What would happen if I dispatch_barrier_(a)sync to the queue that target to concurrent queue in GCD?

What would happen if I dispatch_barrier_(a)sync to the queue that target
to concurrent queue in GCD?

I have a question about dispatch_barrier and the target queue. I have a
custom serial queue and custom concurrent queue and I set the target queue
of the serial to concurrent queue
(serial queue) -> (concurrent queue) -> (global concurrent queue)
What happen when I dispatch_barrier blocks on the queue that serial queue?
Will I block the execution of the blocks submitted to the targeted queue
too or just the execution blocks in the serial queue only? Or if I
dispatch_barrier blocks to the concurrent queue, will I block the
execution of the blocks submitted to the targeted queue too or just the
execution blocks in the concurrent queue only?
Thank you for your interesting :)