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    SubjectRe: [PATCH 14/16] blkcg: don't allow or retain configuration of missing devices
    On Tue, Jan 24, 2012 at 07:53:09AM -0800, Tejun Heo wrote:
    > On Tue, Jan 24, 2012 at 10:42:24AM -0500, Vivek Goyal wrote:
    > > On Mon, Jan 23, 2012 at 03:09:51PM -0800, Tejun Heo wrote:
    > > > + disk = get_gendisk(dev, &part);
    > > > +
    > > > + if ((!disk || part) && temp) {
    > > > + ret = -ENODEV;
    > > > + goto out;
    > > > + }
    > > > +
    > > > + rcu_read_lock();
    > > > +
    > > > + if (disk && !part) {
    > > > + spin_lock_irq(disk->queue->queue_lock);
    > > > + blkg = blkg_lookup_create(blkcg, disk->queue, plid);
    > > > + spin_unlock_irq(disk->queue->queue_lock);
    > >
    > > If queue is in bypass mode, is group creation and linking allowed when
    > > somebody is trying to set per device rules. (/me is thinking if there are
    > > any potential races between elevator switch and rule setting in cgroup).
    >
    > Yeap, there is. Nice catch. blkg_lookup_create() should be testing
    > blk_queue_bypass() instead of blk_queue_dead() and parse_and_set
    > should probably retry after a bit. Will update.

    So now group creation fails if queue is in bypass mode. So a per device
    rule setting will fail both for cfq and throttling logic while device
    is still around. So a user is supposed to retry? How would a user know
    that elevator switch is happening and he should retry and till when should
    he retry.

    Thanks
    Vivek


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