Re: [PATCH 2/2] cgroup: Use separate work structs on css release path

[Tadeusz Struk]

On 6/1/22 17:07, Tejun Heo wrote:
> Hello,
>
> On Wed, Jun 01, 2022 at 05:00:44PM -0700, Tadeusz Struk wrote:
> > What I'm trying to say is that it's not really a ref imbalance problem.
> > I think once the kill_css() has been called on a css, and it is enqueued to be
> > killed, we shouldn't call css_put(css) on it anymore outside of the "killed call
> > flow path". It will all be handled by the css_killed_ref_fn() function.
> >
> > The fact the css_release() is called (via cgroup_kn_unlock()) just after
> > kill_css() causes the css->destroy_work to be enqueued twice on the same WQ
> > (cgroup_destroy_wq), just with different function. This results in the
> > BUG: corrupted list in insert_work issue.
>
> I have a hard time following here. The kill / release paths relationship
> isn't that complicated. The kill path is invoked when the percpu_ref's base
> ref is killed and holds an extra ref so that it's guaranteed that release
> can't happen before the kill path is done with the css. When the final put
> happens - whether that's from the kill path or someone else, which often is
> the case - the release path triggers. If we have release getting scheduled
> while the kill path isn't finished, it is a reference counting problem,
> right?
>
> Can you elaborate the exact scenario that you think is happening? Please
> feel free to omit the function calls and all that. Just lay out who's doing
> what.

Ok the problem is that

1. kill_css() triggers css_killed_ref_fn(), which enqueues &css->destroy_work on cgroup_destroy_wq
2. Last put_css() calls css_release(), which enqueues &css->destroy_work on cgroup_destroy_wq

We have two instances of the same work struct enqueued on the same WQ (cgroup_destroy_wq),
which causes "BUG: corrupted list in insert_work"

So I think the easiest way to solve this would be to have two separate work_structs,
one for the killed_ref path and css_release path as in:

https://lore.kernel.org/all/20220523212724.233314-1-tadeusz.struk@xxxxxxxxxx/

--
Thanks,
Tadeusz