Hi readers

Just an experience I wanted to share : the guilty guy is not always the one you're told :-)

We've been told that our seam application was causing 100% CPU usage (never ending).
Context : JBoss 4.2.2, JDK 1.5.0_03, Seam 2.0.1 GA
After loosing a bit time trying reproduce, we did a monkey test (press F5 for 5 seconds) on the home page of our webapp and ... bingo ! cpu locked at 100% !

Okay : our application can cause an infinite loop...
After deeper investigation on the root cause of this infinite loop, we found that it was the JVM !!  (http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6241823)


Thanks to open source concept : we could investigate on the jvm source code, we found the bug, identified the root cause, and fixed the problem (by updating our JDK).


Rgds


For those who want to see stacktrace of infinite loop :
ReentrantLock$FairSync(AbstractQueuedSynchronizer).fullGetFirstQueuedThread()
line: 1246 [local variables unavailable] 
ReentrantLock$FairSync(AbstractQueuedSynchronizer).getFirstQueuedThread() line:
1233   
ReentrantLock$FairSync.tryAcquire(int) line: 208       
ReentrantLock$FairSync(AbstractQueuedSynchronizer).tryAcquireNanos(int, long)
line: 1087       
ReentrantLock.tryLock(long, TimeUnit) line: 416
ConversationEntry.lock() line: 204     
FacesManager(Manager).restoreAndLockConversation(ConversationEntry) line: 490  
FacesManager(Manager).restoreConversation() line: 485  
SeamPhaseListener.afterRestoreView(FacesContext) line: 377     
SeamPhaseListener.afterServletPhase(PhaseEvent) line: 216      
SeamPhaseListener.afterPhase(PhaseEvent) line: 182     
LifecycleImpl.phase(PhaseId, Phase, FacesContext) line: 280    
LifecycleImpl.execute(FacesContext) line: 117  
FacesServlet.service(ServletRequest, ServletResponse) line: 244
ApplicationFilterChain.internalDoFilter(ServletRequest, ServletResponse) line:
290     
ApplicationFilterChain.doFilter(ServletRequest, ServletResponse) line: 206     
SeamFilter$FilterChainImpl.doFilter(ServletRequest, ServletResponse) line: 83  
MultipartFilter.doFilter(ServletRequest, ServletResponse, FilterChain) line: 85
SeamFilter$FilterChainImpl.doFilter(ServletRequest, ServletResponse) line: 69  
ExceptionFilter.doFilter(ServletRequest, ServletResponse, FilterChain) line: 64
SeamFilter$FilterChainImpl.doFilter(ServletRequest, ServletResponse) line: 69  
RedirectFilter.doFilter(ServletRequest, ServletResponse, FilterChain) line: 45 
SeamFilter$FilterChainImpl.doFilter(ServletRequest, ServletResponse) line: 69  
ConfigurableXMLFilter(BaseXMLFilter).doXmlFilter(FilterChain,
HttpServletRequest, HttpServletResponse) line: 154       
Filter(BaseFilter).handleRequest(HttpServletRequest, HttpServletResponse,
FilterChain) line: 260       
Filter(BaseFilter).processUploadsAndHandleRequest(HttpServletRequest,
HttpServletResponse, FilterChain) line: 366      
Filter(BaseFilter).doFilter(ServletRequest, ServletResponse, FilterChain) line:
493    
Ajax4jsfFilter.doFilter(ServletRequest, ServletResponse, FilterChain) line: 60 
SeamFilter$FilterChainImpl.doFilter(ServletRequest, ServletResponse) line: 69  
LoggingFilter.doFilter(ServletRequest, ServletResponse, FilterChain) line: 58  
SeamFilter$FilterChainImpl.doFilter(ServletRequest, ServletResponse) line: 69  
SeamFilter.doFilter(ServletRequest, ServletResponse, FilterChain) line: 158    
ApplicationFilterChain.internalDoFilter(ServletRequest, ServletResponse) line:
235     
ApplicationFilterChain.doFilter(ServletRequest, ServletResponse) line: 206     
ReplyHeaderFilter.doFilter(ServletRequest, ServletResponse, FilterChain) line:
96      
ApplicationFilterChain.internalDoFilter(ServletRequest, ServletResponse) line:
235     
ApplicationFilterChain.doFilter(ServletRequest, ServletResponse) line: 206     
StandardWrapperValve.invoke(Request, Response) line: 230       
StandardContextValve.invoke(Request, Response) line: 175       
SecurityAssociationValve.invoke(Request, Response) line: 179   
BasicAuthenticator(AuthenticatorBase).invoke(Request, Response) line: 432      
JaccContextValve.invoke(Request, Response) line: 84    
StandardHostValve.invoke(Request, Response) line: 127  
ErrorReportValve.invoke(Request, Response) line: 102   
CachedConnectionValve.invoke(Request, Response) line: 157      
AccessLogValve.invoke(Request, Response) line: 562     
StandardEngineValve.invoke(Request, Response) line: 109
CoyoteAdapter.service(Request, Response) line: 262     
Http11Processor.process(Socket) line: 844      
Http11Protocol$Http11ConnectionHandler.process(Socket) line: 583       
JIoEndpoint$Worker.run() line: 446     
Thread.run() line: 595 

Faulty code from jvm:
It's in jdk :
package java.util.concurrent.locks;
/**
     * Version of getFirstQueuedThread called when fastpath fails
     */
    private Thread fullGetFirstQueuedThread() {
        /*
         * This loops only if the queue changes while we read sets of
         * fields.
         */
        for (;;) {
            Node h = head;
            if (h == null)                    // No queue
                return null;

            /*
             * The first node is normally h.next. Try to get its
             * thread field, ensuring consistent reads: If thread
             * field is nulled out or s.prev is no longer head, then
             * some other thread(s) concurrently performed setHead in
             * between some of our reads, so we must reread.
             */
            Node s = h.next;
            if (s != null) {
                Thread st = s.thread;
                Node sp = s.prev;
                if (st != null && sp == head)
                    return st;
            }

            /*
             * Head's next field might not have been set yet, or may
             * have been unset after setHead. So we must check to see
             * if tail is actually first node, in almost the same way
             * as above.
             */
            Node t = tail;
            if (t == h)                       // Empty queue
                return null;

            if (t != null) {
                Thread tt = t.thread;
                Node tp = t.prev;
                if (tt != null && tp == head)
                    return tt;
            }
        }
    }