8328235: GenShen: Robustify ShenandoahGCSession and fix missing use

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

@@ -170,6 +170,10 @@
                     ShenandoahAsserts::assert_heaplocked_or_fullgc_safepoint(__FILE__, __LINE__)
 #define shenandoah_assert_control_or_vm_thread() \
                     assert(Thread::current()->is_VM_thread() || Thread::current() == ShenandoahHeap::heap()->control_thread(), "Expected control thread or vm thread")
+// A stronger version of the above that checks that we are at a safepoint if the vm thread
+#define shenandoah_assert_control_or_vm_thread_at_safepoint() \
+                    assert(Thread::current() == ShenandoahHeap::heap()->control_thread() || (SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread()), \
+                    "Expected control thread, or vm thread at a safepoint") 
 
 #define shenandoah_assert_generational() \
                     assert(ShenandoahHeap::heap()->mode()->is_generational(), "Must be in generational mode here.")
@@ -236,6 +240,7 @@
 #define shenandoah_assert_heaplocked_or_safepoint()
 #define shenandoah_assert_heaplocked_or_fullgc_safepoint()
 #define shenandoah_assert_control_or_vm_thread()
+#define shenandoah_assert_control_or_vm_thread_at_safepoint()
 #define shenandoah_assert_generational()
 #define shenandoah_assert_generations_reconciled()                                                             \
 

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

@@ -1574,10 +1574,8 @@ void ShenandoahHeap::print_tracing_info() const {
 }
 
 void ShenandoahHeap::set_gc_generation(ShenandoahGeneration* generation) {
+  shenandoah_assert_control_or_vm_thread_at_safepoint();
   _gc_generation = generation;
-  assert(Thread::current() == ShenandoahController::thread() ||
-         (SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread()),
-         "Unexpected thread or condition");
 }
 
 // Active generation may only be set by the VM thread at a safepoint.