Found one Java-level deadlock:
=============================
"default task-1":
  waiting to lock monitor 0xc6b5f124 (object 0xe2fa9ee8, a io.undertow.protocols.http2.Http2Channel),
  which is held by "default I/O-5"
"default I/O-5":
  waiting to lock monitor 0xc6b5f190 (object 0xe3048fa8, a java.lang.Object),
  which is held by "default task-1"

Java stack information for the threads listed above:
===================================================
"default task-1":
	at io.undertow.server.protocol.framed.AbstractFramedChannel.markWritesBroken(AbstractFramedChannel.java:884)
	- waiting to lock <0xe2fa9ee8> (a io.undertow.protocols.http2.Http2Channel)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.channelForciblyClosed(AbstractFramedStreamSinkChannel.java:562)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.close(AbstractFramedStreamSinkChannel.java:539)
	at org.xnio.IoUtils.safeClose(IoUtils.java:152)
	at io.undertow.server.protocol.framed.AbstractFramedChannel.queueFrame(AbstractFramedChannel.java:709)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.queueFinalFrame(AbstractFramedStreamSinkChannel.java:257)
	- locked <0xe3048f28> (a java.lang.Object)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.shutdownWrites(AbstractFramedStreamSinkChannel.java:240)
	- locked <0xe3048f28> (a java.lang.Object)
	at io.undertow.protocols.http2.Http2Channel.sendGoAway(Http2Channel.java:822)
	at io.undertow.protocols.http2.Http2Channel.sendGoAway(Http2Channel.java:809)
	at io.undertow.protocols.http2.Http2Channel.removeStreamSink(Http2Channel.java:968)
	at io.undertow.protocols.http2.Http2StreamSinkChannel.channelForciblyClosed(Http2StreamSinkChannel.java:68)
	at io.undertow.protocols.http2.Http2DataStreamSinkChannel.channelForciblyClosed(Http2DataStreamSinkChannel.java:322)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.close(AbstractFramedStreamSinkChannel.java:539)
	at org.xnio.IoUtils.safeClose(IoUtils.java:152)
	at io.undertow.server.protocol.framed.AbstractFramedChannel.queueFrame(AbstractFramedChannel.java:709)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.queueFinalFrame(AbstractFramedStreamSinkChannel.java:257)
	- locked <0xe3048fa8> (a java.lang.Object)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.shutdownWrites(AbstractFramedStreamSinkChannel.java:240)
	- locked <0xe3048fa8> (a java.lang.Object)
	at org.xnio.conduits.StreamSinkChannelWrappingConduit.terminateWrites(StreamSinkChannelWrappingConduit.java:75)
	at org.xnio.conduits.ConduitStreamSinkChannel.shutdownWrites(ConduitStreamSinkChannel.java:178)
	at io.undertow.channels.DetachableStreamSinkChannel.shutdownWrites(DetachableStreamSinkChannel.java:79)
	at io.undertow.io.UndertowOutputStream.close(UndertowOutputStream.java:347)
	at io.undertow.io.UndertowOutputStream.updateWritten(UndertowOutputStream.java:266)
	at io.undertow.io.UndertowOutputStream.write(UndertowOutputStream.java:209)
	at io.undertow.io.UndertowOutputStream.write(UndertowOutputStream.java:105)
	at io.undertow.io.BlockingSenderImpl.send(BlockingSenderImpl.java:131)
	at io.undertow.io.BlockingSenderImpl.send(BlockingSenderImpl.java:166)
	at io.undertow.server.handlers.error.SimpleErrorPageHandler$1.handleDefaultResponse(SimpleErrorPageHandler.java:68)
	at io.undertow.server.HttpServerExchange.endExchange(HttpServerExchange.java:1608)
	at io.undertow.server.Connectors.executeRootHandler(Connectors.java:401)
	at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:830)
	at org.jboss.threads.ContextClassLoaderSavingRunnable.run(ContextClassLoaderSavingRunnable.java:35)
	at org.jboss.threads.EnhancedQueueExecutor.safeRun(EnhancedQueueExecutor.java:1982)
	at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.doRunTask(EnhancedQueueExecutor.java:1486)
	at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1377)
	at java.lang.Thread.run(Thread.java:748)
"default I/O-5":
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.close(AbstractFramedStreamSinkChannel.java:525)
	- waiting to lock <0xe3048fa8> (a java.lang.Object)
	at org.xnio.IoUtils.safeClose(IoUtils.java:152)
	at io.undertow.protocols.http2.Http2Channel.closeSubChannels(Http2Channel.java:645)
	at io.undertow.server.protocol.framed.AbstractFramedChannel.close(AbstractFramedChannel.java:824)
	at org.xnio.IoUtils.safeClose(IoUtils.java:152)
	at io.undertow.protocols.http2.Http2Channel.lastDataRead(Http2Channel.java:593)
	at io.undertow.server.protocol.framed.AbstractFramedChannel.receive(AbstractFramedChannel.java:379)
	- locked <0xe2fa9ee8> (a io.undertow.protocols.http2.Http2Channel)
	at io.undertow.server.protocol.http2.Http2ReceiveListener.handleEvent(Http2ReceiveListener.java:118)
	at io.undertow.server.protocol.http2.Http2ReceiveListener.handleEvent(Http2ReceiveListener.java:67)
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
	at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:951)
	at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:932)
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
	at org.xnio.conduits.ReadReadyHandler$ChannelListenerHandler.readReady(ReadReadyHandler.java:66)
	at org.xnio.nio.NioSocketConduit.handleReady(NioSocketConduit.java:89)
	at org.xnio.nio.WorkerThread.run(WorkerThread.java:591)

Found 1 deadlock.