2012/6/5  [off-list ref]:

If you ever tried to implement unplug function in umem, you would find the code was almost identical as in dm/raid driver.
For other components which also need such unplug mechanism, it will much more convenient to have such facilities.

It's just several lines of code and your usage actually doesn't need
the counter.
It's unclear to me your usage requires to move the callback to list head.

PS: What's your specific concern about blk_plug_cb handles each request_queue?

the blk_plug_cb stores request_queue, which is incorrect to me. Why can't
one plug cb unplug two or more request_queues?