# HG changeset patch
# User eosterlund
# Date 1573588883 0
# Node ID b9a42ca342dbb594e4e6fc5dd7b95d0a44b95db6
# Parent  3d2575331a41f56dbbe41ea2bf886ae48c62dae7
8233061: ZGC: Enforce memory ordering in segmented bit maps
Reviewed-by: pliden, stefank

diff -r 3d2575331a41 -r b9a42ca342db src/hotspot/share/gc/z/zLiveMap.cpp
--- a/src/hotspot/share/gc/z/zLiveMap.cpp	Tue Nov 12 10:45:23 2019 -0800
+++ b/src/hotspot/share/gc/z/zLiveMap.cpp	Tue Nov 12 20:01:23 2019 +0000
@@ -54,7 +54,9 @@
 
   // Multiple threads can enter here, make sure only one of them
   // resets the marking information while the others busy wait.
-  for (uint32_t seqnum = _seqnum; seqnum != ZGlobalSeqNum; seqnum = _seqnum) {
+  for (uint32_t seqnum = OrderAccess::load_acquire(&_seqnum);
+       seqnum != ZGlobalSeqNum;
+       seqnum = OrderAccess::load_acquire(&_seqnum)) {
     if ((seqnum != seqnum_initializing) &&
         (Atomic::cmpxchg(seqnum_initializing, &_seqnum, seqnum) == seqnum)) {
       // Reset marking information
@@ -65,13 +67,13 @@
       segment_live_bits().clear();
       segment_claim_bits().clear();
 
-      // Make sure the newly reset marking information is
-      // globally visible before updating the page seqnum.
-      OrderAccess::storestore();
+      assert(_seqnum == seqnum_initializing, "Invalid");
 
-      // Update seqnum
-      assert(_seqnum == seqnum_initializing, "Invalid");
-      _seqnum = ZGlobalSeqNum;
+      // Make sure the newly reset marking information is ordered
+      // before the update of the page seqnum, such that when the
+      // up-to-date seqnum is load acquired, the bit maps will not
+      // contain stale information.
+      OrderAccess::release_store(&_seqnum, ZGlobalSeqNum);
       break;
     }
 
@@ -93,10 +95,6 @@
   if (!claim_segment(segment)) {
     // Already claimed, wait for live bit to be set
     while (!is_segment_live(segment)) {
-      // Busy wait. The loadload barrier is needed to make
-      // sure we re-read the live bit every time we loop.
-      OrderAccess::loadload();
-
       // Mark reset contention
       if (!contention) {
         // Count contention once
diff -r 3d2575331a41 -r b9a42ca342db src/hotspot/share/gc/z/zLiveMap.inline.hpp
--- a/src/hotspot/share/gc/z/zLiveMap.inline.hpp	Tue Nov 12 10:45:23 2019 -0800
+++ b/src/hotspot/share/gc/z/zLiveMap.inline.hpp	Tue Nov 12 20:01:23 2019 +0000
@@ -30,6 +30,7 @@
 #include "gc/z/zOop.inline.hpp"
 #include "gc/z/zUtils.inline.hpp"
 #include "runtime/atomic.hpp"
+#include "runtime/orderAccess.hpp"
 #include "utilities/bitMap.inline.hpp"
 #include "utilities/debug.hpp"
 
@@ -38,7 +39,7 @@
 }
 
 inline bool ZLiveMap::is_marked() const {
-  return _seqnum == ZGlobalSeqNum;
+  return OrderAccess::load_acquire(&_seqnum) == ZGlobalSeqNum;
 }
 
 inline uint32_t ZLiveMap::live_objects() const {
@@ -68,15 +69,15 @@
 }
 
 inline bool ZLiveMap::is_segment_live(BitMap::idx_t segment) const {
-  return segment_live_bits().at(segment);
+  return segment_live_bits().par_at(segment);
 }
 
 inline bool ZLiveMap::set_segment_live_atomic(BitMap::idx_t segment) {
-  return segment_live_bits().par_set_bit(segment);
+  return segment_live_bits().par_set_bit(segment, memory_order_release);
 }
 
 inline bool ZLiveMap::claim_segment(BitMap::idx_t segment) {
-  return segment_claim_bits().par_set_bit(segment);
+  return segment_claim_bits().par_set_bit(segment, memory_order_acq_rel);
 }
 
 inline BitMap::idx_t ZLiveMap::first_live_segment() const {