MetaspaceGC::inc_capacity_until_GC presently has

2420 size_t capacity_until_GC = _capacity_until_GC;
2421 size_t new_value = capacity_until_GC + v;
  ...
2428 size_t expected = _capacity_until_GC;
2429 size_t actual = Atomic::cmpxchg(new_value, &_capacity_until_GC, expected);

If a different thread were to modify _capacity_until_GC between line 2420 and 2428, that modification could be quietly lost.

The refetch of _capacity_until_GC at line 2428 is the culprit. It should be removed, and all uses of "expected" should be replaced with "capacity_until_GC" (which should perhaps be renamed "old_value").