8005489: VM hangs during GC with ParallelGC and ParallelGCThreads=0

Summary: Print an error message and exit the VM if UseParallalGC is combined with ParllelGCThreads==0. Also reviewed by vitalyd@gmail.com.
Reviewed-by: stefank, ehelin

src/share/vm/runtime/arguments.cpp

@@ -1454,9 +1454,14 @@ void Arguments::set_parallel_gc_flags() {
 
   // If no heap maximum was requested explicitly, use some reasonable fraction
   // of the physical memory, up to a maximum of 1GB.
-  if (UseParallelGC) {
   FLAG_SET_DEFAULT(ParallelGCThreads,
                    Abstract_VM_Version::parallel_worker_threads());
+  if (ParallelGCThreads == 0) {
+    jio_fprintf(defaultStream::error_stream(),
+        "The Parallel GC can not be combined with -XX:ParallelGCThreads=0\n");
+    vm_exit(1);
+  }
+
 
   // If InitialSurvivorRatio or MinSurvivorRatio were not specified, but the
   // SurvivorRatio has been set, reset their default values to SurvivorRatio +
@@ -1479,7 +1484,6 @@ void Arguments::set_parallel_gc_flags() {
       FLAG_SET_DEFAULT(MarkSweepDeadRatio, 1);
     }
   }
-  }
 }
 
 void Arguments::set_g1_gc_flags() {