Found one Java-level deadlock:
=============================
"Handling POST /jenkins/queue/cancelItem from 192.168.23.74 : RequestHandlerThread[#1775]":
  waiting to lock monitor 0x00007fd2fb61f838 (object 0x000000070b2b2a28, a hudson.model.queue.FutureImpl),
  which is held by "Executor #-1 for node : executing some-job-flow #97"
"Executor #-1 for node : executing some-job-flow #97":
  waiting for ownable synchronizer 0x00000006c03be958, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
  which is held by "Handling POST /jenkins/queue/cancelItem from 192.168.23.74 : RequestHandlerThread[#1775]"

"Executor #-1 for node : executing some-job-flow #97":
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x00000006c03be958> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
	at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:834)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:867)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1197)
	at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:214)
	at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:290)
	at hudson.model.Queue.cancel(Queue.java:725)
	at hudson.model.queue.FutureImpl.cancel(FutureImpl.java:82)
	- locked <0x000000070b2b2a28> (a hudson.model.queue.FutureImpl)
	- locked <0x00000006c03bdd38> (a hudson.model.Queue)
	at java_util_concurrent_Future$cancel.call(Unknown Source)
	at com.cloudbees.plugins.flow.JobInvocation.abort(JobInvocation.groovy:113)
	at com.cloudbees.plugins.flow.JobInvocation$abort.call(Unknown Source)
	at com.cloudbees.plugins.flow.FlowDSL$_killRunningJobs_closure3.doCall(FlowDSL.groovy:111)

"Handling POST /jenkins/queue/cancelItem from 192.168.23.74 : RequestHandlerThread[#1775]":
	at hudson.model.queue.FutureImpl.setAsCancelled(FutureImpl.java:89)
	- waiting to lock <0x000000070b2b2a28> (a hudson.model.queue.FutureImpl)
	at hudson.model.Queue$Item.cancel(Queue.java:2111)
	at hudson.model.Queue.cancel(Queue.java:748)
	at hudson.model.Queue.doCancelItem(Queue.java:761)