dispatch_barrier_async

dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(queue, ^{
    NSLog(@"Before Barrier Block - 1");
});
dispatch_async(queue, ^{
    NSLog(@"Before Barrier Block - 2");
});
dispatch_async(queue, ^{
    NSLog(@"Before Barrier Block - 3");
});
dispatch_async(queue, ^{
    NSLog(@"Before Barrier Block - 4");
});
dispatch_async(queue, ^{
    NSLog(@"Before Barrier Block - 5");
});

dispatch_barrier_async(queue, ^{
    NSLog(@"Barrier Block");
});

dispatch_async(queue, ^{
    NSLog(@"After Barrier Block - 1");
});
dispatch_async(queue, ^{
    NSLog(@"After Barrier Block - 2");
});
dispatch_async(queue, ^{
    NSLog(@"After Barrier Block - 3");
});
dispatch_async(queue, ^{
    NSLog(@"After Barrier Block - 4");
});
dispatch_async(queue, ^{
    NSLog(@"After Barrier Block - 5");
}); 

Before Barrier Block - 1
Before Barrier Block - 2
Before Barrier Block - 3
Before Barrier Block - 5
Before Barrier Block - 4
Barrier Block
After Barrier Block - 1
After Barrier Block - 3
After Barrier Block - 2
After Barrier Block - 5
After Barrier Block - 4