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From Rich Feit <richf...@gmail.com>
Subject Re: page flow destroy, controls, and deadlock
Date Fri, 07 Apr 2006 04:51:03 GMT
Funny, I'd drafted a longish email earlier asking about the BeanContext 
lock, because it sounded like the *fundamental* issue (ordering problems 
in locking that and the control bean instance itself).  But it's not, or 
at least it's unrelated.  In a way, that's good.  :)

Regarding this:
> Thread A is execution an action on JPF X
> Thread B is destroying JPF X en route to running JPF Y
>
> From the code in PageFlowController.persistInSession (transitively
> down to StorageHandler.removeAttribute), it seems like a lock is held
> on the "current" JPF that would prevent this.  Right?
That's correct.  The only time onDestroy() should ever run without a 
lock on the page flow instance is when the session is expiring.  That's 
the situation where the session itself is already locked, so it's 
dangerous to subsequently lock the page flow instance (while another 
thread may be accessing the session through synchronized get() after 
already locking the page flow instance).  The user could also simulate 
this situation by explicitly removing the page flow session attribute, 
but of course that's not something we should try to support.

Let me know if you see/find any holes in that, though...

Rich

Eddie O'Neil wrote:
> Rich--
>
>   Having looked into this more now, I've got a little more detail
> about what's happening:
>
>   There are two issues intertwined here.  The first issue is related
> to the ControlBeanContext implementation class and its usage of a
> global, VM-wide lock.  At issue is the fact that all
> ControlContainerContext implementations inherit from a
> BeanContextServicesSupport base class.  This class (and another in
> that hierarchy) use a the VM-wide object
> BeanContext.globalHierarchyLock for synchronization in two situations:
>
>   - adding / removing a control to / from a context
>   - adding / removing / getting a "service" from a context
>
> This acts as a choke point for all Control references and is what
> shows up in the locking traces in a previous mail.  This is
> problematic, but it's also orthogonal (I believe) to the issue with
> the "destroy" lifecycle method on a JPF.  Ultimately, I think that the
> JDK assumed that the classes in java.beans.beancontext.* would be used
> to run a single hierarchy of controls on a VM.  Beehive's usage
> pattern is a little different in that in enterprise applications, many
> hierarchies of beans run on a VM (one per JPF, in NetUI's case).  As
> such, a global lock doesn't fit well.
>
>   So, hopefully that provides some background on the
> BeanContext.globalHierarchyLock.
>
>   That being said, I think that the deadlock is orthogonal to this
> problem as the "destroy" lifecycle method on a JPF can be invoked
> without waiting for a lock on the JPF instance itself.  The result is
> that two threads can actually run through a JPF simultaneously.
>
>   With Daryl's changes in February, the ControlContainerContext object
> for a Page Flow is a class-level field, and locking is performed on
> this object at:
>
>   - create time
>   - action execution time
>   - page rendering time (JSP and Faces)
>
> Guess that the question is whether this situation is protected:
>
> Thread A is execution an action on JPF X
> Thread B is destroying JPF X en route to running JPF Y
>
> >From the code in PageFlowController.persistInSession (transitively
> down to StorageHandler.removeAttribute), it seems like a lock is held
> on the "current" JPF that would prevent this.  Right?
>
>   If that's true, then I've got a little more info to go on in
> investigating this problem.
>
>   Thanks for any info...  :)
>
> Eddie
>
>
>
>
> On 3/31/06, Eddie O'Neil <ekoneil@gmail.com> wrote:
>   
>>   Good questions -- let's see if I can explain the threads more
>> clearly in ASCII.  :)
>>
>>   What's happening is this (note, both threads operate on the *same*
>> instance of a JPF):
>>
>> Thread 1 (executing a page flow)              Thread2 (destroying a page flow)
>> acquire lock on FooPage Flow
>>                                                              acquire BC.gHL
>> acquire lock on BarControlBean
>>                                                              wait for
>> lock on BarControlBean
>> wait for lock on BC.gHL
>>
>> Because the Controls infrastructure is based on the BeanContext
>> (Services|Support|etc) code in the java.beans package of the JDK, it
>> uses the BeanContextServicesSupport class as a base class for the
>> ControlBeanContext object in a Control.  This JDK class (BCSS) uses
>> the BeanContext.globalHierarchyLock (also in the JDK) to serialize
>> access to shared data structures like the list of "service" objects in
>> a BeanContext, etc.  This is a lock that is static throughout the JDK
>> (!).
>>
>> To answer your questions:
>>
>>     
>>> What grabs the lock on BeanContextServicesSupport.globalHierarchyLock?
>>>       
>> The base classes for the ControlBeanContext object which delegates up
>> to it's "super" in order to serialize changes and service requests to
>> the classes that implement event listener support, etc.
>>
>>     
>>> What prevents deadlock in general between locking on
>>> BeanContextServicesSupport.globalHierarchyLock and BarControlBean?
>>>       
>> Good question -- this seems like the crux of the issue.  In general,
>> this is protected by a Control container serializing access to the
>> container's ControlContainerContext object.  In this case, the problem
>> occurs because two threads are trying to setup and destroy the same
>> context object simultaneously.  If those setup and destroy calls were
>> serialized against the ControlContainerContext object stored in the
>> HttpSession, presumably, the semantics would be maintained.
>>
>> My guess is that this deadlock occurred with a double page submit or
>> quick refresh that caught the tail end of the previous thread and the
>> beginning of the next request such that the threads interleaved in
>> this way.
>>
>> So, hopefully that helps explain what's happening.
>>
>> Ultimately, I'm not sure how to fix this yet and would appreciate
>> thoughts on the topic.  Would it be safe to acquire a lock on the JPF
>> instance during onDestroy?  That would serialize the access to the
>> session scoped CCC object.
>>
>> Another way to go (and it's a lot of work) is to build a NetUI /
>> web-tier specific control container that doesn't leverage the *Support
>> classes in the JDK.  This would allow the container's implementation
>> to be tailored to the single-threaded nature of the web tier and would
>> remove a lot of the performance implications around locking and
>> synchronized data structures.  But, I've not thought through this yet,
>> either.  :)
>>
>> Eddie
>>
>>
>>
>> On 3/30/06, Rich Feit <richfeit@gmail.com> wrote:
>>     
>>> Looks like a tough one.  First of all, the problem with synchronizing in
>>> onDestroy is also a deadlock issue: since onDestroy is called from an
>>> HttpSessionBindingListener, the Servlet container may have a lock on the
>>> session itself when onDestroy is called.  I don't think it's mandated by
>>> the Servlet spec, but I don't think it's forbidden either.
>>> Unfortunately, this means that a deadlock can occur if code in another
>>> thread synchronizes on the page flow instance first (as in an action
>>> method invocation), then calls a method on HttpSession that synchronizes
>>> on the session object.  Again, I don't think this is mandated by the
>>> spec, but it happens.
>>>
>>> Nothing strikes me off the bat, but I actually don't understand the
>>> deadlock sequence below (I don't disagree -- just don't have enough info
>>> to see it).  What grabs the lock on
>>> BeanContextServicesSupport.globalHierarchyLock?  What prevents deadlock
>>> in general between locking on
>>> BeanContextServicesSupport.globalHierarchyLock and BarControlBean?
>>>
>>> Also, how is the same JPF being *created* by two separate threads?
>>>
>>> Rich
>>>
>>> Eddie O'Neil wrote:
>>>       
>>>> Rich/Daryl--
>>>>
>>>>   Hey, I've run into a thread deadlock problem in the interaction
>>>> between Controls and Page Flow that happens in the following
>>>> circumstance:
>>>>
>>>> thread1: acquire lock on FooPageFlow (during FlowController.execute)
>>>> thread2: acquire lock on
>>>> BeanContextServicesSupport.globalHierarchyLock (JDK class)
>>>> thread1: acquire lock on BarControlBean (ControlBean.ensureControl())
>>>> thread2: wait for lock on BarControlBean (BeanContextSupport.remove())
>>>> thread1: wait for lock on
>>>> BeanContextServicesSupport.globalHierarchyLock (JDK class)
>>>>
>>>> So, the problem is that the same JPF is being both created and
>>>> destroyed by two different threads.  It appears that the "destroy"
>>>> phase of the JPF lifecycle is entirely unsynchronized and can freely
>>>> acquire Control locks without having serialized access to the Page
>>>> Flow itself.
>>>>
>>>>   My thought is to add a synchronization point in
>>>> JavaControlUtils.uninitJavaControls that locks on the
>>>> PageFlowManagedObject as this would serialize access to the Page Flow.
>>>>  But, I seem to recall some threading issues with locking on a JPF
>>>> during the "destroy" part of the lifecycle and don't want to resurrect
>>>> those.
>>>>
>>>>   Any suggestions about how best to make this fix?
>>>>
>>>> Eddie
>>>>
>>>>
>>>>         
>
>   

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