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Commit 32fab448 authored by Kiyoshi Ueda's avatar Kiyoshi Ueda Committed by Jens Axboe
Browse files

block: add request update interface 

This patch adds blk_update_request(), which updates struct request
with completing its data part, but doesn't complete the struct
request itself.
Though it looks like end_that_request_first() of older kernels,
blk_update_request() should be used only by request stacking drivers.

Request-based dm will use it in bio->bi_end_io callback to update
the original request when a data part of a cloned request completes.
Followings are additional background information of why request-based
dm needs this interface.

  - Request stacking drivers can't use blk_end_request() directly from
    the lower driver's completion context (bio->bi_end_io or rq->end_io),
    because some device drivers (e.g. ide) may try to complete
    their request with queue lock held, and it may cause deadlock.
    See below for detailed description of possible deadlock:
    <http://marc.info/?l=linux-kernel&m=120311479108569&w=2

>

  - To solve that, request-based dm offloads the completion of
    cloned struct request to softirq context (i.e. using
    blk_complete_request() from rq->end_io).

  - Though it is possible to use the same solution from bio->bi_end_io,
    it will delay the notification of bio completion to the original
    submitter.  Also, it will cause inefficient partial completion,
    because the lower driver can't perform the cloned request anymore
    and request-based dm needs to requeue and redispatch it to
    the lower driver again later.  That's not good.

  - So request-based dm needs blk_update_request() to perform the bio
    completion in the lower driver's completion context, which is more
    efficient.

Signed-off-by: default avatarKiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: default avatarJun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
parent e3335de9

block/blk-core.c

+48 −9
Original line number Diff line number Diff line
@@ -1806,6 +1806,22 @@ void end_request(struct request *req, int uptodate) 
}

EXPORT_SYMBOL(end_request);



static int end_that_request_data(struct request *rq, int error,

				 unsigned int nr_bytes, unsigned int bidi_bytes)

{

	if (rq->bio) {

		if (__end_that_request_first(rq, error, nr_bytes))

			return 1;



		/* Bidi request must be completed as a whole */

		if (blk_bidi_rq(rq) &&

		    __end_that_request_first(rq->next_rq, error, bidi_bytes))

			return 1;

	}



	return 0;

}



/**

 * blk_end_io - Generic end_io function to complete a request.

 * @rq:           the request being processed
@@ -1832,16 +1848,9 @@ static int blk_end_io(struct request *rq, int error, unsigned int nr_bytes, 
	struct request_queue *q = rq->q;

	unsigned long flags = 0UL;



	if (rq->bio) {

		if (__end_that_request_first(rq, error, nr_bytes))

	if (end_that_request_data(rq, error, nr_bytes, bidi_bytes))

		return 1;



		/* Bidi request must be completed as a whole */

		if (blk_bidi_rq(rq) &&

		    __end_that_request_first(rq->next_rq, error, bidi_bytes))

			return 1;

	}



	/* Special feature for tricky drivers */

	if (drv_callback && drv_callback(rq))

		return 1;
@@ -1922,6 +1931,36 @@ int blk_end_bidi_request(struct request *rq, int error, unsigned int nr_bytes, 
}

EXPORT_SYMBOL_GPL(blk_end_bidi_request);



/**

 * blk_update_request - Special helper function for request stacking drivers

 * @rq:           the request being processed

 * @error:        %0 for success, < %0 for error

 * @nr_bytes:     number of bytes to complete @rq

 *

 * Description:

 *     Ends I/O on a number of bytes attached to @rq, but doesn't complete

 *     the request structure even if @rq doesn't have leftover.

 *     If @rq has leftover, sets it up for the next range of segments.

 *

 *     This special helper function is only for request stacking drivers

 *     (e.g. request-based dm) so that they can handle partial completion.

 *     Actual device drivers should use blk_end_request instead.

 */

void blk_update_request(struct request *rq, int error, unsigned int nr_bytes)

{

	if (!end_that_request_data(rq, error, nr_bytes, 0)) {

		/*

		 * These members are not updated in end_that_request_data()

		 * when all bios are completed.

		 * Update them so that the request stacking driver can find

		 * how many bytes remain in the request later.

		 */

		rq->nr_sectors = rq->hard_nr_sectors = 0;

		rq->current_nr_sectors = rq->hard_cur_sectors = 0;

	}

}

EXPORT_SYMBOL_GPL(blk_update_request);



/**

 * blk_end_request_callback - Special helper function for tricky drivers

 * @rq:           the request being processed

include/linux/blkdev.h

+2 −0
Original line number Diff line number Diff line
@@ -791,6 +791,8 @@ extern void blk_complete_request(struct request *); 
extern void __blk_complete_request(struct request *);

extern void blk_abort_request(struct request *);

extern void blk_abort_queue(struct request_queue *);

extern void blk_update_request(struct request *rq, int error,

			       unsigned int nr_bytes);



/*

 * blk_end_request() takes bytes instead of sectors as a complete size.