Re: [PATCH v2] mm/list_lru: Fix possible race in memcg_reparent_list_lru_node()

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, Mar 30, 2022 at 01:26:46PM -0400, Waiman Long wrote:
> Muchun Song found out there could be a race between list_lru_add()
> and memcg_reparent_list_lru_node() causing the later function to miss
> reparenting of a lru entry as shown below:
> 
> 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++
> 
> Though it is not likely that a list_lru_node that has 0 item suddenly
> has a newly added lru entry at the end of its life. The race is still
> theoretically possible.
> 
> With the lock/unlock pair used within the percpu_ref_kill() which is
> the last function call of memcg_reparent_objcgs(), any read issued
> in memcg_reparent_list_lru_node() will not be reordered before the
> reparenting of objcgs.
> 
> Adding a !spin_is_locked()/smp_rmb()/!READ_ONCE(nlru->nr_items) check
> to ensure that either the reading of nr_items is valid or the racing
> list_lru_add() will see the reparented objcg.
> 
> Fixes: 405cc51fc104 ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
> Reported-by: Muchun Song <songmuchun@bytedance.com>
> Signed-off-by: Waiman Long <longman@redhat.com>

Acked-by: Roman Gushchin <roman.gushchin@linux.dev>