[Rxtx] Deadlock in CommPortIdentifier.open with 3 threads ?

[Greg Hanna]

I've just struck an issue using rxtx in a multi-threaded environment:

If I have 3 threads trying to open *the same* comm-port I get a deadlock in
CommPortIdentifier.open
If I have just 2, it works fine - I get a PortInUseException which is nice
and I can handle it.

The 3 thread->deadlock thing is causing me issues.
(Sample code below).

This is from the java thread-dump

Found one Java-level deadlock:
=============================
"Thread-2":
  waiting to lock monitor 0x0003e9ec (object 0x240b08e8, a java.lang.Object
),
  which is held by "Thread-1"
"Thread-1":
  waiting to lock monitor 0x0003e9cc (object 0x2450e108, a
gnu.io.CommPortIdentifier),
  which is held by "Thread-2"

Java stack information for the threads listed above:
===================================================
"Thread-2":
        at gnu.io.CommPortIdentifier.open(CommPortIdentifier.java:329)
        - waiting to lock <0x240b08e8> (a java.lang.Object)
        - locked <0x2450e108> (a gnu.io.CommPortIdentifier)
        at OpenThread.run(RXTXLockTest.java:42)
"Thread-1":
        at java.lang.Object.wait(Native Method)
        - waiting on <0x2450e108> (a gnu.io.CommPortIdentifier)
        at gnu.io.CommPortIdentifier.open(CommPortIdentifier.java:332)
        - locked <0x240b08e8> (a java.lang.Object)
        - locked <0x2450e108> (a gnu.io.CommPortIdentifier)
        at OpenThread.run(RXTXLockTest.java:42)


(Apart from this, rxtx is working fantastically for us).

I'm thinking I'll have to implement some sort of locking mechanism that
*doesn't* use java synchronization to prevent this situation.  (Which
doesn't sound like a good idea to me).

Any ideas?

Thanks,

Greg Hanna

=============  RXTXLockTest.java ========================================

import gnu.io.CommPort;
import gnu.io.CommPortIdentifier;

/**
 * @author Greg Hanna
 * Minimal code to create a deadlock with rxtx in multithreaded open
 */
public class RXTXLockTest {

    public static void main(String[] args) {
        try {
            CommPortIdentifier cpiCom1 =
CommPortIdentifier.getPortIdentifier ("COM14");
            OpenThread t1 = new OpenThread ("  one  ", cpiCom1);
            OpenThread t2 = new OpenThread ("  two  ", cpiCom1);
            OpenThread t3 = new OpenThread ("  003  ", cpiCom1);
            t1.start();
            t2.start();
            t3.start();   // comment this out and it runs with (expected)
PortInUseException
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

class OpenThread extends Thread {
    String name;
    CommPortIdentifier cpi;
    CommPort cp;
    public OpenThread (String name, CommPortIdentifier cpi) {
        this.name = name;
        this.cpi = cpi;
        System.out.println (System.currentTimeMillis() + name + " thread
created");
    }
    public void run () {
        try {
            System.out.println (System.currentTimeMillis() + name + "
opening");
            cp = cpi.open(name, 2000);
            System.out.println (System.currentTimeMillis() + name + " opened
- sleeping");
            sleep (2000);  // do the serial comms with the device.
            System.out.println (System.currentTimeMillis() + name + " slept
- closing");
            cp.close();
            System.out.println (System.currentTimeMillis() + name + " closed
- exiting");
        } catch (Exception e) {
            System.out.println ("Error in thread: " + name );
            e.printStackTrace();
        }
    }
}
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