2178143: JVM crashes if the number of bound CPUs changed during runtime

Summary: Supply a new flag -XX:+AssumeMP to workaround the problem. With the flag is turned on, assume VM run on MP platform so is_MP() will return true that sync calls will not skip away.
Reviewed-by: dholmes, acorn, dcubed, jmasa
Contributed-by: yumin.qi@oracle.com

src/share/vm/runtime/arguments.cpp

@@ -3325,6 +3325,13 @@ jint Arguments::parse(const JavaVMInitArgs* args) {
   }
   check_deprecated_gcs();
   check_deprecated_gc_flags();
+  if (AssumeMP && !UseSerialGC) {
+    if (FLAG_IS_DEFAULT(ParallelGCThreads) && ParallelGCThreads == 1) {
+      warning("If the number of processors is expected to increase from one, then"
+              " you should configure the number of parallel GC threads appropriately"
+              " using -XX:ParallelGCThreads=N");
+    }
+  }
 #else // INCLUDE_ALL_GCS
   assert(verify_serial_gc_flags(), "SerialGC unset");
 #endif // INCLUDE_ALL_GCS

src/share/vm/runtime/globals.hpp

@@ -457,6 +457,9 @@ class CommandLineFlags {
   lp64_product(intx, ObjectAlignmentInBytes, 8,                             \
           "Default object alignment in bytes, 8 is minimum")                \
                                                                             \
+  product(bool, AssumeMP, false,                                            \
+          "Instruct the VM to assume multiple processors are available")    \
+                                                                            \
   /* UseMembar is theoretically a temp flag used for memory barrier         \
    * removal testing.  It was supposed to be removed before FCS but has     \
    * been re-added (see 6401008) */                                         \

src/share/vm/runtime/os.hpp

@@ -180,7 +180,7 @@ class os: AllStatic {
   // Interface for detecting multiprocessor system
   static inline bool is_MP() {
     assert(_processor_count > 0, "invalid processor count");
-    return _processor_count > 1;
+    return _processor_count > 1 || AssumeMP;
   }
   static julong available_memory();
   static julong physical_memory();