Found one Java-level deadlock:
=============================
"XNIO-1 task-198":
  waiting to lock monitor 0x00007ff3c41c0498 (object 0x00000006f0aefa68, a io.undertow.protocols.http2.Http2Channel),
  which is held by "XNIO-1 I/O-1"
"XNIO-1 I/O-1":
  waiting to lock monitor 0x00007ff390085128 (object 0x00000006f2286a10, a java.lang.Object),
  which is held by "XNIO-1 task-105"
"XNIO-1 task-105":
  waiting to lock monitor 0x00007ff3c41c0498 (object 0x00000006f0aefa68, a io.undertow.protocols.http2.Http2Channel),
  which is held by "XNIO-1 I/O-1"

Java stack information for the threads listed above:
===================================================
"XNIO-1 task-198":
	at io.undertow.protocols.http2.Http2Channel.updateReceiveFlowControlWindow(Http2Channel.java:789)
	- waiting to lock <0x00000006f0aefa68> (a io.undertow.protocols.http2.Http2Channel)
	at io.undertow.protocols.http2.Http2StreamSourceChannel.updateFlowControlWindow(Http2StreamSourceChannel.java:182)
	at io.undertow.protocols.http2.Http2StreamSourceChannel.read(Http2StreamSourceChannel.java:142)
	at org.xnio.conduits.StreamSourceChannelWrappingConduit.read(StreamSourceChannelWrappingConduit.java:43)
	at org.xnio.conduits.ConduitStreamSourceChannel.read(ConduitStreamSourceChannel.java:127)
	at io.undertow.channels.DetachableStreamSourceChannel.read(DetachableStreamSourceChannel.java:209)
	at io.undertow.server.HttpServerExchange$ReadDispatchChannel.read(HttpServerExchange.java:2287)
	at io.undertow.io.AsyncReceiverImpl.receivePartialBytes(AsyncReceiverImpl.java:579)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.MessageReadHandler.readMessage(MessageReadHandler.java:90)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.MessageReadHandler.handleRequest(MessageReadHandler.java:61)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.TopicHandler.lambda$handleRequest$0(TopicHandler.java:51)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.TopicHandler$$Lambda$124/173628928.accept(Unknown Source)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.TopicHandler.onRequestValid(TopicHandler.java:79)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.TopicHandler.handleRequest(TopicHandler.java:47)
	at io.undertow.security.handlers.AuthenticationCallHandler.handleRequest(AuthenticationCallHandler.java:52)
	at io.undertow.server.Connectors.executeRootHandler(Connectors.java:211)
	at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:809)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)
"XNIO-1 I/O-1":
	at io.undertow.server.protocol.framed.AbstractFramedStreamSourceChannel.markStreamBroken(AbstractFramedStreamSourceChannel.java:658)
	- waiting to lock <0x00000006f2286a10> (a java.lang.Object)
	at io.undertow.server.protocol.framed.AbstractFramedChannel.markReadsBroken(AbstractFramedChannel.java:822)
	at io.undertow.server.protocol.framed.AbstractFramedChannel.receive(AbstractFramedChannel.java:474)
	- locked <0x00000006f0aefa68> (a io.undertow.protocols.http2.Http2Channel)
	at io.undertow.server.protocol.http2.Http2ReceiveListener.handleEvent(Http2ReceiveListener.java:106)
	at io.undertow.server.protocol.http2.Http2ReceiveListener.handleEvent(Http2ReceiveListener.java:57)
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
	at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:931)
	at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:912)
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
	at org.xnio.conduits.ReadReadyHandler$ChannelListenerHandler.readReady(ReadReadyHandler.java:66)
	at io.undertow.protocols.ssl.SslConduit$SslReadReadyHandler.readReady(SslConduit.java:1129)
	at io.undertow.protocols.ssl.SslConduit$1.run(SslConduit.java:168)
	at org.xnio.nio.WorkerThread.safeRun(WorkerThread.java:580)
	at org.xnio.nio.WorkerThread.run(WorkerThread.java:464)
"XNIO-1 task-105":
	at io.undertow.server.protocol.framed.AbstractFramedChannel.notifyClosed(AbstractFramedChannel.java:903)
	- waiting to lock <0x00000006f0aefa68> (a io.undertow.protocols.http2.Http2Channel)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSourceChannel.exitRead(AbstractFramedStreamSourceChannel.java:584)
	- locked <0x00000006f2286a10> (a java.lang.Object)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSourceChannel.read(AbstractFramedStreamSourceChannel.java:531)
	at io.undertow.protocols.http2.Http2StreamSourceChannel.read(Http2StreamSourceChannel.java:141)
	at org.xnio.conduits.StreamSourceChannelWrappingConduit.read(StreamSourceChannelWrappingConduit.java:43)
	at org.xnio.conduits.ConduitStreamSourceChannel.read(ConduitStreamSourceChannel.java:127)
	at io.undertow.channels.DetachableStreamSourceChannel.read(DetachableStreamSourceChannel.java:209)
	at io.undertow.server.HttpServerExchange$ReadDispatchChannel.read(HttpServerExchange.java:2287)
	at io.undertow.io.AsyncReceiverImpl.receivePartialBytes(AsyncReceiverImpl.java:579)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.MessageReadHandler.readMessage(MessageReadHandler.java:90)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.MessageReadHandler.handleRequest(MessageReadHandler.java:61)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.TopicHandler.lambda$handleRequest$0(TopicHandler.java:51)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.TopicHandler$$Lambda$124/173628928.accept(Unknown Source)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.TopicHandler.onRequestValid(TopicHandler.java:79)
	at pl.allegro.tech.hermes.frontend.publishing.handlers.TopicHandler.handleRequest(TopicHandler.java:47)
	at io.undertow.security.handlers.AuthenticationCallHandler.handleRequest(AuthenticationCallHandler.java:52)
	at io.undertow.server.Connectors.executeRootHandler(Connectors.java:211)
	at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:809)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)