safe-restart is waiting to close a registered connection.
but that connection is waiting for being notified forever while the notifier thread either dead or hanging. there is no timeout in wait.
so safe-restart hanging forever.

our environment is jenkins 1.538 and ssh-slave plugin 1.9
but seems the issue i described still hold true in latest code base.
https://github.com/jenkinsci/ssh-slaves-plugin/blob/master/src/main/java/hudson/plugins/sshslaves/PluginImpl.java
/**

• Closes all the registered connections.
  */
  private static synchronized void closeRegisteredConnections() {
  for (Connection connection : activeConnections)
  Unknown macro: { LOGGER.log(Level.INFO, "Forcing connection to {0}}

  activeConnections.clear();
  }

https://github.com/jenkinsci/trilead-ssh2/blob/master/src/com/trilead/ssh2/channel/ChannelManager.java
private void waitUntilChannelOpen(Channel c) throws IOException
{
synchronized (c)
{
while (c.state == Channel.STATE_OPENING)
{
try

{ c.wait(); }

catch (InterruptedException ignore)

{ throw new InterruptedIOException(); }

}

if (c.state != Channel.STATE_OPEN)

{ removeChannel(c.localID); throw ioException("Could not open channel (state:" + c.state + ")", c); }

}
}

stack trace which shows"deadlock"
"safe-restart thread" prio=10 tid=0x00007fd87e4ab800 nid=0x5157 waiting for monitor entry [0x00007fd9aa8e7000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.trilead.ssh2.Connection.getHostname(Connection.java:961)

• waiting to lock <0x0000000682669c88> (a com.trilead.ssh2.Connection)
  at hudson.plugins.sshslaves.PluginImpl.closeRegisteredConnections(PluginImpl.java:70)
• locked <0x0000000674712e38> (a java.lang.Class for hudson.plugins.sshslaves.PluginImpl)
  at hudson.plugins.sshslaves.PluginImpl.stop(PluginImpl.java:61)
  at hudson.PluginWrapper.stop(PluginWrapper.java:376)
  at hudson.PluginManager.stop(PluginManager.java:734)
  at jenkins.model.Jenkins.cleanUp(Jenkins.java:2797)
  at hudson.lifecycle.UnixLifecycle.restart(UnixLifecycle.java:71)
  at jenkins.model.Jenkins$23.run(Jenkins.java:3400)

"Channel reader thread: xyz.com" prio=10 tid=0x00007fd870014800 nid=0xbaec in Object.wait() [0x00007fd80ceb8000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
at java.lang.Object.wait(Object.java:503)
at com.trilead.ssh2.channel.ChannelManager.waitUntilChannelOpen(ChannelManager.java:110)

• locked <0x0000000685743cc0> (a com.trilead.ssh2.channel.Channel)
  at com.trilead.ssh2.channel.ChannelManager.openSessionChannel(ChannelManager.java:584)
  at com.trilead.ssh2.Session.<init>(Session.java:42)
  at com.trilead.ssh2.Connection.openSession(Connection.java:1129)
• locked <0x0000000682669c88> (a com.trilead.ssh2.Connection)
  at com.trilead.ssh2.SFTPv3Client.<init>(SFTPv3Client.java:99)
  at com.trilead.ssh2.SFTPv3Client.<init>(SFTPv3Client.java:119)
  at hudson.plugins.sshslaves.SSHLauncher.afterDisconnect(SSHLauncher.java:1213)
• locked <0x0000000672021fc8> (a hudson.plugins.sshslaves.SSHLauncher)
  at hudson.slaves.SlaveComputer$2.onClosed(SlaveComputer.java:456)
  at hudson.remoting.Channel.terminate(Channel.java:831)
  at hudson.remoting.SynchronousCommandTransport$ReaderThread.run(SynchronousCommandTransport.java:76)