Make WorkerBoundReference initially unfrozen (#4288)

backend.native/tests/runtime/concurrent/worker_bound_reference0.kt

@@ -14,6 +14,7 @@ import kotlin.text.Regex
 
 class A(var a: Int)
 
+@SharedImmutable
 val global1: WorkerBoundReference<A> = WorkerBoundReference(A(3))
 
 @Test
@@ -32,6 +33,7 @@ fun testGlobal() {
     worker.requestTermination().result
 }
 
+@SharedImmutable
 val global2: WorkerBoundReference<A> = WorkerBoundReference(A(3))
 
 @Test
@@ -52,6 +54,7 @@ fun testGlobalDenyAccessOnWorker() {
     worker.requestTermination().result
 }
 
+@SharedImmutable
 val global3: WorkerBoundReference<A> = WorkerBoundReference(A(3).freeze())
 
 @Test
@@ -69,6 +72,7 @@ fun testGlobalAccessOnWorkerFrozenInitially() {
     worker.requestTermination().result
 }
 
+@SharedImmutable
 val global4: WorkerBoundReference<A> = WorkerBoundReference(A(3))
 
 @Test
@@ -86,6 +90,7 @@ fun testGlobalAccessOnWorkerFrozenBeforePassing() {
     worker.requestTermination().result
 }
 
+@SharedImmutable
 val global5: WorkerBoundReference<A> = WorkerBoundReference(A(3))
 
 @Test
@@ -113,6 +118,7 @@ fun testGlobalAccessOnWorkerFrozenBeforeAccess() {
     worker.requestTermination().result
 }
 
+@SharedImmutable
 val global6: WorkerBoundReference<A> = WorkerBoundReference(A(3))
 
 @Test
@@ -140,6 +146,7 @@ fun testGlobalModification() {
     worker.requestTermination().result
 }
 
+@SharedImmutable
 val global7: WorkerBoundReference<A> = WorkerBoundReference(A(3))
 
 @Test
@@ -162,6 +169,13 @@ fun testLocal() {
     val local = WorkerBoundReference(A(3))
     assertEquals(3, local.value.a)
     assertEquals(3, local.valueOrNull?.a)
+}
+
+@Test
+fun testLocalFrozen() {
+    val local = WorkerBoundReference(A(3)).freeze()
+    assertEquals(3, local.value.a)
+    assertEquals(3, local.valueOrNull?.a)
 
     val worker = Worker.start()
     val future = worker.execute(TransferMode.SAFE, { local }) { local ->
@@ -175,8 +189,8 @@ fun testLocal() {
 }
 
 @Test
-fun testLocalDenyAccessOnWorker() {
-    val local = WorkerBoundReference(A(3))
+fun testLocalDenyAccessOnWorkerFrozen() {
+    val local = WorkerBoundReference(A(3)).freeze()
     assertEquals(3, local.value.a)
 
     val worker = Worker.start()
@@ -193,8 +207,8 @@ fun testLocalDenyAccessOnWorker() {
 }
 
 @Test
-fun testLocalAccessOnWorkerFrozenInitially() {
-    val local = WorkerBoundReference(A(3).freeze())
+fun testLocalAccessOnWorkerFrozenInitiallyFrozen() {
+    val local = WorkerBoundReference(A(3).freeze()).freeze()
     assertEquals(3, local.value.a)
     assertEquals(3, local.valueOrNull?.a)
 
@@ -209,8 +223,8 @@ fun testLocalAccessOnWorkerFrozenInitially() {
 }
 
 @Test
-fun testLocalAccessOnWorkerFrozenBeforePassing() {
-    val local = WorkerBoundReference(A(3))
+fun testLocalAccessOnWorkerFrozenBeforePassingFrozen() {
+    val local = WorkerBoundReference(A(3)).freeze()
     assertEquals(3, local.value.a)
     local.value.freeze()
 
@@ -225,10 +239,10 @@ fun testLocalAccessOnWorkerFrozenBeforePassing() {
 }
 
 @Test
-fun testLocalAccessOnWorkerFrozenBeforeAccess() {
+fun testLocalAccessOnWorkerFrozenBeforeAccessFrozen() {
     val semaphore: AtomicInt = AtomicInt(0)
 
-    val local = WorkerBoundReference(A(3))
+    val local = WorkerBoundReference(A(3)).freeze()
     assertEquals(3, local.value.a)
 
     val worker = Worker.start()
@@ -250,14 +264,25 @@ fun testLocalAccessOnWorkerFrozenBeforeAccess() {
     worker.requestTermination().result
 }
 
-
 @Test
-fun testLocalDenyAccessOnMainThread() {
+fun testLocalAccessOnMainThread() {
     val worker = Worker.start()
     val future = worker.execute(TransferMode.SAFE, {}) {
         WorkerBoundReference(A(3))
     }
 
+    assertEquals(3, future.result.value.a)
+
+    worker.requestTermination().result
+}
+
+@Test
+fun testLocalDenyAccessOnMainThreadFrozen() {
+    val worker = Worker.start()
+    val future = worker.execute(TransferMode.SAFE, {}) {
+        WorkerBoundReference(A(3)).freeze()
+    }
+
     val value = future.result
     assertFailsWith<IncorrectDereferenceException> {
         value.value
@@ -268,10 +293,10 @@ fun testLocalDenyAccessOnMainThread() {
 }
 
 @Test
-fun testLocalModification() {
+fun testLocalModificationFrozen() {
     val semaphore: AtomicInt = AtomicInt(0)
 
-    val local = WorkerBoundReference(A(3))
+    val local = WorkerBoundReference(A(3)).freeze()
     assertEquals(3, local.value.a)
 
     val worker = Worker.start()
@@ -294,8 +319,8 @@ fun testLocalModification() {
 }
 
 @Test
-fun testLocalGetWorker() {
-    val local = WorkerBoundReference(A(3))
+fun testLocalGetWorkerFrozen() {
+    val local = WorkerBoundReference(A(3)).freeze()
 
     val ownerId = Worker.current.id
     assertEquals(ownerId, local.worker.id)
@@ -326,9 +351,60 @@ fun testLocalForeignGetWorker() {
     worker.requestTermination().result
 }
 
-fun getOwnerAndWeaks(initial: Int): Triple<AtomicReference<WorkerBoundReference<A>?>, WeakReference<WorkerBoundReference<A>>, WeakReference<A>> {
+@Test
+fun testLocalForeignGetWorkerFrozen() {
+    val worker = Worker.start()
+    val ownerId = worker.id
+    val future = worker.execute(TransferMode.SAFE, { ownerId }) { ownerId ->
+        val local = WorkerBoundReference(A(3)).freeze()
+        assertEquals(ownerId, local.worker.id)
+        local
+    }
+
+    val value = future.result
+    assertEquals(ownerId, value.worker.id)
+
+    worker.requestTermination().result
+}
+
+class Wrapper(val ref: WorkerBoundReference<A>)
+
+@Test
+fun testLocalWithWrapperFrozen() {
+    val local = Wrapper(WorkerBoundReference(A(3))).freeze()
+    assertEquals(3, local.ref.value.a)
+
+    val worker = Worker.start()
+    val future = worker.execute(TransferMode.SAFE, { local }) { local ->
+        local
+    }
+
+    val value = future.result
+    assertEquals(3, value.ref.value.a)
+    worker.requestTermination().result
+}
+
+@Test
+fun testLocalDenyAccessWithWrapperFrozen() {
+    val local = Wrapper(WorkerBoundReference(A(3))).freeze()
+    assertEquals(3, local.ref.value.a)
+
+    val worker = Worker.start()
+    val future = worker.execute(TransferMode.SAFE, { local }) { local ->
+        assertFailsWith<IncorrectDereferenceException> {
+            local.ref.value
+        }
+        assertEquals(null, local.ref.valueOrNull)
+        Unit
+    }
+
+    future.result
+    worker.requestTermination().result
+}
+
+fun getOwnerAndWeaks(initial: Int): Triple<FreezableAtomicReference<WorkerBoundReference<A>?>, WeakReference<WorkerBoundReference<A>>, WeakReference<A>> {
     val ref = WorkerBoundReference(A(initial))
-    val refOwner: AtomicReference<WorkerBoundReference<A>?> = AtomicReference(ref)
+    val refOwner: FreezableAtomicReference<WorkerBoundReference<A>?> = FreezableAtomicReference(ref)
     val refWeak = WeakReference(ref)
     val refValueWeak = WeakReference(ref.value)
 
@@ -347,8 +423,29 @@ fun testCollect() {
     assertNull(refValueWeak.value)
 }
 
-fun collectInWorker(worker: Worker, semaphore: AtomicInt): Pair<WeakReference<A>, Future<Unit>> {
-    val (refOwner, _, refValueWeak) = getOwnerAndWeaks(3)
+fun getOwnerAndWeaksFrozen(initial: Int): Triple<AtomicReference<WorkerBoundReference<A>?>, WeakReference<WorkerBoundReference<A>>, WeakReference<A>> {
+    val ref = WorkerBoundReference(A(initial)).freeze()
+    val refOwner: AtomicReference<WorkerBoundReference<A>?> = AtomicReference(ref)
+    val refWeak = WeakReference(ref)
+    val refValueWeak = WeakReference(ref.value)
+
+    return Triple(refOwner, refWeak, refValueWeak)
+}
+
+@Test
+fun testCollectFrozen() {
+    val (refOwner, refWeak, refValueWeak) = getOwnerAndWeaksFrozen(3)
+
+    refOwner.value = null
+    GC.collect()
+
+    // Last reference to WorkerBoundReference is gone, so it and it's referent are destroyed.
+    assertNull(refWeak.value)
+    assertNull(refValueWeak.value)
+}
+
+fun collectInWorkerFrozen(worker: Worker, semaphore: AtomicInt): Pair<WeakReference<A>, Future<Unit>> {
+    val (refOwner, _, refValueWeak) = getOwnerAndWeaksFrozen(3)
 
     val future = worker.execute(TransferMode.SAFE, { Pair(refOwner, semaphore) }) { (refOwner, semaphore) ->
         semaphore.increment()
@@ -370,25 +467,24 @@ fun collectInWorker(worker: Worker, semaphore: AtomicInt): Pair<WeakReference<A>
 }
 
 @Test
-fun testCollectInWorker() {
+fun testCollectInWorkerFrozen() {
     val semaphore: AtomicInt = AtomicInt(0)
 
     val worker = Worker.start()
 
-    val (refValueWeak, future) = collectInWorker(worker, semaphore)
+    val (refValueWeak, future) = collectInWorkerFrozen(worker, semaphore)
     semaphore.increment()
     future.result
 
     // At this point WorkerBoundReference no longer has a reference, so it's referent is destroyed.
-    // WorkerBoundReference, so referent is kept alive.
     GC.collect()
     assertNull(refValueWeak.value)
 
     worker.requestTermination().result
 }
 
-fun doNotCollectInWorker(worker: Worker, semaphore: AtomicInt): Future<WorkerBoundReference<A>> {
-    val ref = WorkerBoundReference(A(3))
+fun doNotCollectInWorkerFrozen(worker: Worker, semaphore: AtomicInt): Future<WorkerBoundReference<A>> {
+    val ref = WorkerBoundReference(A(3)).freeze()
 
     return worker.execute(TransferMode.SAFE, { Pair(ref, semaphore) }) { (ref, semaphore) ->
         semaphore.increment()
@@ -401,12 +497,12 @@ fun doNotCollectInWorker(worker: Worker, semaphore: AtomicInt): Future<WorkerBou
 }
 
 @Test
-fun testDoNotCollectInWorker() {
+fun testDoNotCollectInWorkerFrozen() {
     val semaphore: AtomicInt = AtomicInt(0)
 
     val worker = Worker.start()
 
-    val future = doNotCollectInWorker(worker, semaphore)
+    val future = doNotCollectInWorkerFrozen(worker, semaphore)
     while (semaphore.value < 1) {
     }
     GC.collect()
@@ -423,9 +519,9 @@ class B1 {
 
 data class B2(val b1: WorkerBoundReference<B1>)
 
-fun createCyclicGarbage(): Triple<AtomicReference<WorkerBoundReference<B1>?>, WeakReference<B1>, WeakReference<B2>> {
+fun createCyclicGarbage(): Triple<FreezableAtomicReference<WorkerBoundReference<B1>?>, WeakReference<B1>, WeakReference<B2>> {
     val ref1 = WorkerBoundReference(B1())
-    val ref1Owner: AtomicReference<WorkerBoundReference<B1>?> = AtomicReference(ref1)
+    val ref1Owner: FreezableAtomicReference<WorkerBoundReference<B1>?> = FreezableAtomicReference(ref1)
     val ref1Weak = WeakReference(ref1.value)
 
     val ref2 = WorkerBoundReference(B2(ref1))
@@ -437,26 +533,50 @@ fun createCyclicGarbage(): Triple<AtomicReference<WorkerBoundReference<B1>?>, We
 }
 
 @Test
-fun doesNotCollectCyclicGarbage() {
+fun collectCyclicGarbage() {
     val (ref1Owner, ref1Weak, ref2Weak) = createCyclicGarbage()
 
     ref1Owner.value = null
     GC.collect()
 
+    assertNull(ref1Weak.value)
+    assertNull(ref2Weak.value)
+}
+
+fun createCyclicGarbageFrozen(): Triple<AtomicReference<WorkerBoundReference<B1>?>, WeakReference<B1>, WeakReference<B2>> {
+    val ref1 = WorkerBoundReference(B1()).freeze()
+    val ref1Owner: AtomicReference<WorkerBoundReference<B1>?> = AtomicReference(ref1)
+    val ref1Weak = WeakReference(ref1.value)
+
+    val ref2 = WorkerBoundReference(B2(ref1)).freeze()
+    val ref2Weak = WeakReference(ref2.value)
+
+    ref1.value.b2 = ref2
+
+    return Triple(ref1Owner, ref1Weak, ref2Weak)
+}
+
+@Test
+fun doesNotCollectCyclicGarbageFrozen() {
+    val (ref1Owner, ref1Weak, ref2Weak) = createCyclicGarbageFrozen()
+
+    ref1Owner.value = null
+    GC.collect()
+
     // If these asserts fail, that means WorkerBoundReference managed to clean up cyclic garbage all by itself.
     assertNotNull(ref1Weak.value)
     assertNotNull(ref2Weak.value)
 }
 
-fun createCrossThreadCyclicGarbage(
+fun createCrossThreadCyclicGarbageFrozen(
         worker: Worker
 ): Triple<AtomicReference<WorkerBoundReference<B1>?>, WeakReference<B1>, WeakReference<B2>> {
-    val ref1 = WorkerBoundReference(B1())
+    val ref1 = WorkerBoundReference(B1()).freeze()
     val ref1Owner: AtomicReference<WorkerBoundReference<B1>?> = AtomicReference(ref1)
     val ref1Weak = WeakReference(ref1.value)
 
     val future = worker.execute(TransferMode.SAFE, { ref1 }) { ref1 ->
-        val ref2 = WorkerBoundReference(B2(ref1))
+        val ref2 = WorkerBoundReference(B2(ref1)).freeze()
         Pair(ref2, WeakReference(ref2.value))
     }
     val (ref2, ref2Weak) = future.result
@@ -467,10 +587,10 @@ fun createCrossThreadCyclicGarbage(
 }
 
 @Test
-fun doesNotCollectCrossThreadCyclicGarbage() {
+fun doesNotCollectCrossThreadCyclicGarbageFrozen() {
     val worker = Worker.start()
 
-    val (ref1Owner, ref1Weak, ref2Weak) = createCrossThreadCyclicGarbage(worker)
+    val (ref1Owner, ref1Weak, ref2Weak) = createCrossThreadCyclicGarbageFrozen(worker)
 
     ref1Owner.value = null
     GC.collect()
@@ -493,11 +613,11 @@ class C1 {
 
 data class C2(val c1: AtomicReference<WorkerBoundReference<C1>>)
 
-fun createCyclicGarbageWithAtomics(): Triple<AtomicReference<WorkerBoundReference<C1>?>, WeakReference<C1>, WeakReference<C2>> {
-    val ref1 = WorkerBoundReference(C1())
+fun createCyclicGarbageWithAtomicsFrozen(): Triple<AtomicReference<WorkerBoundReference<C1>?>, WeakReference<C1>, WeakReference<C2>> {
+    val ref1 = WorkerBoundReference(C1()).freeze()
     val ref1Weak = WeakReference(ref1.value)
 
-    val ref2 = WorkerBoundReference(C2(AtomicReference(ref1)))
+    val ref2 = WorkerBoundReference(C2(AtomicReference(ref1))).freeze()
     val ref2Weak = WeakReference(ref2.value)
 
     ref1.value.c2 = AtomicReference(ref2)
@@ -511,8 +631,8 @@ fun dispose(refOwner: AtomicReference<WorkerBoundReference<C1>?>) {
 }
 
 @Test
-fun doesNotCollectCyclicGarbageWithAtomics() {
-    val (ref1Owner, ref1Weak, ref2Weak) = createCyclicGarbageWithAtomics()
+fun doesNotCollectCyclicGarbageWithAtomicsFrozen() {
+    val (ref1Owner, ref1Weak, ref2Weak) = createCyclicGarbageWithAtomicsFrozen()
 
     ref1Owner.value = null
     GC.collect()
@@ -523,8 +643,8 @@ fun doesNotCollectCyclicGarbageWithAtomics() {
 }
 
 @Test
-fun collectCyclicGarbageWithAtomics() {
-    val (ref1Owner, ref1Weak, ref2Weak) = createCyclicGarbageWithAtomics()
+fun collectCyclicGarbageWithAtomicsFrozen() {
+    val (ref1Owner, ref1Weak, ref2Weak) = createCyclicGarbageWithAtomicsFrozen()
 
     dispose(ref1Owner)
     GC.collect()
@@ -533,14 +653,14 @@ fun collectCyclicGarbageWithAtomics() {
     assertNull(ref2Weak.value)
 }
 
-fun createCrossThreadCyclicGarbageWithAtomics(
+fun createCrossThreadCyclicGarbageWithAtomicsFrozen(
         worker: Worker
 ): Triple<AtomicReference<WorkerBoundReference<C1>?>, WeakReference<C1>, WeakReference<C2>> {
-    val ref1 = WorkerBoundReference(C1())
+    val ref1 = WorkerBoundReference(C1()).freeze()
     val ref1Weak = WeakReference(ref1.value)
 
     val future = worker.execute(TransferMode.SAFE, { ref1 }) { ref1 ->
-        val ref2 = WorkerBoundReference(C2(AtomicReference(ref1)))
+        val ref2 = WorkerBoundReference(C2(AtomicReference(ref1))).freeze()
         Pair(ref2, WeakReference(ref2.value))
     }
     val (ref2, ref2Weak) = future.result
@@ -551,10 +671,10 @@ fun createCrossThreadCyclicGarbageWithAtomics(
 }
 
 @Test
-fun doesNotCollectCrossThreadCyclicGarbageWithAtomics() {
+fun doesNotCollectCrossThreadCyclicGarbageWithAtomicsFrozen() {
     val worker = Worker.start()
 
-    val (ref1Owner, ref1Weak, ref2Weak) = createCrossThreadCyclicGarbageWithAtomics(worker)
+    val (ref1Owner, ref1Weak, ref2Weak) = createCrossThreadCyclicGarbageWithAtomicsFrozen(worker)
 
     ref1Owner.value = null
     GC.collect()
@@ -568,10 +688,10 @@ fun doesNotCollectCrossThreadCyclicGarbageWithAtomics() {
 }
 
 @Test
-fun collectCrossThreadCyclicGarbageWithAtomics() {
+fun collectCrossThreadCyclicGarbageWithAtomicsFrozen() {
     val worker = Worker.start()
 
-    val (ref1Owner, ref1Weak, ref2Weak) = createCrossThreadCyclicGarbageWithAtomics(worker)
+    val (ref1Owner, ref1Weak, ref2Weak) = createCrossThreadCyclicGarbageWithAtomicsFrozen(worker)
 
     dispose(ref1Owner)
     // This marks C2 as gone on the main thread
@@ -588,11 +708,11 @@ fun collectCrossThreadCyclicGarbageWithAtomics() {
 }
 
 @Test
-fun concurrentAccess() {
+fun concurrentAccessFrozen() {
     val workerCount = 10
     val workerUnlocker = AtomicInt(0)
 
-    val ref = WorkerBoundReference(A(3))
+    val ref = WorkerBoundReference(A(3)).freeze()
     assertEquals(3, ref.value.a)
 
     val workers = Array(workerCount) {
@@ -621,10 +741,10 @@ fun concurrentAccess() {
 }
 
 @Test
-fun testExceptionMessage() {
+fun testExceptionMessageFrozen() {
     val worker = Worker.start()
     val future = worker.execute(TransferMode.SAFE, {}) {
-        WorkerBoundReference(A(3))
+        WorkerBoundReference(A(3)).freeze()
     }
     val value = future.result
 
@@ -638,3 +758,23 @@ fun testExceptionMessage() {
 
     worker.requestTermination().result
 }
+
+@Test
+fun testDoubleFreeze() {
+    val ref = WorkerBoundReference(A(3))
+    val wrapper = Wrapper(ref)
+    ref.freeze()
+    ref.freeze()
+    wrapper.freeze()
+}
+
+@Test
+fun testDoubleFreezeWithFreezeBlocker() {
+    val ref = WorkerBoundReference(A(3))
+    val wrapper = Wrapper(ref)
+    wrapper.ensureNeverFrozen()
+    assertFailsWith<FreezingException> {
+        wrapper.freeze()
+    }
+    ref.freeze()
+}

runtime/src/main/cpp/Memory.cpp

@@ -748,17 +748,23 @@ inline void traverseReferredObjects(ObjHeader* obj, func process) {
 }
 
 template <typename func>
-inline void traverseContainerObjectFields(ContainerHeader* container, func process) {
+inline void traverseContainerObjects(ContainerHeader* container, func process) {
   RuntimeAssert(!isAggregatingFrozenContainer(container), "Must not be called on such containers");
   ObjHeader* obj = reinterpret_cast<ObjHeader*>(container + 1);
-
-  for (int object = 0; object < container->objectCount(); object++) {
-    traverseObjectFields(obj, process);
+  for (int i = 0; i < container->objectCount(); ++i) {
+    process(obj);
     obj = reinterpret_cast<ObjHeader*>(
       reinterpret_cast<uintptr_t>(obj) + objectSize(obj));
   }
 }
 
+template <typename func>
+inline void traverseContainerObjectFields(ContainerHeader* container, func process) {
+  traverseContainerObjects(container, [process](ObjHeader* obj) {
+    traverseObjectFields(obj, process);
+  });
+}
+
 template <typename func>
 inline void traverseContainerReferredObjects(ContainerHeader* container, func process) {
   traverseContainerObjectFields(container, [process](ObjHeader** location) {
@@ -2582,6 +2588,34 @@ void freezeCyclic(ObjHeader* root,
   }
 }
 
+// These hooks are only allowed to modify `obj` subgraph.
+void runFreezeHooks(ObjHeader* obj) {
+  if (obj->type_info() == theWorkerBoundReferenceTypeInfo) {
+    WorkerBoundReferenceFreezeHook(obj);
+  }
+}
+
+void runFreezeHooksRecursive(ObjHeader* root) {
+  KStdUnorderedSet<KRef> seen;
+  KStdVector<KRef> toVisit;
+  seen.insert(root);
+  toVisit.push_back(root);
+  while (!toVisit.empty()) {
+    KRef obj = toVisit.back();
+    toVisit.pop_back();
+
+    runFreezeHooks(obj);
+
+    traverseReferredObjects(obj, [&seen, &toVisit](ObjHeader* field) {
+      auto wasNotSeenYet = seen.insert(field).second;
+      // Only iterating on unseen objects which containers will get frozen by freezeCyclic or freezeAcyclic.
+      if (wasNotSeenYet && canFreeze(field->container())) {
+        toVisit.push_back(field);
+      }
+    });
+  }
+}
+
 /**
  * Theory of operations.
  *
@@ -2612,6 +2646,12 @@ void freezeSubgraph(ObjHeader* root) {
   ContainerHeader* rootContainer = root->container();
   if (isPermanentOrFrozen(rootContainer)) return;
 
+  MEMORY_LOG("Run freeze hooks on subgraph of %p\n", root);
+
+  // Note: Actual freezing can fail, but these hooks won't be undone, and moreover
+  // these hooks will run again on a repeated freezing attempt.
+  runFreezeHooksRecursive(root);
+
   MEMORY_LOG("Freeze subgraph of %p\n", root)
 
   #if USE_GC

runtime/src/main/cpp/WorkerBoundReference.cpp

@@ -24,9 +24,18 @@ WorkerBoundReference* asWorkerBoundReference(KRef thiz) {
 
 RUNTIME_NOTHROW void DisposeWorkerBoundReference(KRef thiz) {
   // DisposeSharedRef is only called when all references to thiz are gone.
-  auto* holder = asWorkerBoundReference(thiz)->holder;
-  holder->dispose();
-  konanDestructInstance(holder);
+  // Can be null if WorkerBoundReference wasn't frozen.
+  if (auto* holder = asWorkerBoundReference(thiz)->holder) {
+    holder->dispose();
+    konanDestructInstance(holder);
+  }
+}
+
+// Defined in WorkerBoundReference.kt
+extern "C" void Kotlin_WorkerBoundReference_freezeHook(KRef thiz);
+
+RUNTIME_NOTHROW void WorkerBoundReferenceFreezeHook(KRef thiz) {
+  Kotlin_WorkerBoundReference_freezeHook(thiz);
 }
 
 extern "C" {

runtime/src/main/cpp/WorkerBoundReference.h

@@ -13,4 +13,6 @@
 // all types.
 RUNTIME_NOTHROW void DisposeWorkerBoundReference(KRef thiz) NO_INLINE;
 
+RUNTIME_NOTHROW void WorkerBoundReferenceFreezeHook(KRef thiz);
+
 #endif  // RUNTIME_SHAREDREF_H

runtime/src/main/kotlin/kotlin/native/concurrent/WorkerBoundReference.kt

@@ -17,22 +17,22 @@ external private fun derefWorkerBoundReference(ref: NativePtr): Any?
 external private fun describeWorkerBoundReference(ref: NativePtr): String
 
 /**
- * A frozen shared reference to a Kotlin object.
+ * A shared reference to a Kotlin object that doesn't freeze the referred object when it gets frozen itself.
  *
- * Can be safely passed between workers, but [value] can only be accessed on the worker [WorkerBoundReference] was created on,
- * unless the referred object is frozen too.
+ * After freezing can be safely passed between workers, but [value] can only be accessed on
+ * the worker [WorkerBoundReference] was created on, unless the referred object is frozen too.
  *
- * Note: Garbage collector currently cannot free any reference cycles with [WorkerBoundReference] in them.
+ * Note: Garbage collector currently cannot free any reference cycles with frozen [WorkerBoundReference] in them.
  * To resolve such cycles consider using [AtomicReference<WorkerBoundReference?>] which can be explicitly
  * nulled out.
  */
-@Frozen
 @NoReorderFields
 @ExportTypeInfo("theWorkerBoundReferenceTypeInfo")
 public class WorkerBoundReference<out T : Any>(value: T) {
 
-    private val ptr = createWorkerBoundReference(value)
+    private var ptr = NativePtr.NULL
     private val ownerName = Worker.current.name
+    private var valueBeforeFreezing: T? = value
 
     private val valueDescription
         get() = describeWorkerBoundReference(ptr)
@@ -48,10 +48,19 @@ public class WorkerBoundReference<out T : Any>(value: T) {
      * The referenced value or null if referred object is not frozen and current worker is different from the one created [this].
      */
     val valueOrNull: T?
-        get() = @Suppress("UNCHECKED_CAST") (derefWorkerBoundReference(ptr) as T?)
+        get() = valueBeforeFreezing ?: @Suppress("UNCHECKED_CAST") (derefWorkerBoundReference(ptr) as T?)
 
     /**
      * Worker that [value] is bound to.
      */
     val worker: Worker = Worker.current
+
+    @ExportForCppRuntime("Kotlin_WorkerBoundReference_freezeHook")
+    private fun freezeHook() {
+        // If this hook was already run, do nothing.
+        if (valueBeforeFreezing == null)
+            return
+        ptr = createWorkerBoundReference(valueBeforeFreezing!!)
+        valueBeforeFreezing = null
+    }
 }