Concurrent System.gc() is "upgraded" to stop-the-world System.gc()

assert(!GCCause::is_user_requested_gc(gc_cause()) || explicit_gc) failed: invariant

V [libjvm.so+0xb3e62d] report_vm_error(char const*, int, char const*, char const*, ...)+0xdd
V [libjvm.so+0xcc069d] G1CollectedHeap::verify_before_full_collection(bool)+0x4d
V [libjvm.so+0xd078f1] G1FullCollector::prepare_collection()+0x51
V [libjvm.so+0xccc396] G1CollectedHeap::do_full_collection(bool, bool)+0x166
V [libjvm.so+0x1877b55] VM_G1CollectForAllocation::doit()+0x345
V [libjvm.so+0x1875438] VM_Operation::evaluate()+0x208
V [libjvm.so+0x187128e] VMThread::evaluate_operation(VM_Operation*)+0x16e
V [libjvm.so+0x1871e12] VMThread::loop()+0x2e2
V [libjvm.so+0x1872610] VMThread::run()+0xc0
V [libjvm.so+0x14c04ba] thread_native_entry(Thread*)+0xfa