diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index 27f8e66d50..ccefbcd8d6 100644
--- a/src/runtime/mbitmap.go
+++ b/src/runtime/mbitmap.go
@@ -847,10 +847,20 @@ func (s *mspan) countFree() int {
 // malloc does not call heapBitsSetType when there are no pointers,
 // because all free objects are marked as noscan during
 // heapBitsSweepSpan.
+//
 // There can only be one allocation from a given span active at a time,
-// so this code is not racing with other instances of itself, and
-// the bitmap for a span always falls on byte boundaries.
-// Hence, it can access the bitmap with racing.
+// and the bitmap for a span always falls on byte boundaries,
+// so there are no write-write races for access to the heap bitmap.
+// Hence, heapBitsSetType can access the bitmap without atomics.
+//
+// There can be read-write races between heapBitsSetType and things
+// that read the heap bitmap like scanobject. However, since
+// heapBitsSetType is only used for objects that have not yet been
+// made reachable, readers will ignore bits being modified by this
+// function. This does mean this function cannot transiently modify
+// bits that belong to neighboring objects. Also, on weakly-ordered
+// machines, callers must execute a store/store (publication) barrier
+// between calling this function and making the object reachable.
 //
 // TODO: This still has atomic accesses left over from when it could
 // race with GC accessing mark bits in the bitmap. Remove these.