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From "Eddie O'Neil" <ekon...@gmail.com>
Subject Re: page flow destroy, controls, and deadlock
Date Tue, 11 Apr 2006 20:29:28 GMT
Rich--

  Hey; thanks for the response.  Seems like we're thinking the same
thing here, which is good.  :)  Additional comments below...

Eddie

On 4/11/06, Rich Feit <richfeit@gmail.com> wrote:
> expiring.  The one thing I'm still wondering is about the deadlock --
> lack of synchronization can't in itself cause a deadlock.  Is the
> deadlock with the control-bean/BC.gHL?  If so, it still seems like
> there's an ordering issue there.  In general, locks should always be
> acquired in the same order.

[eko] Agreed -- wasn't trying to imply that the lack of
synchronization caused the deadlock.   And, you're right, the deadlock
is with the ControlBean / BC.gHL.  :)  In general, when locks are
taken on the ControlBean / BC.gHL, it's done _after_ a lock on the
current page flow, but in some destroy cases, that synchronize-on-JPF
is missing.  I've simplified and abbreviated a bit here; for example:

create JPF
1) synchronize on JPF, CBC.beginContext(), JPF.onCreate(), CBC.endContext

execute JPF action:
1) synchronize on JPF, CBC.beginContext(), invoke JPF action, CBC.endContext

render page:
1) synchronize on JPF, CBC.beginContext(), render page, CBC.endContext

destroy (most cases):
1) synchronize on JPF, JPF.onDestroy(), CBC.endContext

And, in these cases, all JPF <> Controls interactions are done after
synchronizing on the current page flow.  However, there are cases
where, under load to the same page flow in the same session, the
DeferredSessionStorageHandler can cause the the "destroy" step to run
without synchronizing on the current Page Flow thus violating the
synchronization order used everywhere else.

I think the fix I described in my last mail will cause the locks to be
acquired in the same order and thus prevent the deadlock.  :)

>
> Of course, that doesn't change the fact that there's a fundamental
> problem with the destroy behavior in general.  Just seems like both
> should be addressed (I've never understood why BC.gHL was locked first
> during destroy()).  There might be a reason for doing the locking
> out-of-order... I just can't think of one.

[eko] Definitely!  I think both will be addressed with an atomic call
to DSSH.applyChanges that destroys a JPF explicitly.  Then, the order
will be the same as the working "destroy" step above.

I'm making a simple fix first just to get something that works in the
tree and will look at the longer term solution after that.  Will let
you know how it goes...  :)




>
> About the proposed fix, the only question I have is about this:
> > - make the apply changes and ensure failover stages of
> > DSSH.acceptChanges atomic by synchronizing on a class local.  This
> > would serialize changes on the HttpSession and prevent two threads
> > from stepping on each other's attributes.  I believe that this would
> > preserve the "last-one-wins" semantics described in DSSH's Javadoc.
> Is this a class local in DSSH?  If so, that would create a (short)
> synchronization bottleneck across *all* users, since the DSSH instance
> is used context-wide.  Am I understanding this correctly?
>
> Rich
>
> Eddie O'Neil wrote:
> > Rich--
> >
> >   I think I've got a better idea what's happening now.  Doesn't appear
> > that this is caused by the extra
> > HttpServletBindingListener.valueUnbound event happening when
> > DeferredSessionStorageHandler.acceptChanges (DSSH.aC) is invoked.
> >
> >   A thread dump I've got seems to confirm that the deadlock is caused
> > by an unprotected "destroy" lifecycle method run during
> > "ensureFailover".  While I don't have a concrete repro for this yet,
> > it appears that it's possible to have two threads running through
> > DSSH.aC at once.  When the ServletContainerAdapter is cluster aware
> > and implements failover as a set attribute call on the HttpSession,
> > this can happen (note, this doesn't appear to be a problem on
> > Tomcat!):
> >
> > Thread1, step 1: DSSH.applyChanges -- add JPF A to the session while
> > applying changed attributes
> >
> > Thread2, step 1: DSSH.applyChanges -- add JPF B to the session while
> > applying changed attributes
> >
> > Thread1, step 2: DSSH.applyChanges -- finds JPF B in the session while
> > ensuring failover of JPF A.  Because JPF B is unbound from the
> > HttpSession when JPF A is re-set into the attribute map, the
> > HttpServletBindingListener.valueUnbound event is called and JPF B is
> > destroyed.  This destroy does not synchronize on JPF B which causes
> > the locking problem.  The code that runs while ensuring failover also
> > isn't protected using the ThreadLocal that protects the "destroy"
> > method from running while applying changed attributes
> >
> > So, a proposed fix would do a few things:
> >
> > - make the apply changes and ensure failover stages of
> > DSSH.acceptChanges atomic by synchronizing on a class local.  This
> > would serialize changes on the HttpSession and prevent two threads
> > from stepping on each other's attributes.  I believe that this would
> > preserve the "last-one-wins" semantics described in DSSH's Javadoc.
> >
> > - add a check at the beginning of the method that will call "destroy"
> > on any PageFlowManagedObjects if these objects are still "alive".
> > "liveness" would be tracked by a local in the PFMO class.  This could
> > be replaced with a ServletRequest attribute, I suppose.
> >
> > - protect both the apply changes and ensure failover steps from
> > running "destroy" on any PageFlowManagedObjects using the existing
> > ThreadLocal to wrap the "ensureFailover" call.
> >
> > The latter fix is necessary to ensure that containers don't invoke
> > valueUnbound again during ensure failover.  It appears as though
> > Tomcat and WebLogic both implement the semantics of session attribute
> > binding differently.  Tomcat calls valueUnbound on every setAttribute
> > call while WebLogic only calls valueUnbound when the attribute value
> > is different.  The result is that depending on the
> > ServletContainerAdapter, it is possible to have 3 valueUnbound calls
> > per request on a single Page Flow and to destroy a Page Flow on every
> > request.  Again, this doesn't appear to be an issue on Tomcat...
> >
> >   Am curious about your thoughts on the scenario and proposed fix...
> >
> > In the long run, it might be worth considering changing things to call
> > the PageFlowManagedObject lifecycle methods directly rather than
> > implicitly via valueUnbound because of the differences in how
> > containers manage these objects.  An additional session attribute
> > could be used to listen for session timeout via a valueUnbound call
> > which would cause the rest of the Page Flow objects to be cleaned up.
> > Does this long-term approach sound reasonable?
> >
> >   Anyway, that's what I've found out.  It's similar, but slightly
> > different from the previous situation I described.  Sorry about that.
> > :)
> >
> >   Thoughts welcome...
> >
> > Eddie
> >
> >
> >
> >
> > On 4/7/06, Eddie O'Neil <ekoneil@gmail.com> wrote:
> >
> >> Rich--
> >>
> >>   Hey -- one other question that just struck me about this...
> >>
> >>   In the DeferredSessionStorageHandler, is it possible to have two
> >> threads running there at once such that for a single thread, the
> >> "current" Page Flow changes between changed attributes and failover
> >> attributes?  Seems like this is a possibility.  For example:
> >>
> >> Thread 1                                                   Thread 2
> >>                                                                 1:
> >> apply changed attribute for JPF B
> >> 1: apply changed attribute for JPF A
> >>                                                                 1:
> >> apply failover attribute for new JPF B
> >> 2: apply failover attribute via
> >> session.setAttribute(...) that overwrites
> >> B with A, causing B's onDestroy method
> >> to be invoked
> >>
> >>   In this case, it seems like B.onDestroy(...) would need to be
> >> invoked, so lifecycle methods *should* run when setting failover
> >> changes.
> >>
> >>   Just making sure I've got this right.  ;)
> >>
> >> Eddie
> >>
> >>
> >> On 4/7/06, Eddie O'Neil <ekoneil@gmail.com> wrote:
> >>
> >>> Rich--
> >>>
> >>>   That's close.  :)  My current speculation is that the locking issues
> >>> related to Controls (BH.gHL, etc) are happening because of
> >>> unsynchronized execution of the Page Flow's "destroy" lifecycle method
> >>> from the second valueUnbound event.  Since this is unsynchronized, it
> >>> allows two threads (one running in destroy and another running an
> >>> action) to interact with the locks inside of the ControlBeanContext
> >>> hierarchy.
> >>>
> >>>   Recapping the last example, this is a Page Flow "FooController" with
> >>> an @Control BarControlBean; the same instance is being used in both of
> >>> the threads below.
> >>>
> >>> Thread 1 (run an action a JPF)    Thread 2 (destroying a JPF)
> >>> ===============                  =============
> >>> 1: lock FooController
> >>> 2: lock BarControlBean
> >>>                                                 1: lock BC.gHL
> >>>                                                 2: wait for lock on
> >>> BarControlBean
> >>> 3: wait BC.gHL lock
> >>>
> >>> Thread 2 looks strange because it's destroynig the same JPF instance
> >>> as Thread 1.  I agree that this is odd yet seems possible with "quick
> >>> clicking".
> >>>
> >>>   That seems like the most likely explanation so far.  Does that make
> >>> more sense?
> >>>
> >>> Eddie
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> On 4/7/06, Rich Feit <richfeit@gmail.com> wrote:
> >>>
> >>>> OK, thanks for the info.  It sounds to me like the deadlock is caused
by
> >>>> ordering issues when locking the control bean and BC.gHL, and that the
> >>>> page flow synchronization is simply masking this root problem.  Is that
> >>>> what's going on, or am I misunderstanding?
> >>>>
> >>>> Rich
> >>>>
> >>>> Eddie O'Neil wrote:
> >>>>
> >>>>>   Yeah -- that's right the deadlock is caused by lack of
> >>>>> synchronization during "destroy" on the second valueUnbound event.
 I
> >>>>> think (but haven't confirmed yet) that the same thing could happen
> >>>>> with different implementations of the
> >>>>> ServletContainerAdapter.ensureFailover method if it calls
> >>>>> "session.setAttribute(...)".  Just a hunch, though, and I need to
> >>>>> confirm this.
> >>>>>
> >>>>>   Back to the deadlock, absence of synchronization on the "destroy"
> >>>>> lifecycle call allows a second thread to enter into the
> >>>>> ControlBeanContext which can cause the thread to deadlock with a
> >>>>> second thread that's already in the ControlBeanContext.
> >>>>>
> >>>>>
> >>>>>
> >>>>>> I don't think that's an option here, since it voids the whole
purpose of
> >>>>>> DeferredSessionStorageHandler
> >>>>>> (http://mail-archives.apache.org/mod_mbox/beehive-dev/200601.mbox/%3C43D7AC11.8060207@gmail.com%3E
> >>>>>> ) -- am I understanding you here?
> >>>>>>
> >>>>>>
> >>>>> Yeah, that makes sense -- just wanted to rule this out as an option.
 :)
> >>>>>
> >>>>>
> >>>>>
> >>>>>> Otherwise, I wonder if there's a way to track this without adding
state
> >>>>>> to the page flow instance.  Possibly in a request attribute?
 Just a
> >>>>>> thought...
> >>>>>>
> >>>>>>
> >>>>> Sure -- this would certainly work.  It could take the form of a
set of
> >>>>> attribute names that should be persisted / failed over, but that
> >>>>> should ignore lifecycle methods.  Will look at this and get back
to
> >>>>> you.
> >>>>>
> >>>>> Thanks for the comments...
> >>>>>
> >>>>> Eddie
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>> Rich
> >>>>>>
> >>>>>> Eddie O'Neil wrote:
> >>>>>>
> >>>>>>
> >>>>>>>   Gotcha -- thanks for the info; glad to know I wasn't entirely
out in
> >>>>>>> left field.  :)
> >>>>>>>
> >>>>>>>   Given this and putting the issue of the session expiration
aside for
> >>>>>>> a second, I think that root cause of the problem is that
it is
> >>>>>>> possible for the DeferredSessionStorageHandler to run the
> >>>>>>> "valueUnbound" method code on a PageFlowController twice
in these two
> >>>>>>> circumstances:
> >>>>>>>
> >>>>>>>   - DeferredSessionStorageHandler::setAttribute / remove
Attribute
> >>>>>>>   - DeferredSessionStorageHandler::applyChanges @ line 221
> >>>>>>>
> >>>>>>> The first runs in "real time" with the removal or replacement
of a
> >>>>>>> Page Flow in the session and runs inside of the lock, but
the second
> >>>>>>> is the deferred part that runs at the end of a request to
persist
> >>>>>>> changed attributes in the session.  The "applyChanges" then
makes
> >>>>>>> calls to set / remove the Page Flow attribute from the real
> >>>>>>> HttpSession, and this triggers the "valueUnbound" event
and ultimately
> >>>>>>> an "onDestroy".  This latter step is unsynchronized and
thus causes
> >>>>>>> the deadlock.
> >>>>>>>
> >>>>>>>   If the Page Flow is stored in the session and is an
> >>>>>>> HttpSessionBindingListener, any deferred calls are going
to trigger
> >>>>>>> this event.  In order to keep consecutive JPF invocations
in the same
> >>>>>>> order, the onDestroy of the previous JPF needs to run before
onCreate
> >>>>>>> of the next JPF.  The cheap way to fix this would be to
track the
> >>>>>>> state of "valueUnbound" as a private boolean inside of the
> >>>>>>> PageFlowController.  Then, when the second valueUnbound
call is made,
> >>>>>>> the event will be a no-op on the JPF.
> >>>>>>>
> >>>>>>>   Another solution would be to tunnel down into the real
HttpSession
> >>>>>>> and make JPF changes "live" against the session rather than
deferring
> >>>>>>> them.
> >>>>>>>
> >>>>>>>   Have an opinion either way?
> >>>>>>>
> >>>>>>> Eddie
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>> On 4/6/06, Rich Feit <richfeit@gmail.com> wrote:
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>>> 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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