Core data, threads saving to a persistant store w/out locking, whats could go wrong?

Question

My application downloads and caches photos into coreData in background threads while still allowing the user to poke around in the application. Currently after a photo's data is done downloading I start a thread to save it to coredata using the suggested shared storeCoordinator and thread-owned context then merge on the main thread, I also lock the shared coordinator until just before merging. This locking causes performance issues though on reads by the user.

Do I need to lock here? What are the pitfalls of not locking? The flow is essentially:

-(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>
}
}

Answer 1

No you do not need a lock here, the NSManagedObjectContext will lock the NSPersistentStoreCoordinator and you are probably hindering that.