[jboss-jira] [JBoss JIRA] (DROOLS-1000) Race condition occured when executing Drools

[Thomas Leung (JIRA)]

Thomas Leung created DROOLS-1000:
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             Summary: Race condition occured when executing Drools
                 Key: DROOLS-1000
                 URL: https://issues.jboss.org/browse/DROOLS-1000
             Project: Drools
          Issue Type: Bug
          Components: core engine
    Affects Versions: 6.2.0.Final
            Reporter: Thomas Leung
            Assignee: Mario Fusco


We encountered soft timeout on threads executing Drools in our production environment.
Below is the trimmed thread dump:

"DSLFpmlUniversalContainerCacheWorker:2" id=988 State:RUNNABLE
        at java.util.HashMap.getEntry(HashMap.java:446)
        at java.util.HashMap.containsKey(HashMap.java:434)
        at java.util.HashSet.contains(HashSet.java:201)
        at org.drools.core.impl.KnowledgeBaseImpl.addEventListener(KnowledgeBaseImpl.java:252)
        at org.jbpm.process.instance.ProcessRuntimeImpl.initProcessEventListeners(ProcessRuntimeImpl.java:303)
        at org.jbpm.process.instance.ProcessRuntimeImpl.<init>(ProcessRuntimeImpl.java:115)
        at org.jbpm.process.instance.ProcessRuntimeFactoryServiceImpl.newProcessRuntime(ProcessRuntimeFactoryServiceImpl.java:10)
        at org.jbpm.process.instance.ProcessRuntimeFactoryServiceImpl.newProcessRuntime(ProcessRuntimeFactoryServiceImpl.java:7)
        at org.drools.core.runtime.process.ProcessRuntimeFactory.newProcessRuntime(ProcessRuntimeFactory.java:16)
        at org.drools.core.impl.StatefulKnowledgeSessionImpl.createProcessRuntime(StatefulKnowledgeSessionImpl.java:757)
        at org.drools.core.impl.StatefulKnowledgeSessionImpl.<init>(StatefulKnowledgeSessionImpl.java:393)
        at org.drools.core.impl.StatefulKnowledgeSessionImpl.<init>(StatefulKnowledgeSessionImpl.java:286)
        at org.drools.core.common.PhreakWorkingMemoryFactory.createWorkingMemory(PhreakWorkingMemoryFactory.java:21)
        at org.drools.core.impl.StatelessKnowledgeSessionImpl.newWorkingMemory(StatelessKnowledgeSessionImpl.java:127)
        at org.drools.core.impl.StatelessKnowledgeSessionImpl.execute(StatelessKnowledgeSessionImpl.java:302)

Analysis on the Drools code reveals a possible thread safety issue. A single instance of KnowledgeBaseImpl is shared amongst multiple kSessions but inside KnowledgeBaseImpl, it contains a HashSet storing the listeners:

public final Set<KieBaseEventListener> kieBaseListeners = new HashSet<KieBaseEventListener>();

>From the thread dump, it hanged at addEventListener method:

    public void addEventListener(KieBaseEventListener listener) {
        if (!kieBaseListeners.contains(listener)) {
            eventSupport.addEventListener(listener);
            kieBaseListeners.add(listener);
        }
    }

When 2 threads try to put into a hashmap at the same time and both trigger the map to be resized there is a small chance of created a corrupt internal data structure which results in infinite loops. There are a bunch of references on the net for this, here is one example: 

[http://stackoverflow.com/questions/13695832/explain-the-timing-causing-hashmap-put-to-execute-an-infinite-loop].

Our system heavily rely on Drools and we have high volume everyday, we really need help from Drools dev team and much appreciate if you can provide a patch for us. Thanks in advance.





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