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Date: Tue, 31 Jul 2007 11:10:43 +0400
From: Dmitry Monakhov <dmonakhov@openvz.org>
To: linux-kernel@vger.kernel.org
Subject: bi_end_io question
Message-ID: <20070731071043.GA19019@dnb.sw.ru>
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I want implement some sort of snapshot dev.
In order to do this i've replaced original q->make_request_fn
with specific one which cowed all write requests.
My first attempt looks like this:
static int mysnap_make_request(request_queue_t *q, struct bio *bio)
{
	struct bio cl_bio;
	make_request_fn *fn = get_original_make_fn(q);
	if(bio->bi_rw & (1 << BIO_RW){
	/* This is write request, so we have to cow it first*/
		cl_bio = clone_bio(bio);
		submit_bio(READ, &cl_bio);
		wait_for_completion(&complete); 
		save_cowed_bio(&cl_bio);
		/* At this time old data has successfully saved in cow
		 * area so we may safely perform original request */
	}
	/* call original make_request_fn function for bio*/
	return fn(q, bio);
}
But this implementation has significance performance drawback because
of waiting for read request completion.

So i want use a-sync implementation:
static int my_make_original_request(struct bio *bio, unsigned int bytes_done,
                               int err)
{
	if(bio->bi_size == 0) {
		/* restore original request info */
		struct pending_request*  pr = bio->bi_private;
		/* make original write request */
		ret = pr->fn(pr->q, pr->bio);
		/* error handling logic here*/
	}
}
static int mysnap_make_request_v2(request_queue_t *q, struct bio *bio)
{
	struct bio cl_bio;
	make_request_fn *fn = get_original_make_fn(q);
	if(bio->bi_rw & (1 << BIO_RW){
	/* This is write request, so we have to cow it first*/
		cl_bio = clone_bio(bio);
		cl_bio->bi_private = store_request(fn, q, bio)
		cl_bio->bi_end_io = my_make_original_request;
		/* after read request ended original write request 
		 * will be queued */
		submit_bio(READ, &cl_bio, q);
		/* We dont have to wait submitted bio here any more,
		 * because write request will be automaticaly queued
		 * by cl_bio->bi_end_io callcack.
		 * All job has been done at this moment.
		 */
		return 0;
	}
	/* call original make_request_fn function for bio*/
	return fn(q, bio);
}
My question is following:
1)Can i safely call make_request_fn from ->bi_end_io callback 
  (as it done in my_make_original_request function)
2)May be you have any other sound idea?

thank you.