8367646: [GenShen] Control thread may overwrite gc cancellation cause set by mutator

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

@@ -110,10 +110,9 @@ void ShenandoahGenerationalControlThread::check_for_request(ShenandoahGCRequest&
     // failure (degenerated cycle), or old marking was cancelled to run a young collection.
     // In either case, the correct generation for the next cycle can be determined by
     // the cancellation cause.
-    request.cause = _heap->cancelled_cause();
+    request.cause = _heap->clear_cancellation(GCCause::_shenandoah_concurrent_gc);
     if (request.cause == GCCause::_shenandoah_concurrent_gc) {
       request.generation = _heap->young_generation();
-      _heap->clear_cancelled_gc(false);
     }
   } else {
     request.cause = _requested_gc_cause;

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

@@ -460,6 +460,10 @@ class ShenandoahHeap : public CollectedHeap {
   // old mark does _not_ touch the oom handler).
   inline void clear_cancelled_gc(bool clear_oom_handler = true);
 
+  // Clears the cancellation cause iff the current cancellation reason equals the given
+  // expected cancellation cause. Does not reset the oom handler.
+  inline GCCause::Cause clear_cancellation(GCCause::Cause expected);
+
   void cancel_concurrent_mark();
 
   // Returns true if and only if this call caused a gc to be cancelled.

src/hotspot/share/gc/shenandoah/shenandoahHeap.inline.hpp

@@ -31,6 +31,7 @@
 
 #include "classfile/javaClasses.inline.hpp"
 #include "gc/shared/continuationGCSupport.inline.hpp"
+#include "gc/shared/gcCause.hpp"
 #include "gc/shared/markBitMap.inline.hpp"
 #include "gc/shared/suspendibleThreadSet.hpp"
 #include "gc/shared/threadLocalAllocBuffer.inline.hpp"
@@ -280,6 +281,10 @@ inline void ShenandoahHeap::clear_cancelled_gc(bool clear_oom_handler) {
   }
 }
 
+inline GCCause::Cause ShenandoahHeap::clear_cancellation(GCCause::Cause expected) {
+  return _cancelled_gc.cmpxchg(GCCause::_no_gc, expected);
+}
+
 inline HeapWord* ShenandoahHeap::allocate_from_gclab(Thread* thread, size_t size) {
   assert(UseTLAB, "TLABs should be enabled");