[11u] RFR (XS) 8259271: gc/parallel/TestDynShrinkHeap.java still fails "assert(covered_region.contains(new_memregion)) failed: new region is not in covered_region"

[Aleksey Shipilev]

Original bug:
   https://bugs.openjdk.java.net/browse/JDK-8259271
   https://github.com/openjdk/jdk16/commit/685c03dc

Like with JDK-8260704 backport, we do not have Atomic methods yet, those were moved from
OrderAccess to Atomic in JDK 14 with JDK-8234562. So, 11u patch uses OrderAccess. This backport 
would land after JDK-8260704, so no additional #include is needed.

11u variant:

diff -r 48895ff9213d src/hotspot/share/gc/parallel/mutableSpace.cpp
--- a/src/hotspot/share/gc/parallel/mutableSpace.cpp    Mon Feb 15 18:17:01 2021 +0000
+++ b/src/hotspot/share/gc/parallel/mutableSpace.cpp    Mon Feb 15 19:46:06 2021 +0100
@@ -193,11 +193,15 @@
  }

  // This version is lock-free.
  HeapWord* MutableSpace::cas_allocate(size_t size) {
    do {
-    HeapWord* obj = top();
+    // Read top before end, else the range check may pass when it shouldn't.
+    // If end is read first, other threads may advance end and top such that
+    // current top > old end and current top + size > current end.  Then
+    // pointer_delta underflows, allowing installation of top > current end.
+    HeapWord* obj = OrderAccess::load_acquire(top_addr());
      if (pointer_delta(end(), obj) >= size) {
        HeapWord* new_top = obj + size;
        HeapWord* result = Atomic::cmpxchg(new_top, top_addr(), obj);
        // result can be one of two:
        //  the old top value: the exchange succeeded


Testing: tier{1,2} with Parallel

-- 
Thanks,
-Aleksey