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. James _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
