--- a/src/share/vm/gc_implementation/g1/g1Allocator.cpp	Tue Mar 24 10:04:10 2015 +0000
+++ b/src/share/vm/gc_implementation/g1/g1Allocator.cpp	Wed Mar 25 11:03:16 2015 +0100
@@ -59,7 +59,7 @@
       !(retained_region->top() == retained_region->end()) &&
       !retained_region->is_empty() &&
       !retained_region->isHumongous()) {
-    retained_region->record_top_and_timestamp();
+    retained_region->record_timestamp();
     // The retained region was added to the old region set when it was
     // retired. We have to remove it now, since we don't allow regions
     // we allocate to in the region sets. We'll re-add it later, when
@@ -94,6 +94,9 @@
   // want either way so no reason to check explicitly for either
   // condition.
   _retained_old_gc_alloc_region = old_gc_alloc_region(context)->release();
+  if (_retained_old_gc_alloc_region != NULL) {
+    _retained_old_gc_alloc_region->record_retained_region();
+  }
 
   if (ResizePLAB) {
     _g1h->_survivor_plab_stats.adjust_desired_plab_sz(no_of_gc_workers);

--- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Tue Mar 24 10:04:10 2015 +0000
+++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Wed Mar 25 11:03:16 2015 +0100
@@ -6808,7 +6808,7 @@
       // We really only need to do this for old regions given that we
       // should never scan survivors. But it doesn't hurt to do it
       // for survivors too.
-      new_alloc_region->record_top_and_timestamp();
+      new_alloc_region->record_timestamp();
       if (survivor) {
         new_alloc_region->set_survivor();
         _hr_printer.alloc(new_alloc_region, G1HRPrinter::Survivor);

--- a/src/share/vm/gc_implementation/g1/g1RemSet.cpp	Tue Mar 24 10:04:10 2015 +0000
+++ b/src/share/vm/gc_implementation/g1/g1RemSet.cpp	Wed Mar 25 11:03:16 2015 +0100
@@ -147,11 +147,9 @@
 
     // Set the "from" region in the closure.
     _oc->set_region(r);
-    HeapWord* card_start = _bot_shared->address_for_index(index);
-    HeapWord* card_end = card_start + G1BlockOffsetSharedArray::N_words;
-    Space *sp = SharedHeap::heap()->space_containing(card_start);
-    MemRegion sm_region = sp->used_region_at_save_marks();
-    MemRegion mr = sm_region.intersection(MemRegion(card_start,card_end));
+    MemRegion card_region(_bot_shared->address_for_index(index), G1BlockOffsetSharedArray::N_words);
+    MemRegion pre_gc_allocated(r->bottom(), r->scan_top());
+    MemRegion mr = pre_gc_allocated.intersection(card_region);
     if (!mr.is_empty() && !_ct_bs->is_card_claimed(index)) {
       // We make the card as "claimed" lazily (so races are possible
       // but they're benign), which reduces the number of duplicate

--- a/src/share/vm/gc_implementation/g1/heapRegion.cpp	Tue Mar 24 10:04:10 2015 +0000
+++ b/src/share/vm/gc_implementation/g1/heapRegion.cpp	Wed Mar 25 11:03:16 2015 +0100
@@ -338,7 +338,7 @@
   _orig_end = mr.end();
   hr_clear(false /*par*/, false /*clear_space*/);
   set_top(bottom());
-  record_top_and_timestamp();
+  record_timestamp();
 }
 
 CompactibleSpace* HeapRegion::next_compaction_space() const {
@@ -426,9 +426,9 @@
 
   // If we're within a stop-world GC, then we might look at a card in a
   // GC alloc region that extends onto a GC LAB, which may not be
-  // parseable.  Stop such at the "saved_mark" of the region.
+  // parseable.  Stop such at the "scan_top" of the region.
   if (g1h->is_gc_active()) {
-    mr = mr.intersection(used_region_at_save_marks());
+    mr = mr.intersection(MemRegion(bottom(), scan_top()));
   } else {
     mr = mr.intersection(used_region());
   }
@@ -980,7 +980,7 @@
 
 void G1OffsetTableContigSpace::clear(bool mangle_space) {
   set_top(bottom());
-  set_saved_mark_word(bottom());
+  _scan_top = bottom();
   CompactibleSpace::clear(mangle_space);
   reset_bot();
 }
@@ -1012,41 +1012,42 @@
   return _offsets.threshold();
 }
 
-HeapWord* G1OffsetTableContigSpace::saved_mark_word() const {
+HeapWord* G1OffsetTableContigSpace::scan_top() const {
   G1CollectedHeap* g1h = G1CollectedHeap::heap();
-  assert( _gc_time_stamp <= g1h->get_gc_time_stamp(), "invariant" );
   HeapWord* local_top = top();
   OrderAccess::loadload();
-  if (_gc_time_stamp < g1h->get_gc_time_stamp()) {
+  const unsigned local_time_stamp = _gc_time_stamp;
+  assert(local_time_stamp <= g1h->get_gc_time_stamp(), "invariant");
+  if (local_time_stamp < g1h->get_gc_time_stamp()) {
     return local_top;
   } else {
-    return Space::saved_mark_word();
+    return _scan_top;
   }
 }
 
-void G1OffsetTableContigSpace::record_top_and_timestamp() {
+void G1OffsetTableContigSpace::record_timestamp() {
   G1CollectedHeap* g1h = G1CollectedHeap::heap();
   unsigned curr_gc_time_stamp = g1h->get_gc_time_stamp();
 
   if (_gc_time_stamp < curr_gc_time_stamp) {
-    // The order of these is important, as another thread might be
-    // about to start scanning this region. If it does so after
-    // set_saved_mark and before _gc_time_stamp = ..., then the latter
-    // will be false, and it will pick up top() as the high water mark
-    // of region. If it does so after _gc_time_stamp = ..., then it
-    // will pick up the right saved_mark_word() as the high water mark
-    // of the region. Either way, the behaviour will be correct.
-    Space::set_saved_mark_word(top());
-    OrderAccess::storestore();
+    // Setting the time stamp here tells concurrent readers to look at
+    // scan_top to know the maximum allowed address to look at.
+
+    // scan_top should be bottom for all regions except for the
+    // retained old alloc region which should have scan_top == top
+    HeapWord* st = _scan_top;
+    guarantee(st == _bottom || st == _top, "invariant");
+
     _gc_time_stamp = curr_gc_time_stamp;
-    // No need to do another barrier to flush the writes above. If
-    // this is called in parallel with other threads trying to
-    // allocate into the region, the caller should call this while
-    // holding a lock and when the lock is released the writes will be
-    // flushed.
   }
 }
 
+void G1OffsetTableContigSpace::record_retained_region() {
+  // scan_top is the maximum address where it's safe for the next gc to
+  // scan this region.
+  _scan_top = top();
+}
+
 void G1OffsetTableContigSpace::safe_object_iterate(ObjectClosure* blk) {
   object_iterate(blk);
 }
@@ -1080,6 +1081,8 @@
 void G1OffsetTableContigSpace::initialize(MemRegion mr, bool clear_space, bool mangle_space) {
   CompactibleSpace::initialize(mr, clear_space, mangle_space);
   _top = bottom();
+  _scan_top = bottom();
+  set_saved_mark_word(NULL);
   reset_bot();
 }
 

--- a/src/share/vm/gc_implementation/g1/heapRegion.hpp	Tue Mar 24 10:04:10 2015 +0000
+++ b/src/share/vm/gc_implementation/g1/heapRegion.hpp	Wed Mar 25 11:03:16 2015 +0100
@@ -101,28 +101,25 @@
 // OffsetTableContigSpace.  If the two versions of BlockOffsetTable could
 // be reconciled, then G1OffsetTableContigSpace could go away.
 
-// The idea behind time stamps is the following. Doing a save_marks on
-// all regions at every GC pause is time consuming (if I remember
-// well, 10ms or so). So, we would like to do that only for regions
-// that are GC alloc regions. To achieve this, we use time
-// stamps. For every evacuation pause, G1CollectedHeap generates a
-// unique time stamp (essentially a counter that gets
-// incremented). Every time we want to call save_marks on a region,
-// we set the saved_mark_word to top and also copy the current GC
-// time stamp to the time stamp field of the space. Reading the
-// saved_mark_word involves checking the time stamp of the
-// region. If it is the same as the current GC time stamp, then we
-// can safely read the saved_mark_word field, as it is valid. If the
-// time stamp of the region is not the same as the current GC time
-// stamp, then we instead read top, as the saved_mark_word field is
-// invalid. Time stamps (on the regions and also on the
-// G1CollectedHeap) are reset at every cleanup (we iterate over
-// the regions anyway) and at the end of a Full GC. The current scheme
-// that uses sequential unsigned ints will fail only if we have 4b
+// The idea behind time stamps is the following. We want to keep track of
+// the highest address where it's safe to scan objects for each region.
+// This is only relevant for current GC alloc regions so we keep a time stamp
+// per region to determine if the region has been allocated during the current
+// GC or not. If the time stamp is current we report a scan_top value which
+// was saved at the end of the previous GC for retained alloc regions and which is
+// equal to the bottom for all other regions.
+// There is a race between card scanners and allocating gc workers where we must ensure
+// that card scanners do not read the memory allocated by the gc workers.
+// In order to enforce that, we must not return a value of _top which is more recent than the
+// time stamp. This is due to the fact that a region may become a gc alloc region at
+// some point after we've read the timestamp value as being < the current time stamp.
+// The time stamps are re-initialized to zero at cleanup and at Full GCs.
+// The current scheme that uses sequential unsigned ints will fail only if we have 4b
 // evacuation pauses between two cleanups, which is _highly_ unlikely.
 class G1OffsetTableContigSpace: public CompactibleSpace {
   friend class VMStructs;
   HeapWord* _top;
+  HeapWord* volatile _scan_top;
  protected:
   G1BlockOffsetArrayContigSpace _offsets;
   Mutex _par_alloc_lock;
@@ -166,10 +163,11 @@
   void set_bottom(HeapWord* value);
   void set_end(HeapWord* value);
 
-  virtual HeapWord* saved_mark_word() const;
-  void record_top_and_timestamp();
+  HeapWord* scan_top() const;
+  void record_timestamp();
   void reset_gc_time_stamp() { _gc_time_stamp = 0; }
   unsigned get_gc_time_stamp() { return _gc_time_stamp; }
+  void record_retained_region();
 
   // See the comment above in the declaration of _pre_dummy_top for an
   // explanation of what it is.
@@ -193,6 +191,8 @@
   virtual HeapWord* allocate(size_t word_size);
   HeapWord* par_allocate(size_t word_size);
 
+  HeapWord* saved_mark_word() const { ShouldNotReachHere(); return NULL; }
+
   // MarkSweep support phase3
   virtual HeapWord* initialize_threshold();
   virtual HeapWord* cross_threshold(HeapWord* start, HeapWord* end);