<AWT Dev> [7u6] Review request for 7177040: Deadlock between PostEventQueue.noEvents, EventQueue.isDispatchThread and SwingUtilities.invokeLater

David Holmes david.holmes at oracle.com
Wed Jul 11 18:41:43 PDT 2012

Hi Oleg,

"deadlock" always grabs my attention :)

On 12/07/2012 2:08 AM, Oleg Pekhovskiy wrote:
> Thank you for the review, Anthony,
> please review the next version of fix:
> http://cr.openjdk.java.net/~bagiras/7u6/7177040.2
> I followed your suggestions there.
> I also added 'volatile' for isFlushing because it could be accessed from
> different threads.

I was about to mention that :)

I don't have the complete picture here but the actions of flush() no 
longer "ensure the flush is completed before a new event can be posted 
to this queue". The assurance may still be there in the way things are 
called, but it not ensured by the flush() method.

It also seems that with this fix the interrupted EDT will not detach due 
to the flush being in progress. The EDT will resume the pumpEvents loop 
in its run() method. If the interrupt state of the EDT has not been 
maintained then the fact it was interrupted (and should detach?) is now 
lost. If it is maintained then presumably we will try to 
detachDispatchThread again, in which case the EDT will enter a busy 
polling loop trying, but failing, to detach, until the flush() has 
actually completed.


> Thanks,
> Oleg
> 7/11/2012 7:35 PM, Anthony Petrov wrote:
>> Hi Oleg,
>> 1. I suggest to rename isPending to isFlushing, as it reflects the
>> current state of the PostEventQueue more precisely.
>> 2. I suggest to use the try{}finally{} pattern to set/reset the new
>> flag in the flush() method to ensure the flag is reset even if the
>> while() loop throws an exception.
>> Otherwise the fix looks good.
>> --
>> best regards,
>> Anthony
>> On 7/11/2012 7:24 PM, Oleg Pekhovskiy wrote:
>>> Hi!
>>> Please review the fix for CR:
>>> http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7177040
>>> Webrev:
>>> http://cr.openjdk.java.net/~bagiras/7u6/7177040.1
>>> The idea of the fix is that there are two concurrent threads that try
>>> to get two synchronization objects EventQueue.pushPopLock and
>>> PostEventQueue itself.
>>> For NetBeans these threads are (EDT & WarmUp) or (EDT & TimerQueue).
>>> Problem happens when EDT is interrupted and goes to
>>> EventQueue.detachDispatchThread() where EventQueue.pushPopLock is
>>> owned and
>>> EDT is waiting to own PostEventQueue when calling
>>> SunToolkit.isPostEventQueueEmpty() -> PostEventQueue.noEvents().
>>> At the same time another thread calls EventQueue.postEvent() that
>>> calls EventQueue.flushPendingEvents() -> PostEventQueue.flush() where
>>> PostEventQueue is owned
>>> and another thread is waiting to own EventQueue.pushPopLock when
>>> calling EventQueue.postEvent() -> EventQueue.postEventPrivate().
>>> To avoid potential deadlock I moved synchronization out of
>>> postEvent()'s cycle in PostEventQueue.flush(),
>>> but to be clear about the existence of Events that are not posted to
>>> EventQueue yet I added PostEventQueue.isPending flag that is true
>>> until the end of the cycle.
>>> There are only two classes that utilize PostEventQueue.flush()
>>> method: SunToolkit.flushPendingEvents() and
>>> SunToolkitSubclass.flushPendingEvents().
>>> They are both synchronized by static SunToolkit.flushLock. That
>>> eliminates the situation when we have overlapped calls of
>>> PostEventQueue.flush().
>>> Thanks,
>>> Oleg

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