Re: [PATCH v2] workingset: ensure memcg is valid for recency check

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Aug 18, 2023 at 10:45:56AM -0700, Yosry Ahmed wrote:
> On Fri, Aug 18, 2023 at 10:35 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > On Fri, Aug 18, 2023 at 07:56:37AM -0700, Yosry Ahmed wrote:
> > > If this happens it seems possible for this to happen:
> > >
> > > cpu #1                                  cpu#2
> > >                                              css_put()
> > >                                              /* css_free_rwork_fn is queued */
> > > rcu_read_lock()
> > > mem_cgroup_from_id()
> > >                                              mem_cgroup_id_remove()
> > > /* access memcg */
> >
> > I don't quite see how that'd possible. IDR uses rcu_assign_pointer()
> > during deletion, which inserts the necessary barriering. My
> > understanding is that this should always be safe:
> >
> >   rcu_read_lock()                 (writer serialization, in this case ref count == 0)
> >   foo = idr_find(x)               idr_remove(x)
> >   if (foo)                        kfree_rcu(foo)
> >     LOAD(foo->bar)
> >   rcu_read_unlock()
> 
> How does a barrier inside IDR removal protect against the memcg being
> freed here though?
> 
> If css_put() is executed out-of-order before mem_cgroup_id_remove(),
> the memcg can be freed even before mem_cgroup_id_remove() is called,
> right?

css_put() can start earlier, but it's not allowed to reorder the rcu
callback that frees past the rcu_assign_pointer() in idr_remove().

This is what RCU and its access primitives guarantees. It ensures that
after "unpublishing" the pointer, all concurrent RCU-protected
accesses to the object have finished, and the memory can be freed.