[L0] Add compare and swap for setting the LastCommandEvent in all cases

Signed-off-by: Neil R. Spruit <neil.r.spruit@intel.com>

source/adapters/level_zero/queue.cpp

@@ -1696,6 +1696,7 @@ ur_result_t ur_queue_handle_t_::synchronize() {
     // If event is discarded then it can be in reset state or underlying level
     // zero handle can have device scope, so we can't synchronize the last
     // event.
+    auto savedLastCommandEvent = LastCommandEvent;
     if (isInOrderQueue() && !LastCommandEvent->IsDiscarded) {
       ZE2UR_CALL(zeHostSynchronize, (LastCommandEvent->ZeEvent));
 
@@ -1737,7 +1738,14 @@ ur_result_t ur_queue_handle_t_::synchronize() {
         }
       }
     }
-    LastCommandEvent = nullptr;
+    // If the current version of the LastCommandEvent == savedLastCommandEvent,
+    // then LastCommandEvent = nullptr; Otherwise, if LastCommandEvent !=
+    // savedLastCommandEvent, then LastCommandEvent is unchanged.
+    std::atomic<ur_event_handle_t> LastCommandEventAtomic = LastCommandEvent;
+    if (LastCommandEventAtomic.compare_exchange_strong(savedLastCommandEvent,
+                                                       nullptr)) {
+      LastCommandEvent = LastCommandEventAtomic.load();
+    }
   }
 
   // Since all timestamp recordings should have finished with the