linux-kernel - Re: [PATCH] mm, memcg: assign shrinker

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Date:   Fri, 20 Sep 2019 16:40:06 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     LKML <linux-kernel@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: Re: [PATCH] mm, memcg: assign shrinker_map before kvfree

On Fri, Sep 20, 2019 at 04:21:14PM +0300, 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.

Probably, but as far as I see we're using mutex here to order
requests. I'm not sure, maybe ktkhai@ made the code to use free
before the assign intentionally? As I said there is no bug it
the code right now just forced me to stop and reread it several
times due to this gap. If you look into other code places where
we use similar technique we always assign before free.