In the quest to make G1 heap oop stores not dictate what lock ranks may be used in the VM, or where heap oop stores may be performed, a new "access" rank was created for locks that need to be acquired in order to perform memory accesses. These are 'special' locks that must not check for safepoints and may be acquired in leaf calls and almost any context.

However, the HeapRegionRemSet::_m lock is till "leaf" ranked, and might be called from a heap oop access. Therefore, the heap oop store is not quite free, until this situation is remedied.