Is _thread_in_native safe for handshake/safepoint?

[Reingruber, Richard]

Hi,

I don't understand the method safepoint_safe_with() in safepoint.cpp
Maybe somebody has a quick answer to the questions below...

 648	static bool safepoint_safe_with(JavaThread *thread, JavaThreadState state) {
 649	  switch(state) {
 650	  case _thread_in_native:
 651	    // native threads are safe if they have no java stack or have walkable stack
 652	    return !thread->has_last_Java_frame() || thread->frame_anchor()->walkable();
 653	
 654	  case _thread_blocked:
 655	    // On wait_barrier or blocked.
 656	    // Blocked threads should already have walkable stack.
 657	    assert(!thread->has_last_Java_frame() || thread->frame_anchor()->walkable(), "blocked and not walkable");
 658	    return true;
 659	
 660	  default:
 661	    return false;
 662	  }
 663	}

I thought _thread_in_native and _thread_blocked are both equally safe for
handshakes/safepoints but that method makes a distinction. Looking at line 652
it seems possible that a thread is _thread_in_native without a walkable
stack. The thread is then of course considered not safe. Native wrappers, native
interpreter entries, ThreadToNativeFromVM change the state to _thread_in_native
after making the stack walkable though (I looked also at the old sparc versions).

So how can a thread be _thread_in_native without a walkable stack? And why does
that state make sense when it blocks handshakes/safepoints?

I found JavaThreadInVMAndNative which seems to set the state to
_thread_in_native without making the stack walkable. It seems though that from
there ThreadInVMfromNative is reachable which can trigger an assertion in
JavaThread::check_safepoint_and_suspend_for_native_trans() if the stack isn't
walkable.

JfrRecorderService::invoke_safepoint_write() : void
	JfrRecorderService::write() : void
		JfrRecorderService::finalize_current_chunk() : void
			JfrRecorderService::chunk_rotation() : void
				JfrRecorderService::rotate(int) : void
					JfrEmergencyDump::on_vm_shutdown(bool) : void

Thanks, Richard.