diff --git a/src/common.h b/src/common.h
index b9990a674f..535e231b6c 100644
--- a/src/common.h
+++ b/src/common.h
@@ -325,6 +325,14 @@ markBit(uintptr_t* map, unsigned i)
   map[wordOf(i)] |= static_cast<uintptr_t>(1) << bitOf(i);
 }
 
+inline void
+markBitAtomic(uintptr_t* map, unsigned i)
+{
+  uintptr_t* p = map + wordOf(i);
+  uintptr_t v = static_cast<uintptr_t>(1) << bitOf(i);
+  while (not __sync_bool_compare_and_swap(p, *p, *p | v)) { }
+}
+
 inline void
 clearBit(uintptr_t* map, unsigned i)
 {
diff --git a/src/heap.cpp b/src/heap.cpp
index 5067909498..11c3c4ceda 100644
--- a/src/heap.cpp
+++ b/src/heap.cpp
@@ -303,6 +303,13 @@ class Segment {
       if (child) child->set(p, v);
     }
 
+    void markAtomic(void* p) {
+      assert(segment->context, bitsPerRecord == 1);
+      markBitAtomic(data, indexOf(p));
+      assert(segment->context, getBit(data, indexOf(p)));
+      if (child) child->markAtomic(p);
+    }
+
     unsigned get(void* p) {
       return getBits(data, bitsPerRecord, indexOf(p));
     }
@@ -1013,8 +1020,12 @@ void
 markDirty(Context* c, Fixie* f)
 {
   if (not f->dirty) {
-    f->dirty = true;
-    f->move(c, &(c->dirtyTenuredFixies));
+    ACQUIRE(c->lock);
+
+    if (not f->dirty) {
+      f->dirty = true;
+      f->move(c, &(c->dirtyTenuredFixies));
+    }
   }
 }
 
@@ -1784,7 +1795,7 @@ class MyHeap: public Heap {
             Fixie(&c, sizeInWords, objectMask, 0, true))->body();
   }
 
-  virtual bool needsMark(void* p) {
+  bool needsMark(void* p) {
     assert(&c, c.client->isFixed(p) or (not immortalHeapContains(&c, p)));
 
     if (c.client->isFixed(p)) {
@@ -1794,11 +1805,6 @@ class MyHeap: public Heap {
     }
   }
 
-  virtual bool needsMark(void* p, unsigned offset) {
-    return needsMark(p) and targetNeedsMark
-      (mask(*(static_cast<void**>(p) + offset)));
-  }
-
   bool targetNeedsMark(void* target) {
     return target
       and not c.gen2.contains(target)
@@ -1809,38 +1815,41 @@ class MyHeap: public Heap {
   }
 
   virtual void mark(void* p, unsigned offset, unsigned count) {
-    if (c.client->isFixed(p)) {
-      Fixie* f = fixie(p);
-      assert(&c, offset == 0 or f->hasMask);
+    if (needsMark(p)) {
+      if (c.client->isFixed(p)) {
+        Fixie* f = fixie(p);
+        assert(&c, offset == 0 or f->hasMask);
 
-      bool dirty = false;
-      for (unsigned i = 0; i < count; ++i) {
-        void** target = static_cast<void**>(p) + offset + i;
-        if (targetNeedsMark(mask(*target))) {
-          if (DebugFixies) {
-            fprintf(stderr, "dirty fixie %p at %d (%p): %p\n",
-                    f, offset, f->body() + offset, mask(*target));
+        bool dirty = false;
+        for (unsigned i = 0; i < count; ++i) {
+          void** target = static_cast<void**>(p) + offset + i;
+          if (targetNeedsMark(mask(*target))) {
+            if (DebugFixies) {
+              fprintf(stderr, "dirty fixie %p at %d (%p): %p\n",
+                      f, offset, f->body() + offset, mask(*target));
+            }
+
+            dirty = true;
+            markBitAtomic(f->mask(), offset + i);
+            assert(&c, getBit(f->mask(), offset + i));
           }
-
-          dirty = true;
-          markBit(f->mask(), offset + i);
         }
-      }
 
-      if (dirty) markDirty(&c, f);
-    } else {
-      Segment::Map* map;
-      if (c.gen2.contains(p)) {
-        map = &(c.heapMap);
+        if (dirty) markDirty(&c, f);
       } else {
-        assert(&c, c.nextGen2.contains(p));
-        map = &(c.nextHeapMap);
-      }
+        Segment::Map* map;
+        if (c.gen2.contains(p)) {
+          map = &(c.heapMap);
+        } else {
+          assert(&c, c.nextGen2.contains(p));
+          map = &(c.nextHeapMap);
+        }
 
-      for (unsigned i = 0; i < count; ++i) {
-        void** target = static_cast<void**>(p) + offset + i;
-        if (targetNeedsMark(mask(*target))) {
-          map->set(target);
+        for (unsigned i = 0; i < count; ++i) {
+          void** target = static_cast<void**>(p) + offset + i;
+          if (targetNeedsMark(mask(*target))) {
+            map->markAtomic(target);
+          }
         }
       }
     }
diff --git a/src/heap.h b/src/heap.h
index 28f1533d8f..769bc1ec4b 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -59,8 +59,6 @@ class Heap: public Allocator {
   virtual void* allocateImmortalFixed(Allocator* allocator,
                                       unsigned sizeInWords, bool objectMask,
                                       unsigned* totalInBytes) = 0;
-  virtual bool needsMark(void* p) = 0;
-  virtual bool needsMark(void* p, unsigned offset) = 0;
   virtual void mark(void* p, unsigned offset, unsigned count) = 0;
   virtual void pad(void* p) = 0;
   virtual void* follow(void* p) = 0;
diff --git a/src/machine.h b/src/machine.h
index 1bc04a9e9e..598a72e424 100644
--- a/src/machine.h
+++ b/src/machine.h
@@ -1584,19 +1584,13 @@ allocate(Thread* t, unsigned sizeInBytes, bool objectMask)
 inline void
 mark(Thread* t, object o, unsigned offset, unsigned count)
 {
-  if (t->m->heap->needsMark(o)) {
-    ACQUIRE_RAW(t, t->m->heapLock);
-    t->m->heap->mark(o, offset / BytesPerWord, count);
-  }
+  t->m->heap->mark(o, offset / BytesPerWord, count);
 }
 
 inline void
 mark(Thread* t, object o, unsigned offset)
 {
-  if (t->m->heap->needsMark(o, offset / BytesPerWord)) {
-    ACQUIRE_RAW(t, t->m->heapLock);
-    t->m->heap->mark(o, offset / BytesPerWord, 1);
-  }
+  t->m->heap->mark(o, offset / BytesPerWord, 1);
 }
 
 inline void