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Date:   Wed, 2 Sep 2020 15:52:58 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Pavel Tatashin <pasha.tatashin@soleen.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>, Roman Gushchin <guro@fb.com>,
        Bharata B Rao <bharata@linux.ibm.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        Yafang Shao <laoar.shao@gmail.com>,
        stable <stable@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sasha Levin <sashal@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v2 00/28] The new cgroup slab memory controller
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On Wed 02-09-20 08:53:49, Pavel Tatashin wrote:
> On Wed, Sep 2, 2020 at 7:32 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Wed 02-09-20 11:53:00, Vlastimil Babka wrote:
> > > >> > > Thread #2: ccs killer kthread
> > > >> > >    css_killed_work_fn
> > > >> > >      cgroup_mutex  <- Grab this Mutex
> > > >> > >      mem_cgroup_css_offline
> > > >> > >        memcg_offline_kmem.part
> > > >> > >           memcg_deactivate_kmem_caches
> > > >> > >             get_online_mems
> > > >> > >               mem_hotplug_lock <- waits for Thread#1 to get read access
> >
> > And one more thing. THis has been brought up several times already.
> > Maybe I have forgoten but why do we take hotplug locks in this path in
> > the first place? Memory hotplug notifier takes slab_mutex so this
> > shouldn't be really needed.
> 
> Good point, it seems this lock can be completely removed from
> memcg_deactivate_kmem_caches

I am pretty sure we have discussed that in the past. But I do not
remember the outcome. Either we have concluded that this is indeed the
case but nobody came up with a patch or we have hit some obscure
issue... Maybe David/Roman rememeber more than I do.

-- 
Michal Hocko
SUSE Labs