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Subject: Re: [PATCH] mm, memcg: assign shrinker_map before kvfree
To:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Cyrill Gorcunov <gorcunov@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>
References: <20190920122907.GG2507@uranus.lan>
 <20190920132114.ofzphp53vqqjb3fs@box>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <55c22ce3-5e42-f7a1-3861-8e0cb73d26de@virtuozzo.com>
Date:   Fri, 20 Sep 2019 17:20:14 +0300
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On 20.09.2019 16:21, Kirill A. Shutemov wrote:
> On Fri, Sep 20, 2019 at 03:29:07PM +0300, Cyrill Gorcunov wrote:
>> Currently there is a small gap between fetching pointer, calling
>> kvfree and assign its value to nil. In current callgraph it is
>> not a problem (since memcg_free_shrinker_maps is running from
>> memcg_alloc_shrinker_maps and mem_cgroup_css_free only) still
>> this looks suspicious and we can easily eliminate the gap at all.
> 
> With this logic it will still look suspicious since you don't wait a grace
> period before freeing the map.

This freeing occurs in the moment, when nobody can dereference shrinker_map
in parallel:

memcg is either not yet online or its css->refcnt is already dead.
This NULLifying is needed just to prevent double freeing of shrinker_map.

Please, see the explanation in my email to our namesake.

Kirill