On Fri, Apr 13, 2012 at 05:33:44PM -0400, Vivek Goyal 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? > > Well, blkcg_deactivate_policy() frees the policy data in a non-rcu > manner. So group is around but policy data is gone. So technically if some > IO submitter does not see the queue bypass flag, he might still try to > access blkg->pd[pol->plid] after being freed. No, we always go through blkg_destroy_all() and each blkg along with any attached policy_data will go through RCU grace period before getting destroyed. It is stupid subtle but nevertheless correct. > Having said that, in this case we are probably fine as blk_release_queue() > is executed after last reference to queue is dropped and no more IO can > come. May be a 2 line comment will help. Yeah, we're guaranteed that by the time blk_release_queue() executes nobody is traversing the queue. Hmmm... right, this is much easier to wrap one's head around. I'll use this explanation in the comment. > BTW, looks like blkio_exit_group_fn() probably is not a good name anymore > as it is not even called when policy is being deactivated. It should > probably be now .blkio_exit_policy_data_fn() or something like that. Heh, I'm brewing mass blkcg API rename patch as we speak. Thanks. -- tejun _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
