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

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Mar 28, 2022 at 8:58 AM Waiman Long <longman@redhat.com> 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.
>
> Adding a spin_is_locked() check will likely be enough for x86, but it
> is less certain for other arches with a more relaxed memory semantics
> like arcm64 and ppc. To avoid race, this patch moves the nr_items check
> to within the lock critical section.
>
> Fixes: 405cc51fc104 ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
> Signed-off-by: Waiman Long <longman@redhat.com>

How about the following patch?  It is low overhead on x86_64. Even on
relaxed memory mode, I think it is also lower overhead since it avoid a
store operation to nlru->lock.

We do not need to insert a smp_wmb() into the list_lru_add() since
spin_lock() always implies at least a load acquiring semantics.

Thanks.

diff --git a/mm/list_lru.c b/mm/list_lru.c
index c669d87001a6..0e58374b629b 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -397,8 +397,11 @@ static void memcg_reparent_list_lru_node(struct
list_lru *lru, int nid,
        /*
         * If there is no lru entry in this nlru, we can skip it immediately.
         */
-       if (!READ_ONCE(nlru->nr_items))
-               return;
+       if (!READ_ONCE(nlru->nr_items)) {
+               smp_rmb();
+               if (!spin_is_locked(&nlru->lock))
+                       return;
+       }

        /*
         * Since list_lru_{add,del} may be called under an IRQ-safe lock,