Re: Use after free with BFQ and cgroups

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On Mon 29-11-21 07:12:42, Tejun Heo wrote:
> On Fri, Nov 26, 2021 at 03:47:24PM +0100, Michal Koutný wrote:
> > The question here is how long would stay the offlined blkcgs around if
> > they were directly pinned upon the IO submission. If it's unbound, then
> > reparenting makes more sense.
> 
> It should be fine to pin whatever's necessary while related IOs are in
> flight and percpu_ref used for css refcnting isn't gonna make any noticeable
> difference in terms of overhead.

Yes, holding cgroup ref from IO would be fine. But that is not really our
problem.

The problem is bfq_queue associated with a task effectively holds a
reference to the potentially dead cgroup and the reference can stay there
until the task (that itself got reparented to the root cgroup) exits. So I
think we need to reparent these bfq_queue structures as well to avoid
holding cgroup in zombie state excessively long.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR



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