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    SubjectRe: [PATCH UPDATED 11/11] blkcg: unify blkg's for blkcg policies
    On Fri, Feb 03, 2012 at 01:47:19PM -0800, Tejun Heo wrote:
    > On Fri, Feb 03, 2012 at 04:44:35PM -0500, Vivek Goyal wrote:
    > > Ok, looks like now it is needed because blkcg lock will just gurantee that
    > > blkg is around but blkg->pd[plid] can disappear if you are not holding
    > > blkio_list lock (update_root_blkgs).
    > >
    > > I am wondering if we should take blkcg->lock if blkg is on blkcg list and
    > > is being modified in place. That way, once we are switching elevator,
    > > we should be able to shoot down the policy data without taking blkio_list
    > > lock.
    >
    > I think it gotta become something per-queue, not global, and if we
    > make it per-queue, it should be able to live inside queue_lock.

    Hmm... then blkiocg_reset_stats() will run into lock ordering issue. Can't
    hold queue lock inside blkcg lock. I guess you will do some kind of
    locking trick again as you did for io context logic.

    Thanks
    Vivek


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