iOS address book on different queue - ios

Question

Below is some of my code that doesn't block the main thread when loading in contacts to a tableview. It seems to work fine but I am wondering if this will eventually cause some erradic behavior after dispatch_async is called following if(accessGranted). If what I am doing is good practice then maybe this will help some people because I have seen a lot of people with questions about AB with many different answers (probably due to the poor documentation of AB) but if it's not then hopefully someone can tell me what I am doing wrong.
My understanding is that this would be thread safe because it is all being done on the same thread.
CFErrorRef error = nil;
ABAddressBookRef addressBook = ABAddressBookCreateWithOptions(NULL, &error);
__block BOOL accessGranted = NO;
if (ABAddressBookRequestAccessWithCompletion != NULL) { // we're on iOS 6 or later
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
ABAddressBookRequestAccessWithCompletion(addressBook, ^(bool granted, CFErrorRef error) {
accessGranted = granted;
dispatch_semaphore_signal(sema);
});
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
}
else { // we're on iOS 5 or older
accessGranted = YES;
}
if (accessGranted) {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
CFArrayRef contacts = ABAddressBookCopyArrayOfAllPeople(addressBook);
CFMutableArrayRef peopleMutable = CFArrayCreateMutableCopy(kCFAllocatorDefault,
CFArrayGetCount(contacts),
contacts);
CFArraySortValues(peopleMutable,
CFRangeMake(0, CFArrayGetCount(peopleMutable)),
(CFComparatorFunction) ABPersonComparePeopleByName,
kABPersonSortByFirstName);
CFRelease(contacts);
for (CFIndex i = 0; i < CFArrayGetCount(peopleMutable); i++)
{
ABRecordRef record = CFArrayGetValueAtIndex(peopleMutable, i);
ABMultiValueRef phoneNumberProperty = ABRecordCopyValue(record, kABPersonPhoneProperty);
if (CFBridgingRelease(ABMultiValueCopyValueAtIndex(phoneNumberProperty, 0)) != nil) {
NSString *firstName = CFBridgingRelease(ABRecordCopyValue(record, kABPersonFirstNameProperty));
NSString *lastName = CFBridgingRelease(ABRecordCopyValue(record, kABPersonLastNameProperty));
NSString *phoneNumbers = (__bridge NSString*)ABMultiValueCopyValueAtIndex(phoneNumberProperty, 0);
}
CFRelease(phoneNumberProperty);
}
CFRelease(peopleMutable);
dispatch_async(dispatch_get_main_queue(), ^{
// Update UI
[self.tableView reloadData];
});
});
}