[jira] [Created] (FELIX-4462) Classloader deadlock in Java6 between two bundles

["Stefan Egli (JIRA)" <ji...@apache.org>]

Stefan Egli created FELIX-4462:
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             Summary: Classloader deadlock in Java6 between two bundles
                 Key: FELIX-4462
                 URL: https://issues.apache.org/jira/browse/FELIX-4462
             Project: Felix
          Issue Type: Bug
          Components: Framework
    Affects Versions: framework-4.2.1
         Environment: MacOS 10.7.5, java version "1.6.0_65", Java(TM) SE Runtime Environment (build 1.6.0_65-b14-462-11M4609), Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-462, mixed mode)
            Reporter: Stefan Egli


As discussed on the user list ([0]) there is a deadlock in Java 6 (if bug 4670071 [2] is not fixed). The problem is the hidden, implicit ClassLoader.loadClassInternal which is is synchronized(this) in that case. 

Consider the following scenario:
 * bundle 2 wants to load a class of bundle 1 - hence is not synchronized with bundle 1's classloader - thus uses the m_classLocks locking mechanism to lock bundle 1's classloader for the particular class being loaded. Then calls defineClass (with bundle 1's classloader)
 * before it can do so, bundle 1 wants to load the same class (of bundle 1) - hence does a synchronized loadClassInternal. then reaches the m_classLocks locking mechanism and notices that there's another thread loading the class

-> this results in the deadlock reported.

There's multiple fixes for this:
 * use Java 7
 * use -XX:+UnlockDiagnosticVMOptions -XX:+UnsyncloadClass
 * create a special Java 6 classloader (Java 7 would use the current one) which does a plain synchronized findClass() - and replaces all synchronized(m_classLocks) with synchronized(this)

[0] http://markmail.org/thread/crwqzqobxgob7q3n
[1] http://bugs.java.com/bugdatabase/view_bug.do?bug_id=4670071



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