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Date: Tue, 2 Mar 2010 10:47:14 +0800
Message-ID: <9c483daa1003011847l120a889ake5c524a537e68cb3@mail.gmail.com>
Subject: Re: Tomcat Acceptor Thread goes into wait() to never come back -
	Tomcat 6.0
From: Hae Loong Chan <haeloong@gmail.com>
To: Tomcat Users List <users@tomcat.apache.org>
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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 <hnkamat@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 ha=
s
> been refactored).
> I could not find any reference to it in the Bug database or the mailing l=
ist
> archives.
>
> Consider a tomcat instance with maxThreads set to 2, i.e. you have 2 tomc=
at
> 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 processi=
ng
> the two requests.
> 5. Here is the race condition in method getWorkerThread() in the code sho=
wn
> below
>
> protected Worker getWorkerThread(){
> ...
> Worker workerThread =3D createWorkerThread();
> =A0 =A0 =A0 =A0while (workerThread =3D=3D null) {
> =A0 =A0 =A0 =A0 =A0 =A0try {
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0synchronized (workers) {
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0workers.wait();
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 }
> =A0 =A0 =A0 =A0 =A0 =A0}
> ...
> }
>
> The acceptor thread executes the "while(workerThread =3D=3D null)" statem=
ent 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 synchroniz=
ed
> block and goes into the wait().
>
> At this point, there aren't any Worker threads out there processing reque=
sts
> 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=3D1578 in WAITING on
> lock=3Dorg.apache.tomcat.util.net.JIoEndpoint$Worker@13aa4ee3^M
> =A0 =A0at java.lang.Object.wait(Native Method)^M
> =A0 =A0at java.lang.Object.wait(Object.java:485)^M
> =A0 =A0at
> org.apache.tomcat.util.net.JIoEndpoint$Worker.await(JIoEndpoint.java:416)=
^M
> =A0 =A0at
> org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:442)^M
> =A0 =A0at java.lang.Thread.run(Thread.java:619)^M
>
> "http-8091-Acceptor-0" id=3D43 in WAITING on
> lock=3Dorg.apache.tomcat.util.net.JIoEndpoint$WorkerStack@13bd7b6a^M
> =A0 =A0at java.lang.Object.wait(Native Method)^M
> =A0 =A0at java.lang.Object.wait(Object.java:485)^M
> =A0 =A0at
> org.apache.tomcat.util.net.JIoEndpoint.getWorkerThread(JIoEndpoint.java:7=
00)^M
> =A0 =A0at
> org.apache.tomcat.util.net.JIoEndpoint.processSocket(JIoEndpoint.java:731=
)^M
> =A0 =A0at
> org.apache.tomcat.util.net.JIoEndpoint$Acceptor.run(JIoEndpoint.java:313)=
^M
> =A0 =A0at java.lang.Thread.run(Thread.java:619)^M
>

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