https://issues.apache.org/bugzilla/show_bug.cgi?id=48843
Summary: Tomcat Acceptor Thread goes into wait() and it will
never come back
Product: Tomcat 6
Version: 6.0.18
Platform: All
OS/Version: All
Status: NEW
Severity: major
Priority: P2
Component: Catalina
AssignedTo: dev@tomcat.apache.org
ReportedBy: hnka...@gmail.com
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.
A simple solution would be to check if curThreadsBusy > 0 in the
synchronized block before going into wait() in method getWorkerThread()
OR increase the scope of the critical section to include the while loop.
Thanks,
Harshad
Stack Traces below:
"bda19102143" id=1578 in WAITING on
lock=org.apache.tomcat.util.net.jioendpoint$wor...@13aa4ee3^m
at java.lang.Object.wait(Native Method)^M
at java.lang.Object.wait(Object.java:485)^M
at
org.apache.tomcat.util.net.JIoEndpoint$Worker.await(JIoEndpoint.java:416)^M
at
org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:442)^M
at java.lang.Thread.run(Thread.java:619)^M
"http-8091-Acceptor-0" id=43 in WAITING on
lock=org.apache.tomcat.util.net.jioendpoint$workerst...@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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