Protecting critical code from being called again

Question

I need to protect a critical area of my code, which is multi-threaded. I want to prevent it from being called multiple times before the other thread is finished. This is what I am working with:

- (void) filterAllEventsIntoDictionary{

    // start critical area
    if (self.sortedKeys.count != 0) {
        [self.sortedKeys removeAllObjects];
    }
    dispatch_async(self.filterMainQueue, ^{

        [self internal_filterAllEventsIntoDictionary]; 

        dispatch_sync(dispatch_get_main_queue(), ^{
            [self.tableView reloadData];
        });
    });
}

Since the internal_filterAllEventsIntoDictionary method also accesses self.sortedKeys, if this code is called twice, it crashes because of removeAllObjects at the start.

I still need to call the internal... method in another thread since I don't want to block the UI. So what's the best way to block on the start of this method while the dispatch_async call is still not finished?

Answer 1

While I am far from being a concurrency expert, it sounds to me like you need a lock on your sortedKeys object. If you used a traditional lock, though, you'd end up blocking the main thread.

The recommended replacement for locks in the world of Grand Central Dispatch is to put critical sections of code on a serial queue. See "Eliminating Lock-Based Code" in the Concurrency Programming Guide.

If you put the [self.sortedKeys removeAllObjects]; call onto the same queue that the block with the internal... call is scheduled on, you guarantee that it won't happen until after that block completes:

// start critical area
dispatch_async(self.filterMainQueue, ^{
    if (self.sortedKeys.count != 0) {
        [self.sortedKeys removeAllObjects];
    }
});

This assumes that filterMainQueue is serial. Using dispatch_async for the critical section ensures that the main thread will not be blocked. Also note the warning in "Dispatch Queues and Thread Safety":

Do not call the dispatch_sync function from a task that is executing on the same queue that you pass to your function call. Doing so will deadlock the queue.

Although this will only be an issue if the internal... method does something that causes this method to be called again.