There are some safepoint checking locks that pass allow_vm_block as true that aren't actually taken out by the VM thread.  If allow_vm_block is true, that causes a NoSafepointVerifier after the lock is taken, which could unnecessarily delay safepoints.

We can't really change below to 'if !thread->is_Java_thread() && SafepointSynchronize::is_at_safepoint()' because G1 concurrent threads run through safepoints.  So it can be true that it's at a safepoint but trying to acquire a lock, such as the Heap_lock will not block the execution of the safepoint. So the check is somewhat narrow and special but it's still worth keeping.

     if (!_allow_vm_block && thread->is_VM_thread()) {
    // JavaThreads are checked to make sure that they do not hold _allow_vm_block locks during operations
    // that could safepoint.  Make sure the vm thread never uses locks with _allow_vm_block == false.
    fatal("VM thread could block on lock that may be held by a JavaThread during safepoint: %s", name());
     }