Fix PowerPC specific issues

 -Replace do_{_if_(condition)_{_..._continute;_}_}while_{false}; with
  for(;;)_{_if_(condition)_{...}_else{break;_}_} for an infinite loop
  with a break logic to handle waiting concurrent gc finish during
  continuation mounting. compiler might generate wrong logic for
  do_while infinite loop.

 -fix small hole during mounting synchronization with concurrent
  scanning(for the case concurrent scanning start just before resetting
  pendingmount flag-before reading the value of state for atomic update)

- new assertion check for catching issue earlier.

signed-off-by: hulin <[email protected]>

runtime/vm/ContinuationHelpers.cpp

@@ -152,6 +152,7 @@ synchronizeWithConcurrentGCScan(J9VMThread *currentThread, j9object_t continuati
 {
 	ContinuationState oldContinuationState = 0;
 	ContinuationState returnContinuationState = 0;
+
 	/* Set pending state and carrier id, GC need to distinguish between fully mounted (don't scan) vs pending to be mounted (do scan) cases. */
 	do {
 		oldContinuationState = *continuationStatePtr;
@@ -167,10 +168,11 @@ synchronizeWithConcurrentGCScan(J9VMThread *currentThread, j9object_t continuati
 	 * but only up to small finite number of times.
 	 */
 	do {
-		if (VM_ContinuationHelpers::isConcurrentlyScanned(returnContinuationState)) {
+		oldContinuationState = *continuationStatePtr;
+		if (VM_ContinuationHelpers::isConcurrentlyScanned(oldContinuationState)) {
 			omrthread_monitor_enter(currentThread->publicFlagsMutex);
 			/* Wait till potentially concurrent GC scans are complete */
-			do {
+			for(;;) {
 				oldContinuationState = *continuationStatePtr;
 				if (VM_ContinuationHelpers::isConcurrentlyScanned(oldContinuationState)) {
 					PUSH_OBJECT_IN_SPECIAL_FRAME(currentThread, continuationObject);
@@ -183,20 +185,21 @@ synchronizeWithConcurrentGCScan(J9VMThread *currentThread, j9object_t continuati
 					continuationObject = POP_OBJECT_IN_SPECIAL_FRAME(currentThread);
 					/* Object might have moved - update its address and the address of the state slot. */
 					continuationStatePtr = VM_ContinuationHelpers::getContinuationStateAddress(currentThread, continuationObject);
-					continue;
+				} else {
+					break;
 				}
-			} while (false);
+			}
 			omrthread_monitor_exit(currentThread->publicFlagsMutex);
 		}
 		/* Remove pending state */
-		oldContinuationState = *continuationStatePtr;
 		Assert_VM_true(VM_ContinuationHelpers::isMountedWithCarrierThread(oldContinuationState, currentThread));
 		Assert_VM_true(VM_ContinuationHelpers::isPendingToBeMounted(oldContinuationState));
 		ContinuationState newContinuationState = oldContinuationState;
 		VM_ContinuationHelpers::resetPendingState(&newContinuationState);
 		returnContinuationState = VM_AtomicSupport::lockCompareExchange(continuationStatePtr, oldContinuationState, newContinuationState);
 	} while (oldContinuationState != returnContinuationState);
 	Assert_VM_false(VM_ContinuationHelpers::isPendingToBeMounted(*continuationStatePtr));
+	Assert_VM_false(VM_ContinuationHelpers::isConcurrentlyScanned(*continuationStatePtr));
 
 	return continuationObject;
 }