核心数据、保存到持久存储的线程(不带锁),有什么可能出错?

我的应用程序在后台线程中下载照片并将其缓存到coredata中,同时仍允许用户在应用程序中浏览。当前,在下载完照片的数据后,我启动一个线程,使用建议的共享存储协调器和线程拥有的上下文将其保存到coredata,然后在主线程上合并,我还锁定共享协调器,直到在合并之前。这种锁定会导致用户在读取时出现性能问题。

我需要锁在这里吗?不锁闭的陷阱是什么?该流程基本上是:

-(void)saveThreaded:(args)args {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[NSThread setThreadPriority:.5];

[_appDelegate.persistentStoreCoordinator lock];  //necessary?
NSManagedObjectContext *_moc = [[NSManagedObjectContext alloc] init];
[_moc setPersistentStoreCoordinator: [_appDelegate persistentStoreCoordinator]];

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(threadControllerContextDidSave:)
                                             name:NSManagedObjectContextDidSaveNotification object:_moc];
...<blah blah>...
}

- (void)threadControllerContextDidSave:(NSNotification*)saveNotification {
// need to unlock before we let main thread merge  
[_appDelegate.persistentStoreCoordinator unlock];
[self performSelectorOnMainThread:@selector(mergeToMainContext:) withObject:saveNotification waitUntilDone:YES];
}

- (void)mergeToMainContext:(NSNotification*)saveNotification {
NSError *error;
[_appDelegate.managedObjectContext mergeChangesFromContextDidSaveNotification:saveNotification];  
if (![_appDelegate.managedObjectContext save:&error]) {
<handle error>
}
}