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    SubjectRe: [PATCH 07/11] blkcg: make request_queue bypassing on allocation
    On Fri, 2012-04-13 at 14:16 -0700, Tejun Heo wrote:
    > On Fri, Apr 13, 2012 at 02:05:48PM -0700, Tejun Heo wrote:
    > > On Fri, Apr 13, 2012 at 04:55:01PM -0400, Vivek Goyal wrote:
    > > > But neither seems to be the case here. So to make sure that blkg_lookup()
    > > > under rcu will see the updated value of queue flag (bypass), are we
    > > > relying on the fact that caller should see the DEAD flag and not go
    > > > ahead with blkg_lookup()? If yes, atleast it is not obivious.
    > >
    > > We're relying on the fact that it doesn't matter anymore because all
    > > blkgs will be shoot down in queue cleanup path which goes through rcu
    > > free, which is different from deactivating individual policies. It
    > > indeed is subtle. Umm... this is starting to get ridiculous. Why the
    > > hell was megaraid messing with so many queues anyways?
    > I suppose megaraid depends on sequential LUN scan which SCSI
    > implements by creating sdev for each LUN, trying to see whether it
    > actually exists and then destroys the sdev if not. Urgh.... so, we
    > seem to be stuck with it.

    Right, sorry ... it's not just megaraid, it's any SCSI-2 device. The
    standard says we have to probe the LUNs one at a time to see if they're
    there. SCSI-3 on supports the REPORT LUNS command which just returns a
    list which obviates the need to probe on every one but not all older
    (and USB to be frank) devices support this.

    > So, the current code is technically correct although subtle like hell.
    > We can RCU defer blk_put_queue() from blk_cleanup_queue() using
    > call_rcu() to make clear that RCU grace period is necessary there.
    > Any better ideas?

    Not really ... except that perhaps we might redo LUN scanning to use
    just a single queue, so repurpose the LUN underneath, but not destroy
    the old queue and setup the new one? It's a bit counter intuitive, but
    it shouldn't be impossible.


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