8253778: ShenandoahSafepoint::is_at_shenandoah_safepoint should not access VMThread state from other threads

Reviewed-by: jiefu, rehn, rkennke

Author: shipilev

src/hotspot/share/gc/shenandoah/shenandoahUtils.hpp

@@ -141,10 +141,17 @@ class ShenandoahGCPauseMark : public StackObj {
 
 class ShenandoahSafepoint : public AllStatic {
 public:
-  // check if Shenandoah GC safepoint is in progress
+  // Check if Shenandoah GC safepoint is in progress. This is nominally
+  // equivalent to calling SafepointSynchronize::is_at_safepoint(), but
+  // it also checks the Shenandoah specifics, when it can.
   static inline bool is_at_shenandoah_safepoint() {
     if (!SafepointSynchronize::is_at_safepoint()) return false;
 
+    // This is not VM thread, cannot see what VM thread is doing,
+    // so pretend this is a proper Shenandoah safepoint
+    if (!Thread::current()->is_VM_thread()) return true;
+
+    // Otherwise check we are at proper operation type
     VM_Operation* vm_op = VMThread::vm_operation();
     if (vm_op == NULL) return false;