Messages in this thread | | | Date | Fri, 17 Feb 2012 13:08:00 -0500 | From | Vivek Goyal <> | Subject | Re: [PATCH 2/9] blkcg: drop unnecessary RCU locking |
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On Fri, Feb 17, 2012 at 09:43:04AM -0800, Tejun Heo wrote: > Hello, > > On Fri, Feb 17, 2012 at 12:28:57PM -0500, Vivek Goyal wrote: > > I am kind of confused that what are the semantics of calling > > blkg_lookup_create(). Given the fact that it traverses the > > blkcg->blkg_list which is rcu protected, so either we should have > > rcu read lock held or we should have blkcg->lock held. > > Modifying blkgs require both blkcg and queue locks,
> so read access can be done holding any lock.
This is the point I am not getting. How blkg_lookup() is safe just under queue lock. What stops freeing up blkg associated with other queues. I thought caller needs to hold rcu_read_lock() also to make sure it can safely compare blkg->q == q and return the blkg belonging to the queue in question.
> > > > Can pre_destroy() and blkio_policy_parse_and_set() make progress in > > parallel for same cgroup(blkcg) but different queue. > > > > If yes, blkg_lookup() might be doing blkg->q == q check and pre_destroy > > might delete that group and free it up. > > And that's why __blkg_release() is RCU free'ing blkgs, no?
Yes. And you are not holding rcu_read_lock() while doing blkg_lookup() in blkio_policy_parse_and_set(). Just queue lock will not be enough?
Thanks Vivek
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