Re: [PATCH block/for-3.4/core] cfq: fix cfqg ref handling when BLK_CGROUP && !CFQ_GROUP_IOSCHED

Tejun Heo
2012-03-15T17:18:15Z

On Thu, Mar 15, 2012 at 01:01:00PM -0400, Vivek Goyal wrote:
> On Thu, Mar 15, 2012 at 09:50:57AM -0700, Tejun Heo wrote:
>
> > > [..]
> > > > @@ -3533,7 +3551,7 @@ static int cfq_init_queue(struct request
> > > >
> > > > spin_lock_irq(q->queue_lock);
> > > > cfq_link_cfqq_cfqg(&cfqd->oom_cfqq, cfqd->root_group);
> > > > - blkg_put(cfqg_to_blkg(cfqd->root_group));
> > > > + cfqg_put(cfqd->root_group);
> > >
> > > This seems to be a spurious cfqg_put()? Which reference we are putting
> > > down here?
> >
> > The extra ref from cfq_link_cfqq_cfqg() for oom_cfqq; otherwise, we
> > need an extra cfq_put() in cfq_exit_queue(). I thought I wrote
> > comment about that somewhere. Hmmm.... apparently not. The thing is
> > that oom_cfqq doesn't go through proper cfqq destruction and thus
> > never puts the extra ref to root cfqg.
>
> Ok. Is cfq_exit_queue() a better place to put down this reference
> explicitly with a comment. Even if you keep it here, atleast a comment
> is required. It is not obvious at all (atleast to me).

Ah... the comment actually already is there.

/*
* Our fallback cfqq if cfq_find_alloc_queue() runs into OOM issues.
* Grab a permanent reference to it, so that the normal code flow
* will not attempt to free it. oom_cfqq is linked to root_group
* but shouldn't hold a reference as it'll never be unlinked. Lose
* the reference from linking right away.
*/

--
tejun