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From "Sam hendley (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (AMQ-4512) MemoryUsage left in inconsistent state
Date Mon, 06 May 2013 22:16:16 GMT

     [ https://issues.apache.org/jira/browse/AMQ-4512?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Sam hendley updated AMQ-4512:
-----------------------------

    Attachment: MemUsage2.PNG
                MemUsage1.PNG
    
> MemoryUsage left 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
>         Attachments: MemUsage1.PNG, MemUsage2.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 disappointing thing is the locks are still being explicitly
held while calling fireEvent anyways.
> I will be attaching more details once ticket is created.

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