More fences and commentary

scala · SethTisue · Aug 18, 2018 · May 4, 2018 · May 4, 2018 · Aug 16, 2018
commit b6d74ff9a52f15eef0a7455bf1fcfba7be824178
@@ -565,6 +565,8 @@ sealed abstract class List[+A]
 
 }
 
+// Internal code that mutates `next` _must_ call `ScalaRunTime.releaseFence()` if either immediately, or
+// before a newly-allocated, thread-local :: instance is aliased (e.g. in ListBuffer.toList)
 final case class :: [+A](override val head: A, private[scala] var next: List[A @uncheckedVariance]) // sound because `next` is used only locally
   extends List[A] {
   ScalaRunTime.releaseFence()

@@ -71,7 +71,12 @@ final class Vector[+A] private[immutable] (private[collection] val startIndex: I
 
   override def iterableFactory: SeqFactory[Vector] = Vector
 
+  // Code paths that mutates `dirty` _must_ call `ScalaRunTime.releaseFence()` before returning from
+  // the public method.
   private[immutable] var dirty = false
+  // While most JDKs would implicit add this fence because of >= 1 final field, the spec only mandates
+  // it if all fields are final, so let's add this in explicitly.
+  ScalaRunTime.releaseFence()
 
   def length: Int = endIndex - startIndex
 

@@ -62,6 +62,9 @@ class ListBuffer[A]
   // Avoids copying where possible.
   override def toList: List[A] = {
     aliased = nonEmpty
+    // We've accumulated a number of mutations to `List.tail` by this stage.
+    // Make sure they are visible to threads that the client of this ListBuffer might be about
+    // to share this List with.
     ScalaRunTime.releaseFence()
     first
   }