[rvm-research] Bug in Classpath's VMProcess w.r.t. use of notify() instead of notifyAll()

Hi,

I've been chasing a bug which causes benchmarks from Dacapo 2006 to
hang with the concurrent collector we are implementing.  I've tracked
the source of the bug to VMProcess in Classpath.  VMProcess performs a
notify() on state changes rather than notifyAll(), this means that if
one thread calls waitFor() on a Process object waiting for its
termination and another calls destroy() then only one of the two will
be woken up.

This occurs during stress testing with Dacapo 2006's Eclipse benchmark
which calls external commands with a timeout.  Under high load the
external processes do not finish within the timeout period and
destroy() is called.  At the same time Eclipse's process monitor
thread is waiting in waitFor().  Only the waitFor() thread detects
termination, the destroy() thread is never resumed.

The attached program can reproduce the bug in miniature with any
version of Jikes compiled against Classpath.  This can be fixed by
applying the attached patch to classpath any version.

Cheers,

Carl

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--- a/vm/reference/java/lang/VMProcess.java
+++ b/vm/reference/java/lang/VMProcess.java
@@ -152,7 +152,7 @@ final class VMProcess extends Process
                     {
                       process.exitValue = exitValue;
                       process.state = TERMINATED;
-                      process.notify();
+                      process.notifyAll();
                     }
                 }
               else
@@ -214,7 +214,7 @@ final class VMProcess extends Process
               process.state = TERMINATED;
               process.exception = t;
             }
-          process.notify();
+          process.notifyAll();
         }
     }
   }
@@ -243,7 +243,7 @@ final class VMProcess extends Process
           }
         else
           {
-            workList.notify();
+            workList.notifyAll();
           }
       }