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    SubjectRe: [PATCH 05/10] block: remove per-queue plugging
    On Mon, 18 Apr 2011 08:38:24 +0200 Jens Axboe <jaxboe@fusionio.com> wrote:

    > On 2011-04-18 00:19, NeilBrown wrote:
    > > On Mon, 11 Apr 2011 14:11:58 +0200 Jens Axboe <jaxboe@fusionio.com> wrote:
    > >
    > >>> Yes. But I need to know when to release the requests that I have stored.
    > >>> I need to know when ->write_pages or ->read_pages or whatever has finished
    > >>> submitting a pile of pages so that I can start processing the request that I
    > >>> have put aside. So I need a callback from blk_finish_plug.
    > >>
    > >> OK fair enough, I'll add your callback patch.
    > >>
    > >
    > > But you didn't did you? You added a completely different patch which is
    > > completely pointless.
    > > If you don't like my patch I would really prefer you said so rather than
    > > silently replace it with something completely different (and broken).
    >
    > First of all, you were CC'ed on all that discussion, yet didn't speak up
    > until now. This was last week. Secondly, please change your tone.

    Yes, I was CC'ed on a discussion. In that discussion it was never mentioned
    that you had completely changed the patch I sent you, and it never contained
    the new patch in-line for review. Nothing that was discussed was
    particularly relevant to md's needs so there was nothing to speak up about.

    Yes- there were 'git pull' requests and I could have done a pull myself to
    review the code but there seemed to be no urgency because you had already
    agreed to apply my patch.
    When I did finally pull the patches (after all the other issues had settle
    down and I had time to finish of the RAID side) I found ... what I found.

    I apologise for my tone, but I was very frustrated.

    >
    > > I'll try to explain again.
    > >
    > > md does not use __make_request. At all.
    > > md does not use 'struct request'. At all.
    > >
    > > The 'list' in 'struct blk_plug' is a list of 'struct request'.
    >
    > I'm well aware of how these facts, but thanks for bringing it up.
    >
    > > Therefore md cannot put anything useful on the list in 'struct blk_plug'.
    > >
    > > So when blk_flush_plug_list calls queue_unplugged() on a queue that belonged
    > > to a request found on the blk_plug list, that queue cannot possibly ever be
    > > for an 'md' device (because no 'struct request' ever belongs to an md device,
    > > because md doesn't not use 'struct request').
    > >
    > > So your patch (commit f75664570d8b) doesn't help MD at all.
    > >
    > > For md, I need to attach something to blk_plug which somehow identifies an md
    > > device, so that blk_finish_plug can get to that device and let it unplug.
    > > The most sensible thing to have is a completely generic callback. That way
    > > different block devices (which choose not to use __make_request) can attach
    > > different sorts of things to blk_plug.
    > >
    > > So can we please have my original patch applied? (Revised version using
    > > list_splice_init included below).
    > >
    > > Or if not, a clear explanation of why not?
    >
    > So correct me if I'm wrong here, but the _only_ real difference between
    > this patch and the current code in the tree, is the checking of the
    > callback list indicating a need to flush the callbacks. And that's
    > definitely an oversight. It should be functionally equivelant if md
    > would just flag this need to get a callback, eg instead of queueing a
    > callback on the list, just set plug->need_unplug from md instead of
    > queuing a callback and have blk_needs_flush_plug() do:
    >
    > return plug && (!list_empty(&plug->list) || plug->need_unplug);
    >
    > instead. Something like the below, completely untested.
    >

    No, that is not the only real difference.

    The real difference is that in the current code, md has no way to register
    anything with a blk_plug because you can only register a 'struct request' on a
    blk_plug, and md doesn't make any use of 'struct request'.

    As I said in the Email you quote above:

    > > Therefore md cannot put anything useful on the list in 'struct blk_plug'.

    That is the heart of the problem.

    NeilBrown




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