bio_list_get(&primary_pe->origin_bios));
+		free_pending_exception(primary_pe);
+		/* If we got here, pe_queue is necessarily empty. */
+		goto out;
+	}
+
 	/*
 	 * Now that we have a complete pe list we can start the copying.
 	 */
 	list_for_each_entry_safe(pe, next_pe, &pe_queue, list)
 		start_copy(pe);
 
+ out:
 	return r;
 }