<AWT Dev> [8] Review request for 7186109: Simplify lock machinery for PostEventQueue & EventQueue

[Anthony Petrov]

Hi David,

On 8/29/2012 3:45 PM, David Holmes wrote:
> I took a look at the last incarnation of this so let me see if I can 
> follow the new scheme.
> 
> On 29/08/2012 9:08 PM, Anthony Petrov wrote:
>> Hi Oleg,
>>
>> I'm still concerned about the following:
>>
>> detachDispatchThread()
>> {
>> flush();
>> lock();
>> // possibly detach
>> unlock();
>> }
>>
>> at EventQueue.java. What if an even get posted to the queue after the
>> flush() returns but before we even acquired the lock? We may still end
>> up with a situation when we detach the thread while in fact there are
>> some pending events present, which actually contradicts the current
>> logic of the detach() method. I see that you say "Minimize discard
>> possibility" in the comment instead of "Prevent ...", but I feel
>> uncomfortable with this actually.
> 
> If a new event is posted before the lock() then within the locked region 
> won't peekEvent() see it and so avoid the detach?

peekEvent() checks the event queue only, while the posted event may be 
stuck in the PostEventQueue. The flushPendingEvents() actually posts the 
events from the PostEventQueue to the EventQueue.


>> What exactly prevents us from adding some synchronization to ensure that
>> the detaching only happens when there's really no pending events?
>>
>> SunToolkit.java:
>>> 2120 Boolean b = isThreadLocalFlushing.get();
>>> 2121 if (b != null && b) {
>>> 2122 return;
>>> 2123 }
>>> 2124 2125 isThreadLocalFlushing.set(true);
>>> 2126 try {
>>
>> How does access to the isThreadLocalFlushing synchronized? What happens
>> if the flush() is being invoked from two different threads for the same
>> post event queue? Why do we have two "isFlushing" flags? Can we collapse
>> them into one? Why is the isFlushing set/reset in two disjunct
>> synchronized(){} blocks?
> 
> isThreadLocalFlushing is a ThreadLocal so no synchronization is needed. 
> I presume it is used to prevent re-entrant/recursive calls to flush() 
> when calling postEvent.
> 
> The isFlushing variable is the global flag to coordinate flushing across 
> multiple threads. It has to be set and cleared in synchronized blocks, 
> but the synchronization lock has to be dropped when calling postEvent to 
> avoid deadlocks. So a thread acquires the lock and checks if flushing is 
> in progress, and if so it waits. Else/then it updates isFlushing to 
> indicate if that thread is doing flushing and releases the lock. It then 
> does any flushing needed, reacquires the lock, sets isFlushing to false 
> and notifies any other threads who may be waiting.
> 
>> Overall, I find the current synchronization scheme in flush() very,
>> *very* (and I mean it) confusing. Can we simplify it somehow?
> 
> This seems like a reasonable protocol to coordinate multiple flushers 
> whilst dropping the synchronization lock when posting events. The actual 
> coordination might be simpler to understand if expressed using a 
> Semaphore - but I think the semantics would be the same.

Ah, I see. Thanks for the insight. It now looks much clearer. I think 
that the final isThreadLocalFlushing.set(false); must be in the 
finally{} block, though.

--
best regards,
Anthony


> 
> David
> 
>> -- 
>> best regards,
>> Anthony
>>
>> On 8/28/2012 6:33 PM, Oleg Pekhovskiy wrote:
>>> Hi Artem, Anthony,
>>>
>>> thank you for your proposals!
>>>
>>> We with Artem also had off-line discussion,
>>> so as a result I prepared improved version of fix:
>>> http://cr.openjdk.java.net/~bagiras/8/7186109.3/
>>>
>>> What was done:
>>> 1. EventQueue.detachDispatchThread(): moved
>>> SunToolkit.flushPnedingEvents() above the comments and added a
>>> separate comment to it.
>>> 2. Moved SunToolkitSubclass.flushPendingEvents(AppContext) method to
>>> SunToolkit. Deleted SunToolkitSubclass.
>>> 3. Moved isFlushingPendingEvents to PostEventQueue with the new name -
>>> isThreadLocalFlushing and made it ThreadLocal.
>>> 4. Left PostEventQueue.flush() unsynchronized and created
>>> wait()-notifyAll() synchronization mechanism to avoid blocking of
>>> PostEventQueue.postEvent().
>>>
>>> Looking forward to your comments!
>>>
>>> Thanks,
>>> Oleg
>>>
>>> 20.08.2012 20:20, Artem Ananiev wrote:
>>>> Hi, Oleg,
>>>>
>>>> here are a few comments:
>>>>
>>>> 1. What is the reason of keeping "isFlushingPendingEvents" in
>>>> SunToolkit, given that PEQ.flush() is synchronized (and therefore
>>>> serialized) anyway?
>>>>
>>>> 2. flushPendingEvents(AppContext) may be moved directly to
>>>> SunToolkit, so we don't need a separate sun-class for that.
>>>>
>>>> 3. EQ.java:1035-1040 - this comment is obsolete and must be replaced
>>>> by another one.
>>>>
>>>> Thanks,
>>>>
>>>> Artem
>>>>
>>>> On 8/17/2012 4:49 PM, Oleg Pekhovskiy wrote:
>>>>> Hi!
>>>>>
>>>>> Please review the fix for CR:
>>>>> http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7186109
>>>>>
>>>>> Webrev:
>>>>> http://cr.openjdk.java.net/~bagiras/8/7186109.1/
>>>>>
>>>>> The following changes were made:
>>>>> 1. Removed flushLock from SunToolkit.flushPendingEvent()
>>>>> 2. Returned method PostEventQueue.flush() as 'synchronized' back
>>>>> 3. Added call of SunToolkit.flushPendingEvents() to
>>>>> EventQueue.detachDispatchThread(),
>>>>> right before pushPopLock.lock()
>>>>> 4. Removed !SunToolkit.isPostEventQueueEmpty() check from
>>>>> EventQueue.detachDispatchThread()
>>>>> 5. Removed SunToolkit.isPostEventQueueEmpty() &
>>>>> PostEventQueue.noEvents();
>>>>>
>>>>> Thanks,
>>>>> Oleg
>>>>> <http://cr.openjdk.java.net/%7Ebagiras/8/7186109.1/>
>>>
>>>