@@ -50,6 +50,7 @@
                        struct list_head *a, struct list_head *b)
 {
        struct list_head *tail = head;
+       u8 count = 0;
 
        for (;;) {
                /* if equal, take 'a' -- important for sort stability */
@@ -75,6 +76,15 @@
        /* Finish linking remainder of list b on to tail */
        tail->next = b;
        do {
+               /*
+                * If the merge is highly unbalanced (e.g. the input is
+                * already sorted), this loop may run many iterations.
+                * Continue callbacks to the client even though no
+                * element comparison is needed, so the client's cmp()
+                * routine can invoke cond_resched() periodically.
+                */
+               if (unlikely(!++count))
+                       cmp(priv, b, b);
                b->prev = tail;
                tail = b;
                b = b->next;