Re: [PATCH scsi-misc-2.6 01/05] scsi: make blk layer set REQ_SOFTBARRIER when a request is dispatched

Jens Axboe
2005-04-20T06:30:55Z

On Wed, Apr 20 2005, Tejun Heo wrote:
> 01_scsi_blk_make_started_requests_ordered.patch
>
> Reordering already started requests is without any real
> benefit and causes problems if the request has its
> driver-specific resources allocated (as in SCSI). This patch
> makes elv_next_request() set REQ_SOFTBARRIER automatically
> when a request is dispatched.
>
> As both as and cfq schedulers don't allow passing requeued
> requests, the only behavior change is that requests deferred
> by prep_fn won't be passed by other requests. This change
> shouldn't cause any problem. The only affected driver other
> than SCSI is i2o_block.
>
> Signed-off-by: Tejun Heo <htejun@gmail.com>
>
> elevator.c | 8 ++++----
> 1 files changed, 4 insertions(+), 4 deletions(-)
>
> Index: scsi-reqfn-export/drivers/block/elevator.c
> ===================================================================
> --- scsi-reqfn-export.orig/drivers/block/elevator.c 2005-04-20 08:13:01.000000000 +0900
> +++ scsi-reqfn-export/drivers/block/elevator.c 2005-04-20 08:13:33.000000000 +0900
> @@ -370,11 +370,11 @@ struct request *elv_next_request(request
>
> while ((rq = __elv_next_request(q)) != NULL) {
> /*
> - * just mark as started even if we don't start it, a request
> - * that has been delayed should not be passed by new incoming
> - * requests
> + * just mark as started even if we don't start it.
> + * also, as a request that has been delayed should not
> + * be passed by new incoming requests, set softbarrier.
> */
> - rq->flags |= REQ_STARTED;
> + rq->flags |= REQ_STARTED | REQ_SOFTBARRIER;
>
> if (rq == q->last_merge)
> q->last_merge = NULL;

Do it on requeue, please - not on the initial spotting of the request.

--
Jens Axboe