Hi Hannes,
On Tue, 11 Sep 2007 12:44:46 +0200, Hannes Reinecke <[EMAIL PROTECTED]> wrote:
> this is a proposal for a different implementation of request
> callbacks. The existing ->endio callback of a request is actually a
> destructor function, to be called to terminate a request and free all
> structures.
>
> However, on certain occasions (like request-based multipathing) it is
> desirable to have a callback function for a request which is called
> right after the request is finished, ie in end_that_request_first()
> before any bio->bi_endio callback is called.
>
> So a simple solution for this is to clone the request and add a new
> 'marker' bio in front of the bio list of the request. This callback
> will be attached a structure in bi_private which keeps a pointer to
> the cloned and the original request, thus serving as a callback for
> the request itself.
Thank you for another idea for request-based multipath.
However, I disagree with the idea.
I think the design (bio->bi_end_io() completes a request) is complex
a little bit and is breaking layer stacking.
Also, I think that the design of completion handling by 2 hooks,
bio->bi_end_io() and rq->end_io(), makes error handing in dm-multipath
complex.
e.g. Even if we detect an error on the first bio->bi_end_io() hook,
the request can't be retried until the second rq->end_io() hook
is called, because the ownership of the request is still on
low level drivers until the low level driver calls
end_that_request_last().
Although this is just implementation issue, I think that the patch
can't handle leftover.
SCSI, scsi_end_request(), may have some leftovers in the request.
Then, the marker bio is freed in bio->bi_end_io() and the request
is resubmitted in SCSI level. So the resubmitted request
doesn't have the marker bio any more.
Thanks,
Kiyoshi Ueda
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