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https://issues.apache.org/jira/browse/AMQ-4512?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13650927#comment-13650927
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Sam hendley edited comment on AMQ-4512 at 5/7/13 2:56 PM:
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Test case showing failure. Remember that there are many other functions that
have the same broken pattern of letting the "percentUsage" escape the
synchronized block and they should be fixed across the board, not just for
MemoryUsage.
was (Author: shendley):
test case showing failure
> MemoryUsage waitForSpace in inconsistent state
> ----------------------------------------------
>
> Key: AMQ-4512
> URL: https://issues.apache.org/jira/browse/AMQ-4512
> Project: ActiveMQ
> Issue Type: Bug
> Components: Broker
> Affects Versions: 5.6.0, 5.7.0
> Reporter: Sam hendley
> Fix For: 5.9.0
>
> Attachments: MemoryUsageTest.java, MemUsage1.PNG, MemUsage2.PNG,
> QueueStats.PNG
>
>
> There is a race condition in MemoryUsage which makes it possible for it to be
> left in an inconsistent state and thereby hang any clients in waitForSpace().
> The core issue is in the following block of code:
> {code:java}
> public void decreaseUsage(long value) {
> if (value == 0) {
> return;
> }
> int percentUsage;
> synchronized (usageMutex) {
> usage -= value;
> percentUsage = caclPercentUsage();
> }
> setPercentUsage(percentUsage);
> if (parent != null) {
> parent.decreaseUsage(value);
> }
> }
> {code}
> The bug occurs when multiple threads are calling increment/decrement at same
> time. Since the field "usage" is protected with the usageMutex each
> writer/reader will see the correct and current value for usage and calculate
> the right value for percentUsage at that instant. "setPercentUsage" is also
> protected by the same usageMutex so we resyncronize on usageMutex to set the
> percentUsage field as well. The issue is that threads may enter the
> setPercentUsage synchronized block in a different order than they entered the
> percentUsage "calculating" block. Since percentUsage is carried between the
> two blocks, a reordering of threads can allow the wrong final percentUsage
> value to be set.
> Possible threading (imagine usage starts at 0 and limit is 100).
> Thread #1 - usage += 150; percentUsage = 150;
> Thread #1 - suspended before called setPercentUsage
> Thread #2 - usage -= 150; percentUsage = 0;
> Thread #2 - setPercentUsage(0);
> Thread #1 - resumed, can now call setPercentUsage
> Thread #1 - setPercentUsage(150);
> Final value = 150
> This same pattern of synchronizing to calculate the percentUsage and then
> setting the value later is repeated in all of the Usage objects I looked at.
> My guess it was written this way to avoid holding locks while making calls
> out to "untrusted code" but it is a very dangerous way to do the
> calculations. The most surprising thing is the locks are still being
> explicitly held while calling fireEvent anyways.
> I have attached two screenshots taken using a debugger of two threads that
> have both been stalled for multiple minutes on "waitForSpace" trying to
> publish to the same queue. Notice they both have a "usage" of 0 but a
> "percentUsage" > 100, this should be impossible. To get the system into this
> state I was using JmsTemplate and a CachingConnectionFactory to publish on 8
> threads and a single DefaultMessageListenerContainer who is pulling the
> messages off as fast as possible. The test publishes 100000 measurements and
> around ~75% of the time atleast a few producers end up stalled in
> waitForSpace() even though the queue is ready for more messages. I can also
> reproduce these results using JmsTemplate and PooledConnectionFactory so I
> don't believe it's an issue in the pooling implementations.
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