[jetty-discuss] deadlock between org.mortbay.component.Container and org.mortbay.management.MBeanContainer

[Henning B. <hen...@gm...>]

Hi,

  I observed a deadlock in Jetty 6.1.0, actually twice, but 
unfortunately I didn't manage to reproduce it today and didn't keep the 
stack traces (but at least I saw them). Thought I can explain it 
nevertheless. Btw. I think the relevant code did not change in later 
6.1.x releases. Ok, here it is:

  When using Jetty's org.mortbay.management.MBeanContainer, e.g. via a 
jetty.xml like this one:

<Configure class="org.mortbay.jetty.Server">
<!-- ... -->

    <!-- initialize the Jetty MBean container -->
    <Get name="container">
        <Call name="addEventListener">
            <Arg>
                <New class="org.mortbay.management.MBeanContainer">
                    <Arg>
                        <Ref id="MBeanServer" />
                    </Arg>
                    <Call name="start" />
                </New>
            </Arg>
        </Call>
    </Get>

<!-- .. -->

the MBean container gets registered with an 
org.mortbay.component.Container. Now, in the MBeanContainer 
implementation, at line 171, within the synchronized removeBean() 
method, it calls the Container instances synchronized method  update(...).

It seems that when starting a web app, while another webapp is stopping, 
this can lead to a deadlock, as AbstractHandler calls 
Container.removeBean (which is not synchronized, just as 
Container.addBean) that calls MBeanContainer.removeBean (that is 
synchronized) which in turn calls Container.update (which is 
synchronized). So that thread locks MBeanContainer and tries to lock 
Container.

Another thread may have entered the scenery via Container.update 
(synchronized), for example via ContextHandler, that calls 
Container.remove/addBean and consequently will try to lock 
MBeanContainer by calling MBeanContainer.removeBean (or addBean) and 
we're stuck, since that thread locks Container and waits to lock 
MBeanContainer.

If the call sequence Container-->MBeanContainer can be guaranteed (not 
sure that is true), then making Container.addBean and 
Container.removeBean synchronized should fix that problem - I would guess.

For now, I will simply remove the MbeanContainer from my setup.

Thanks,
  Henning