--- a/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp	Sat Dec 17 20:50:09 2011 +0100
+++ b/src/share/vm/gc_implementation/parNew/parCardTableModRefBS.cpp	Sat Dec 17 21:40:27 2011 +0100
@@ -33,6 +33,7 @@
 #include "runtime/java.hpp"
 #include "runtime/mutexLocker.hpp"
 #include "runtime/virtualspace.hpp"
+#include "runtime/vmThread.hpp"
 
 void CardTableModRefBS::non_clean_card_iterate_parallel_work(Space* sp, MemRegion mr,
                                                              OopsInGenClosure* cl,
@@ -42,6 +43,11 @@
   assert((n_threads == 1 && ParallelGCThreads == 0) ||
          n_threads <= (int)ParallelGCThreads,
          "# worker threads != # requested!");
+  assert(!Thread::current()->is_VM_thread() || (n_threads == 1), "There is only 1 VM thread");
+  assert(UseDynamicNumberOfGCThreads ||
+         !FLAG_IS_DEFAULT(ParallelGCThreads) ||
+         n_threads == (int)ParallelGCThreads,
+         "# worker threads != # requested!");
   // Make sure the LNC array is valid for the space.
   jbyte**   lowest_non_clean;
   uintptr_t lowest_non_clean_base_chunk_index;
@@ -52,6 +58,8 @@
 
   int n_strides = n_threads * ParGCStridesPerThread;
   SequentialSubTasksDone* pst = sp->par_seq_tasks();
+  // Sets the condition for completion of the subtask (how many threads
+  // need to finish in order to be done).
   pst->set_n_threads(n_threads);
   pst->set_n_tasks(n_strides);
 
@@ -348,15 +356,31 @@
         // cleared before we had a chance to examine it. In that case, the value
         // will have been logged in the LNC for that chunk.
         // We need to examine as many chunks to the right as this object
-        // covers.
-        const uintptr_t last_chunk_index_to_check = addr_to_chunk_index(last_block + last_block_size - 1)
-                                                    - lowest_non_clean_base_chunk_index;
-        DEBUG_ONLY(const uintptr_t last_chunk_index = addr_to_chunk_index(used.last())
-                                                      - lowest_non_clean_base_chunk_index;)
-        assert(last_chunk_index_to_check <= last_chunk_index,
-               err_msg("Out of bounds: last_chunk_index_to_check " INTPTR_FORMAT
-                       " exceeds last_chunk_index " INTPTR_FORMAT,
-                       last_chunk_index_to_check, last_chunk_index));
+        // covers. However, we need to bound this checking to the largest
+        // entry in the LNC array: this is because the heap may expand
+        // after the LNC array has been created but before we reach this point,
+        // and the last block in our chunk may have been expanded to include
+        // the expansion delta (and possibly subsequently allocated from, so
+        // it wouldn't be sufficient to check whether that last block was
+        // or was not an object at this point).
+        uintptr_t last_chunk_index_to_check = addr_to_chunk_index(last_block + last_block_size - 1)
+                                              - lowest_non_clean_base_chunk_index;
+        const uintptr_t last_chunk_index    = addr_to_chunk_index(used.last())
+                                              - lowest_non_clean_base_chunk_index;
+        if (last_chunk_index_to_check > last_chunk_index) {
+          assert(last_block + last_block_size > used.end(),
+                 err_msg("Inconsistency detected: last_block [" PTR_FORMAT "," PTR_FORMAT "]"
+                         " does not exceed used.end() = " PTR_FORMAT ","
+                         " yet last_chunk_index_to_check " INTPTR_FORMAT
+                         " exceeds last_chunk_index " INTPTR_FORMAT,
+                         last_chunk_index_to_check, last_chunk_index));
+          assert(sp->used_region().end() > used.end(),
+                 err_msg("Expansion did not happen: "
+                         "[" PTR_FORMAT "," PTR_FORMAT ") -> [" PTR_FORMAT "," PTR_FORMAT ")",
+                         sp->used_region().start(), sp->used_region().end(), used.start(), used.end()));
+          NOISY(tty->print_cr(" process_chunk_boundary: heap expanded; explicitly bounding last_chunk");)
+          last_chunk_index_to_check = last_chunk_index;
+        }
         for (uintptr_t lnc_index = cur_chunk_index + 1;
              lnc_index <= last_chunk_index_to_check;
              lnc_index++) {