[ mx4j-Bugs-831072 ] ObjectName equals() can deadlock on SMP machines

Bugs item #831072, was opened at 2003-10-27 09:28
Message generated for change (Settings changed) made by wkharold
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Category: JMX implementation
Group: Release 1.1.1 beta1
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Ward K Harold (wkharold)
Assigned to: Ward K Harold (wkharold)
Summary: ObjectName equals() can deadlock on SMP machines

Initial Comment:
The ObjectName equals() method uses the Hashtable
equals method to test the key property sets for
equality. This can result in a deadlock on SMP
machines. For example, assume two threads A and B
compare ObjectNames X and Y as follows:

A calls X.equals(Y)
B calls Y.equals(X)

The first invocation ultimately calls:

X.properties.equals(Y.properties) 

which acquires the monitor lock on X.properties.

The second invocation ultimately calls:

Y.properties.equals(X.properties) 

which acquires the monitor lock on Y.properties.

The implementation of the Hashtable equals() method
calls get(), which is also synchronized. This can lead
to a classic deadlock situation in which A has acquired
X's monitor lock and is waiting on Y's monitor lock
while B holds Y's monitor lock and is waiting on X's
monitor lock.

This is a known liveness issue with Hashtable:

http://developer.java.sun.com/developer/bugParade/bugs/4211002.html
http://java.sun.com/products/jdk/1.2/compatibility.html#api

The simplest solution is to clone the key properties
Hashtable of the ObjectName passed as a parameter to
the equals method.



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Comment By: Eamonn McManus (emcmanus)
Date: 2003-10-27 09:42

Message:
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user_id=770046

Alternatively, use a HashMap internally, and convert to and
from Hashtable where required by the API. HashMap is not
synchronized, and doesn't need to be since the contents will
be immutable.

----------------------------------------------------------------------

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