On Wed, Mar 23, 2022 at 09:06:03AM +0800, Muchun Song wrote:
> On Wed, Mar 9, 2022 at 10:40 PM Waiman Long <longman@xxxxxxxxxx> wrote:
> >
> > Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node()
> > to be race free"), we are tracking the total number of lru
> > entries in a list_lru_node in its nr_items field. In the case of
> > memcg_reparent_list_lru_node(), there is nothing to be done if nr_items
> > is 0. We don't even need to take the nlru->lock as no new lru entry
> > could be added by a racing list_lru_add() to the draining src_idx memcg
> > at this point.
>
> Hi Waiman,
>
> Sorry for the late reply. Quick question: what if there is an inflight
> list_lru_add()? How about the following race?
>
> CPU0: CPU1:
> list_lru_add()
> spin_lock(&nlru->lock)
> l = list_lru_from_kmem(memcg)
> memcg_reparent_objcgs(memcg)
> memcg_reparent_list_lrus(memcg)
> memcg_reparent_list_lru()
> memcg_reparent_list_lru_node()
> if (!READ_ONCE(nlru->nr_items))
> // Miss reparenting
> return
> // Assume 0->1
> l->nr_items++
> // Assume 0->1
> nlru->nr_items++
>
> IIUC, we use nlru->lock to serialise this scenario.
Thank you for bringing this up, really cool race!
