--- old/src/share/vm/gc_implementation/g1/heapRegion.inline.hpp	2014-06-25 13:43:35.811525370 +0200
+++ new/src/share/vm/gc_implementation/g1/heapRegion.inline.hpp	2014-06-25 13:43:35.707521129 +0200
@@ -31,38 +31,43 @@
 #include "memory/space.hpp"
 #include "runtime/atomic.inline.hpp"
 
-inline HeapWord* G1OffsetTableContigSpace::cas_allocate_inner(size_t size) {
+// This version requires locking.
+inline HeapWord* G1OffsetTableContigSpace::allocate_impl(size_t size,
+                                                HeapWord* const end_value) {
   HeapWord* obj = top();
+  if (pointer_delta(end_value, obj) >= size) {
+    HeapWord* new_top = obj + size;
+    set_top(new_top);
+    assert(is_aligned(obj) && is_aligned(new_top), "checking alignment");
+    return obj;
+  } else {
+    return NULL;
+  }
+}
+
+// This version is lock-free.
+inline HeapWord* G1OffsetTableContigSpace::par_allocate_impl(size_t size,
+                                                    HeapWord* const end_value) {
   do {
-    if (pointer_delta(end(), obj) >= size) {
+    HeapWord* obj = top();
+    if (pointer_delta(end_value, obj) >= size) {
       HeapWord* new_top = obj + size;
-      HeapWord* result = (HeapWord*)Atomic::cmpxchg_ptr(new_top, &_top, obj);
+      HeapWord* result = (HeapWord*)Atomic::cmpxchg_ptr(new_top, top_addr(), obj);
+      // result can be one of two:
+      //  the old top value: the exchange succeeded
+      //  otherwise: the new value of the top is returned.
       if (result == obj) {
         assert(is_aligned(obj) && is_aligned(new_top), "checking alignment");
         return obj;
       }
-      obj = result;
     } else {
-      break;
+      return NULL;
     }
   } while (true);
-  return NULL;
 }
 
-inline HeapWord* G1OffsetTableContigSpace::allocate_inner(size_t size) {
-  HeapWord* obj = top();
-  if (pointer_delta(end(), obj) >= size) {
-    HeapWord* new_top = obj + size;
-    assert(is_aligned(obj) && is_aligned(new_top), "checking alignment");
-    set_top(new_top);
-    return obj;
-  }
-  return NULL;
-}
-
-
 inline HeapWord* G1OffsetTableContigSpace::allocate(size_t size) {
-  HeapWord* res = allocate_inner(size);
+  HeapWord* res = allocate_impl(size, end());
   if (res != NULL) {
     _offsets.alloc_block(res, size);
   }
@@ -104,12 +109,12 @@
 
 inline HeapWord* HeapRegion::par_allocate_no_bot_updates(size_t word_size) {
   assert(is_young(), "we can only skip BOT updates on young regions");
-  return cas_allocate_inner(word_size);
+  return par_allocate_impl(word_size, end());
 }
 
 inline HeapWord* HeapRegion::allocate_no_bot_updates(size_t word_size) {
   assert(is_young(), "we can only skip BOT updates on young regions");
-  return allocate_inner(word_size);
+  return allocate_impl(word_size, end());
 }
 
 inline void HeapRegion::note_start_of_marking() {