From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [RFC][PATCH] cgroup_threadgroup_rwsem - affects scalability and
 OOM
Date: Tue, 9 Aug 2016 10:26:03 -0400
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To: Balbir Singh <bsingharora-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, "linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org" <linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>

Hello, Balbir.

On Tue, Aug 09, 2016 at 05:02:47PM +1000, Balbir Singh wrote:
> > Hmm? Where does mem_cgroup_iter grab cgroup_mutex?  cgroup_mutex nests
> > outside cgroup_threadgroup_rwsem or most other mutexes for that matter
> > and isn't exposed from cgroup core.
> > 
> 
> I based my theory on the code
> 
> mem_cgroup_iter -> css_next_descendant_pre which asserts
> 
> cgroup_assert_mutex_or_rcu_locked(), 
> 
> although you are right, we hold RCU lock while calling css_* routines.

That's "or".  The iterator can be called either with RCU lock or
cgroup_mutex.  cgroup core may use it under cgroup_mutex.  Everyone
else uses it with rcu.

Thanks.

-- 
tejun