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From:   Kirill Tkhai <tkhai@ya.ru>
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To:     Johannes Weiner <hannes@cmpxchg.org>,
        Yang Shi <shy828301@gmail.com>
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References: <20201214223722.232537-1-shy828301@gmail.com>
         <20201214223722.232537-4-shy828301@gmail.com> <20201215123802.GA379720@cmpxchg.org>
Subject: Re: [v2 PATCH 3/9] mm: vmscan: guarantee shrinker_slab_memcg() sees valid shrinker_maps for online memcg
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Date:   Tue, 15 Dec 2020 15:58:06 +0300
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15.12.2020, 15:40, "Johannes Weiner" <hannes@cmpxchg.org>:
> On Mon, Dec 14, 2020 at 02:37:16PM -0800, Yang Shi wrote:
>>  The shrink_slab_memcg() races with mem_cgroup_css_online(). A visibility of CSS_ONLINE flag
>>  in shrink_slab_memcg()->mem_cgroup_online() does not guarantee that we will see
>>  memcg->nodeinfo[nid]->shrinker_maps != NULL. This may occur because of processor reordering
>>  on !x86.
>>
>>  This seems like the below case:
>>
>>             CPU A CPU B
>>  store shrinker_map load CSS_ONLINE
>>  store CSS_ONLINE load shrinker_map
>>
>>  So the memory ordering could be guaranteed by smp_wmb()/smp_rmb() pair.
>>
>>  The memory barriers pair will guarantee the ordering between shrinker_deferred and CSS_ONLINE
>>  for the following patches as well.
>>
>>  Signed-off-by: Yang Shi <shy828301@gmail.com>
>
> As per previous feedback, please move the misplaced shrinker
> allocation callback from .css_online to .css_alloc. This will get you
> the necessary ordering guarantees from the cgroup core code.


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