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SubjectRe: [PATCH] mm/list_lru: Fix possible race in memcg_reparent_list_lru_node()
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On 3/27/22 20:57, 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.
>
> 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()")

Sorry, I should have added

Reported-by: Muchun Song <songmuchun@bytedance.com>

> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> mm/list_lru.c | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index c669d87001a6..8aec8ebd5995 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -394,18 +394,18 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid,
> int dst_idx = dst_memcg->kmemcg_id;
> struct list_lru_one *src, *dst;
>
> - /*
> - * If there is no lru entry in this nlru, we can skip it immediately.
> - */
> - if (!READ_ONCE(nlru->nr_items))
> - return;
> -
> /*
> * Since list_lru_{add,del} may be called under an IRQ-safe lock,
> * we have to use IRQ-safe primitives here to avoid deadlock.
> */
> spin_lock_irq(&nlru->lock);
>
> + /*
> + * If there is no lru entry in this nlru, we can skip it immediately.
> + */
> + if (!nlru->nr_items)
> + goto out;
> +
> src = list_lru_from_memcg_idx(lru, nid, src_idx);
> if (!src)
> goto out;
Cheers,
Longman

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