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    SubjectRe: [PATCH] io-controller: Add io group reference handling for request
    Nauman Rafique wrote:
    > On Fri, May 8, 2009 at 6:57 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
    >> On Fri, May 08, 2009 at 05:45:32PM +0800, Gui Jianfeng wrote:
    >>> Hi Vivek,
    >>>
    >>> This patch adds io group reference handling when allocating
    >>> and removing a request.
    >>>
    >> Hi Gui,
    >>
    >> Thanks for the patch. We were thinking that requests can take a reference
    >> on io queues and io queues can take a reference on io groups. That should
    >> make sure that io groups don't go away as long as active requests are
    >> present.
    >>
    >> But there seems to be a small window while allocating the new request
    >> where request gets allocated from a group first and then later it is
    >> mapped to that group and queue is created. IOW, in get_request_wait(),
    >> we allocate a request from a particular group and set rq->rl, then
    >> drop the queue lock and later call elv_set_request() which again maps
    >> the request to the group saves rq->iog and creates new queue. This window
    >> is troublesome because request can be mapped to a particular group at the
    >> time of allocation and during set_request() it can go to a different
    >> group as queue lock was dropped and group might have disappeared.
    >>
    >> In this case probably it might make sense that request also takes a
    >> reference on groups. At the same time it looks too much that request takes
    >> a reference on queue as well as group object. Ideas are welcome on how
    >> to handle it...
    >
    > IMHO a request being allocated on the wrong cgroup should not be a big
    > problem as such. All it means is that the request descriptor was
    > accounted to the wrong cgroup in this particular corner case. Please
    > correct me if I am wrong.
    >
    > We can also get rid of rq->iog pointer too. What that means is that
    > request is associated with ioq (rq->ioq), and we can use
    > ioq_to_io_group() function to get the io_group. So the request would
    > only be indirectly associated with an io_group i.e. the request is
    > associated with an io_queue and the io_group for the request is the
    > io_group associated with io_queue. Do you see any problems with that
    > approach?

    That sounds reasonable to get rid of rq->iog, and rq->rl is also dead.
    Hope to see the patch soon. ;)

    --
    Regards
    Gui Jianfeng



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