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Date:   Fri, 1 Apr 2022 13:28:32 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Shakeel Butt <shakeelb@google.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Waiman Long <longman@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Muchun Song <songmuchun@bytedance.com>
Subject: Re: [PATCH v2] mm/list_lru: Fix possible race in
 memcg_reparent_list_lru_node()
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On Thu 31-03-22 18:11:26, Andrew Morton wrote:
[...]
> Yup.  I did this:
> 
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: mm/list_lru.c: revert "mm/list_lru: optimize memcg_reparent_list_lru_node()"
> 
> 405cc51fc1049c73 ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
> has subtle races which are proving ugly to fix.  Revert the original
> optimization.  If quantitative testing indicates that we have a
> significant problem here then other implementations can be looked at.
> 
> Fixes: 405cc51fc1049c73 ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
> Cc: Waiman Long <longman@redhat.com>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Muchun Song <songmuchun@bytedance.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Shakeel Butt <shakeelb@google.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
> 
>  mm/list_lru.c |    6 ------
>  1 file changed, 6 deletions(-)
> 
> --- a/mm/list_lru.c~revert-1
> +++ a/mm/list_lru.c
> @@ -395,12 +395,6 @@ static void memcg_reparent_list_lru_node
>  	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.
>  	 */
> _

-- 
Michal Hocko
SUSE Labs