linux-kernel - Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from

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Message-ID: <20191114193340.GA24848@dhcp22.suse.cz>
Date:   Thu, 14 Nov 2019 20:33:40 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Roman Gushchin <guro@...com>,
        Michal Koutný <mkoutny@...e.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in
 get_mem_cgroup_from_mm()

On Thu 14-11-19 11:20:18, Tejun Heo wrote:
> Hello,
> 
> On Thu, Nov 14, 2019 at 08:16:57PM +0100, Michal Hocko wrote:
> > Then what is the point of this function and what about all other users?
> 
> It is useful for controlling admissions of new userspace visible uses
> - e.g. a tracepoint shouldn't be allowed to be attached to a cgroup
> which has already been deleted.

I am not sure I understand. Roman says that the cgroup can get offline
right after the function returns. How is "already deleted" different
from "just deleted"? I thought that the state is preserved at least
while the rcu lock is held but my memory is dim here.

> We're just using it too liberally.

Can we get a doc update to be explicit about sensible usecases so that
others can be dropped accordingly? 
-- 
Michal Hocko
SUSE Labs