[jdk16] RFR: 8259271: gc/parallel/TestDynShrinkHeap.java still fails "assert(covered_region.contains(new_memregion)) failed: new region is not in covered_region"

[Kim Barrett]

Please review this fix for an intermittent crash when using ParallelGC on
aarch64.  The problem is a mis-ordered pair of reads that permit an
algorithmic invariant to be violated.  The mis-ordering is due to the lack
any explicit ordering request (a missing barrier).

In MutableSpace::cas_allocate, we had

    HeapWord* obj = top();
    if (pointer_delta(end(), obj) >= size) {
      ... space available, attempt the CAS to claim it ...
    }

If end is read before top, other threads may advance top and end between
those reads.  If, when top is read, current top > old end and current top +
size > current end, the range check will unexpectedly pass because of
underflow in pointer_delta.  This will allow top to be advanced to a value
which is > current end, violating the algorithmic invariant, and likely
leading to crashes or memory corruption.

gcc for x86 doesn't reorder the reads, but for aarch64 it does, and is
permitted to do so.  Even if it didn't, there is nothing to prevent the
hardware from doing so.  The solution is to use a load_acquire for top, to
ensure the needed order.

This may have been the actual root cause of JDK-8257999.  However, the
change made there was and still is needed for the reasons described.

Testing:
mach5 tier1-3

Even with knowledge of the failure mode it's very hard to reproduce.  I was
unable to catch the underflow case in over 1K attempts using machines in our
test farm, though StefanK caught it a few times on a personal machine.

-------------

Commit messages:
 - Use load_acquire to order reads of top and end.

Changes: https://git.openjdk.java.net/jdk16/pull/127/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=127&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259271
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/127.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/127/head:pull/127

PR: https://git.openjdk.java.net/jdk16/pull/127