Dispatch to a serial queue without deadlocking

gistfile1.m

static const char *kMyQueueSentinel = "🐔";

- (instancetype)init {
  ...
  _serialQueue = dispatch_queue_crete("com.my.serialqueue", DISPATCH_QUEUE_SERIAL);
  dispatch_queue_set_specific(self.serialQueue, kMyQueueSentinel, (void *)1, NULL);
  ...
}

static inline void _performOnSerialQueue(dispatch_block_t myBlock) {
    if (dispatch_get_specific(kMyQueueSentinel) != NULL) {
        // We're already on the serial queue
        myBlock();
    } else {
        dispatch_sync(self.serialQueue, myBlock);
    }
}