[XMLBEANS-388] XmlBeans can Deadlock Threads

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Version 2.3, Version 2.3.1, Version 2.4
• Fix Version/s: Version 2.4.1
• Component/s: XmlObject
• Environment: Windows XP, Java 1.6
• Assignee: Cezar Cristian Andrei
• Reporter: Paul Hepworth
• Created: 04/Nov/08 17:37
• Updated: 14/Jan/09 17:08
• Resolved: 14/Jan/09 17:08

Description

The situation can occur when calling a setter on an XmlObject which synchronizes on Locale A, to set an XmlObject which synchronizes on Locale B at the same time as calling a setter on an XmlObject which synchronizes on Locale B to set an XmlObject which synchronizes on Locale A.

The code within XmlObjectBase is fine, but the problem lies in the generated code compiled from a schema.

The code that is generated contains a synchronized block on the monitor (Locale A or B depending on the object), and then delegates to the XmlObjectBase set method, which detects that you need to lock more than 1 Locale and so acquires a GlobalLock and then synchronizes on both monitors.

As another thread can be doing the reverse, you can end up with 2 threads, one with Locale A locked, and one with Locale B locked. The first one then acquires a GlobalLock, but can never obtain a lock on the other Locale.

The attached code can produce the following:
run:
[java] Thread:main:RUNNABLE:true
[java] java.lang.Thread.dumpThreads:-2
[java] java.lang.Thread.getAllStackTraces:1487
[java] Deadlock.main:21
[java] Thread:Signal Dispatcher:RUNNABLE:true
[java] Thread:Finalizer:WAITING:true
[java] java.lang.Object.wait:-2
[java] java.lang.ref.ReferenceQueue.remove:116
[java] java.lang.ref.ReferenceQueue.remove:132
[java] java.lang.ref.Finalizer$FinalizerThread.run:159
[java] Thread:Reference Handler:WAITING:true
[java] java.lang.Object.wait:-2
[java] java.lang.Object.wait:485
[java] java.lang.ref.Reference$ReferenceHandler.run:116
[java] Thread:Attach Listener:RUNNABLE:true
[java] Thread:Thread-0:WAITING:true
[java] java.lang.Object.wait:-2
[java] java.lang.Object.wait:485
[java] org.apache.xmlbeans.impl.common.Mutex.acquire:33
[java] org.apache.xmlbeans.impl.common.GlobalLock.acquire:27
[java] org.apache.xmlbeans.impl.values.XmlObjectBase.set:2049
[java] noNamespace.impl.ObjectBTypeImpl.setObjectA:-1
[java] MyThread.run:49
[java] Thread:Thread-1:BLOCKED:true
[java] org.apache.xmlbeans.impl.values.XmlObjectBase.set:2054
[java] noNamespace.impl.ObjectATypeImpl.setObjectB:-1
[java] MyThread.run:48
[java] Java Result: 1

As you can see, one thread is waiting on the GlobalLock, and the other has the lock and is waiting for a lock on the other Locale (XmlObjectBase.set:2054).