Re: Tomcat Acceptor Thread goes into wait() to never come back - Tomcat 6.0

[Hae Loong Chan <ha...@gmail.com>]

Hi,

If the threads are in the race condition, can it be any possibility
the Tomcat will become responsive again - say if the current
processing request thread is dropped due to request timeout and the
browser is closed?



On Tue, Mar 2, 2010 at 10:27 AM, Harshad Kamat <hn...@gmail.com> wrote:
> Hi,
>
> I believe I've found a race condition in Tomcat that causes the http port to
> be non-responsive. It exists in 6.0 and also in 5.5 (although the code has
> been refactored).
> I could not find any reference to it in the Bug database or the mailing list
> archives.
>
> Consider a tomcat instance with maxThreads set to 2, i.e. you have 2 tomcat
> threads to service incoming requests.
> The sequence of events is as follows:
> 1. Thread 1 and Thread 2 are both servicing a request each.
> 2. A third request comes in.
> 3. In class JIOEndpoint.java, the acceptor thread calls methods
> processSocket() which then calls getWorkerThread() which then calls
> createWorkerThread().
> 4. createWorkerThread() returns null since both threads are busy processing
> the two requests.
> 5. Here is the race condition in method getWorkerThread() in the code shown
> below
>
> protected Worker getWorkerThread(){
> ...
> Worker workerThread = createWorkerThread();
>        while (workerThread == null) {
>            try {
>                synchronized (workers) {
>                    workers.wait();
>                 }
>            }
> ...
> }
>
> The acceptor thread executes the "while(workerThread == null)" statement and
> is then switched out by the CPU.
> The two threads executing the two requests complete and go into
> Worker.await() waiting for the next job after executing method
> recycleWorkerThread().
> The acceptor thread is switched back into CPU and executes the synchronized
> block and goes into the wait().
>
> At this point, there aren't any Worker threads out there processing requests
> and therefore there isn't any thread to wake up the acceptor thread.
> The application is non-responsive after this.
>
> Thoughts?
>
> A simple solution would be to check if curThreadsBusy > 0 in the
> synchronized block before going into wait() in method getWorkerThread().
>
> Thanks,
> Harshad
>
>
> Stack Traces below:
>
> "bda19102143" id=1578 in WAITING on
> lock=org.apache.tomcat.util.net.JIoEndpoint$Worker@13aa4ee3^M
>    at java.lang.Object.wait(Native Method)^M
>    at java.lang.Object.wait(Object.java:485)^M
>    at
> org.apache.tomcat.util.net.JIoEndpoint$Worker.await(JIoEndpoint.java:416)^M
>    at
> org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:442)^M
>    at java.lang.Thread.run(Thread.java:619)^M
>
> "http-8091-Acceptor-0" id=43 in WAITING on
> lock=org.apache.tomcat.util.net.JIoEndpoint$WorkerStack@13bd7b6a^M
>    at java.lang.Object.wait(Native Method)^M
>    at java.lang.Object.wait(Object.java:485)^M
>    at
> org.apache.tomcat.util.net.JIoEndpoint.getWorkerThread(JIoEndpoint.java:700)^M
>    at
> org.apache.tomcat.util.net.JIoEndpoint.processSocket(JIoEndpoint.java:731)^M
>    at
> org.apache.tomcat.util.net.JIoEndpoint$Acceptor.run(JIoEndpoint.java:313)^M
>    at java.lang.Thread.run(Thread.java:619)^M
>

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