Messages in this thread | | | Subject | Re: [PATCH scsi-misc-2.6 01/05] scsi: make blk layer set REQ_SOFTBARRIER when a request is dispatched | From | Nick Piggin <> | Date | Wed, 20 Apr 2005 17:58:19 +1000 |
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On Wed, 2005-04-20 at 16:40 +0900, Tejun Heo wrote: > Hello, Jens. > > On Wed, Apr 20, 2005 at 08:30:10AM +0200, Jens Axboe wrote: > > Do it on requeue, please - not on the initial spotting of the request. > > This is the reworked version of the patch. It sets REQ_SOFTBARRIER > in two places - in elv_next_request() on BLKPREP_DEFER and in > blk_requeue_request(). > > Other patches apply cleanly with this patch or the original one and > the end result is the same, so take your pick. :-) >
I'm not sure that you need *either* one.
As far as I'm aware, REQ_SOFTBARRIER is used when feeding requests into the top of the block layer, and is used to guarantee the device driver gets the requests in a specific ordering.
When dealing with the requests at the other end (ie. elevator_next_req_fn, blk_requeue_request), then ordering does not change.
That is - if you call elevator_next_req_fn and don't dequeue the request, then that's the same request you'll get next time.
And blk_requeue_request will push the request back onto the end of the queue in a LIFO manner.
So I think adding barriers, apart from not doing anything, confuses the issue because it suggests there *could* be reordering without them.
Or am I completely wrong? It's been a while since I last got into the code.
-- SUSE Labs, Novell Inc.
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