device plugging

Why do we need a dummy req for plugging.
As i understood only thing plugging does is to, on arrival of new req if the dev queue is empty, puts a dummy req on the queue, and schedules the unplug routine on tq_disk, which on being scheduled calls the strategy routine.
So we can as well put the new req on the queue without dummy req.

If i'm incorrect please explain what's exact process and the reason

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thanks
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You are reading 2.2 sources, maybe try the 2.4 sources as they are easier to follow in this regard IMHO.

What happens is that we assign dev->current_request &dev->plug, so that the front request is recognizable as the 'plug' and not a valid request. You seem to completely misunderstand plugging -- if we just put the new request on the queue and scheduled tq_disk, then there would be no plugging going on at all. What we do instead is add this specific 'plug request' and queue lots of stuff behind that. Not until someone needs the data on the queue is the 'plug request' removed and the request_fn run. This happens for instance if _someone else_ calls tq_disk, typically __wait_on_buffer.

Wrong

See?

Jens Axboe

Hello,
Can some one provide online pointers for the description and features of linux 2.4 kernel.
The rubini 2nd edn is not avlbl in india and most of the books still concentrate on 2.2.
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