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From:   Roman Gushchin <roman.gushchin@linux.dev>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Waiman Long <longman@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH-mm v3] mm/list_lru: Optimize
 memcg_reparent_list_lru_node()
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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@redhat.com> 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!