8267953: restore 'volatile' to ObjectMonitor::_owner field

Reviewed-by: aph

src/hotspot/share/runtime/objectMonitor.hpp

@@ -147,13 +147,13 @@ class ObjectMonitor : public CHeapObj<mtInternal> {
                         sizeof(WeakHandle));
   // Used by async deflation as a marker in the _owner field:
   #define DEFLATER_MARKER reinterpret_cast<void*>(-1)
-  void* _owner;                     // pointer to owning thread OR BasicLock
+  void* volatile _owner;            // pointer to owning thread OR BasicLock
   volatile jlong _previous_owner_tid;  // thread id of the previous owner of the monitor
   // Separate _owner and _next_om on different cache lines since
   // both can have busy multi-threaded access. _previous_owner_tid is only
   // changed by ObjectMonitor::exit() so it is a good choice to share the
   // cache line with _owner.
-  DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(void*) +
+  DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(void* volatile) +
                         sizeof(volatile jlong));
   ObjectMonitor* _next_om;          // Next ObjectMonitor* linkage
   volatile intx _recursions;        // recursion count, 0 for first entry