[rvm-research] Possible race hazard in RVMClass

Hi

I am afraid that we found a possible bug.

There may be a race in org.jikesrvm.classloader.RVMClass.
The addSubClass method can be used by multiple threads simultaneously
causing corruption of the subClasses array.

  /**
   * Add to list of classes that derive from this one.
   */
  private void addSubClass(RVMClass sub) {
    int n = subClasses.length;
    RVMClass[] tmp = new RVMClass[n + 1];

    for (int i = 0; i < n; ++i) {
      tmp[i] = subClasses[i];
    }
    tmp[n] = sub;

    subClasses = tmp;
  }

This bug manifests when testing our concurrent collector with DaCapo 2006 jython.
However it can be reproduced in Jikes tip using the attached example program.
The example program uses a script to create a large number of class files which
are then loaded by separate threads using reflection.

We tested on Linux:
> Linux enju 3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:39:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
with Jikes version:
> e6fab50149e5 tip
The configuration was:
> FastAdaptiveGenImmix_x86_64-linux

This occasionally produces two different error messages:

Exception in thread "Thread-3" java.lang.ArrayIndexOutOfBoundsException: Array index out of range: 2466
   at org.jikesrvm.classloader.RVMClass.addSubClass(RVMClass.java:1646)
   at org.jikesrvm.classloader.RVMClass.<init>(RVMClass.java:1011)
   at org.jikesrvm.classloader.RVMClass.createReflectionClass(RVMClass.java:1935)
   at org.jikesrvm.classloader.RVMMethod.getInvoker(RVMMethod.java:881)
   at java.lang.reflect.VMMethod.<init>(VMMethod.java:42)
   at java.lang.reflect.JikesRVMSupport.createMethod(JikesRVMSupport.java:27)
   at java.lang.Class.getMethods(Class.java:1018)
   at Ref.run(Unknown Source:0)
   at java.lang.Thread.run(Thread.java:745)

Exception in thread "Thread-2" java.lang.InternalError: WeakHashMap in incosistent state
   at java.lang.Error.<init>(Error.java:81)
   at java.lang.VirtualMachineError.<init>(VirtualMachineError.java:71)
   at java.lang.InternalError.<init>(InternalError.java:70)
   at java.util.WeakHashMap.internalRemove(WeakHashMap.java:706)
   at java.util.WeakHashMap.cleanQueue(WeakHashMap.java:595)
   at java.util.WeakHashMap.put(WeakHashMap.java:776)
   at org.jikesrvm.classloader.MethodVector.finish(MethodVector.java:74)
   at org.jikesrvm.classloader.RVMClass.resolve(RVMClass.java:1200)
   at org.jikesrvm.runtime.RuntimeEntrypoints.initializeClassForDynamicLink(RuntimeEntrypoints.java:608)
   at org.jikesrvm.classloader.RVMClass.createReflectionClass(RVMClass.java:1942)
   at org.jikesrvm.classloader.RVMMethod.getInvoker(RVMMethod.java:881)
   at java.lang.reflect.VMMethod.<init>(VMMethod.java:42)
   at java.lang.reflect.JikesRVMSupport.createMethod(JikesRVMSupport.java:27)
   at java.lang.Class.getMethods(Class.java:1018)
   at Ref.run(Unknown Source:0)
   at java.lang.Thread.run(Thread.java:745)

The first of these relates to the addSubClass method and can be fixed by making the
method synchronized.

diff -r fb5276512eae rvm/src/org/jikesrvm/classloader/RVMClass.java
--- a/rvm/src/org/jikesrvm/classloader/RVMClass.java	Fri Feb 15 23:30:49 2013 +0100
+++ b/rvm/src/org/jikesrvm/classloader/RVMClass.java	Tue Feb 19 07:16:34 2013 +0000
@@ -1638,7 +1638,7 @@
   /**
    * Add to list of classes that derive from this one.
    */
-  private void addSubClass(RVMClass sub) {
+  private synchronized void addSubClass(RVMClass sub) {
     int n = subClasses.length;
     RVMClass[] tmp = new RVMClass[n + 1];
 
Attached is a PDF containing graphs of the first run performance for
DaCapo 2006 without patch (prod-tip) and with patch (prod-ascp).
These show this patch does not cause a performance regression.

As for the second message, we have not yet located the cause.

Kind regards
Tomoharu

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