RFR(oneliner): 8205006: Assertion failure when rehashing stringtable

[Robbin Ehn]

Hi all, please review.

When grow completes it poisons the _new_table to make sure no-one uses it after
the table swap. Rehash and grow are mutual exclusive, so the assert tries to
verify that it's ok to rehash. Thus in this case NULL and POISON_PTR are
equivalent.

Reproduced with first forcing table to grow then rehash, with below that works.

Thanks, Robbin

[rehn at rehn-ws open]$ hg diff
diff -r 9fdfe5ca0e5e src/hotspot/share/utilities/concurrentHashTable.inline.hpp
--- a/src/hotspot/share/utilities/concurrentHashTable.inline.hpp	Tue Jun 19 
10:57:13 2018 +0200
+++ b/src/hotspot/share/utilities/concurrentHashTable.inline.hpp	Tue Jun 19 
12:39:17 2018 +0200
@@ -1203,7 +1203,7 @@
    if (!try_resize_lock(thread)) {
      return false;
    }
-  assert(_new_table == NULL, "Must be NULL");
+  assert(_new_table == NULL || _new_table == POISON_PTR, "Must be NULL");
    for (size_t bucket_it = 0; bucket_it < _table->_size; bucket_it++) {
      Bucket* bucket = _table->get_bucket(bucket_it);
      assert(!bucket->have_redirect() && !bucket->is_locked(), "Table must be 
uncontended");