diff --git a/include/ck_ring.h b/include/ck_ring.h
index a9dc373..2c0a990 100644
--- a/include/ck_ring.h
+++ b/include/ck_ring.h
@@ -159,6 +159,7 @@ ck_ring_capacity(struct ck_ring *ring)
 	return ring->size;
 }
 
+/* XXX: MPMC variant is incorrect, replacement in works. */
 CK_CC_INLINE static bool
 ck_ring_enqueue_mpmc(struct ck_ring *ring, void *entry)
 {