FULL PRODUCT VERSION :
java version "1.6.0_18"
Java(TM) SE Runtime Environment (build 1.6.0_18-b07)
Java HotSpot(TM) Server VM (build 16.0-b13, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux hostname 2.6.9-42.EL.vision.1smp #1 SMP Tue Feb 3 09:56:18 EST 2009 i686 i686 i386 GNU/Linux
and
Linux hostname 2.6.18-164.el5PAE #1 SMP Tue Aug 18 15:59:11 EDT 2009 i686 i686 i386 GNU/Linux

EXTRA RELEVANT SYSTEM CONFIGURATION :
Both machines have more than one core.

A DESCRIPTION OF THE PROBLEM :
When multiple threads are logging at the same time, the getLogger() function eventually returns a different logger than the one initialized when it was called the first time. All the configuration done on the first logger is then lost.

The problem started appearing in 1.6.0_18. I validated that it still exists in 1.6.0_20 and that it wasn't in 1.6.0_17.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
I did this test class to reproduce the problem.
It should exit when the reference to the initial logger is lost.
I noticed that it crashes much quickly when the function initRootLogger() is called. It usually takes 1 to 2 seconds to exit with an error. When initRootLogger() isn't called, it can take a couple of seconds to a couple of minutes.

package loggertester;

import java.util.logging.*;

public class LoggerTester {

    public static String LOGGER = "tester";

     public void initRootLogger() {
        Logger.getLogger("").getHandlers();
        Logger rootLogger = Logger.getLogger("");
        rootLogger.setLevel(Level.INFO);
        Handler[] handlers = rootLogger.getHandlers();
        for (Handler handler : handlers) {
            handler.setLevel(Level.INFO);
        }
    }

    public void start() {
        String initialLogger = Logger.getLogger(LOGGER).toString();
        while (true) {
            if (!initialLogger.equals(Logger.getLogger(LOGGER).toString())) {
                System.out.println("Logger changed from " + initialLogger + " to " + Logger.getLogger(LOGGER));
                System.exit(-1);
            }
            Logger.getLogger(LOGGER).fine("LoggerTester log");
        }
    }

    public static void main(String[] args) {
        LoggerTester tester = new LoggerTester();
        tester.initRootLogger();
        Thread thread = new Thread() {
            @Override
            public void run() {
                while (true) {
                    Logger.getLogger(LOGGER).fine("Thread log");
                }
            }
        };
        System.out.println("Starting logger loops");
        thread.start();
        tester.start();
    }
}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The program should never exit as the logger returned by getLogger() should always be the same.
ACTUAL -
The program returns saying that the logger changed in the course of the execution.

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
1.6.0_17 works as expected.

Release Regression From : 6u17
The above release value was the last known release where this 
bug was not reproducible. Since then there has been a regression.