Name: nt126004 Date: 09/09/2002


FULL PRODUCT VERSION :
java version "1.4.1-rc"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.1-rc-b19)
Java HotSpot(TM) Client VM (build 1.4.1-rc-b19, mixed mode)


FULL OPERATING SYSTEM VERSION :
SunOs sss1 5.8 Generic_108528-13 sun4u sparc SUNW,Ultra-80


A DESCRIPTION OF THE PROBLEM :
Closing a SelectableChannel in one thread and calling
Selector.select() in another thread may result in a
deadlock situation (the channel being registred with the
selector). The reason is both SelectableChannel.close() and
Selector.select() synchronize on the same couple of objects
but in a different order as the following stack trace shows:

CLOSE (Thread A):
=================
java.nio.channels.spi.AbstractSelector.cancel() => enter
monitor for a java.util.HashSet (protecting this selector's
cancelled key list)
java.nio.channels.spi.AbstractSelectionKey.cancel()
java.nio.channels.spi.AbstractSelectableChannel.implCloseCha
nnel() => enter monitor for a java.lang.Object (protecting
this channel's keys)
java.nio.channels.spi.AbstractInterruptibleChannel.close()


SELECT (Thread B):
==================
java.nio.channels.spi.AbstractSelectableChannel.removeKey
() => enter monitor for a java.lang.Object (protecting
this channel's keys)
java.nio.channels.spi.AbstractSelector.deregister()
sun.nio.ch.DevPollSelectorImpl.implDereg()
sun.nio.ch.SelectorImpl.processDeregisterQueue() => enter
monitor for a java.util.HashSet (protecting this selector's
cancelled key list)
sun.nio.ch.DevPollSelectorImpl.doSelect()
sun.nio.ch.SelectorImpl.select()


It is possible that Thread A acquires a lock on the Object
then Thread B acquires a lock on the HashSet before Thread
A. This results in a deadlock.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the example code under a Thread Analyser (e.g.
OptimizeIt).

Note: deadlock is unreproductible but thread / source code
analysis shows it can happen.

REPRODUCIBILITY :
This bug can be reproduced rarely.

---------- BEGIN SOURCE ----------
import java.nio.channels.*;

public class Deadlock
{
    public static void main(String[] args) throws Exception
    {
        final Selector selector = Selector.open();
        
        Thread thread = new Thread(new Runnable() {
                public void run()
                {
                    while (true)
                    {
                        try
                        {
                            System.out.println("select");
                            selector.select(100);
                        }
                        catch (Exception e)
                        {}
                    }
                }
            });
        thread.start();
        
        while (true)
        {
            SocketChannel channel = SocketChannel.open();
            channel.configureBlocking(false);
            channel.register(selector, 0, null);
            System.out.println("close");
            channel.close();
        }
    }
}

---------- END SOURCE ----------
(Review ID: 164121)
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