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

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Message-ID: <20191114193736.GL4163745@devbig004.ftw2.facebook.com>
Date:   Thu, 14 Nov 2019 11:37:36 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Michal Hocko <mhocko@...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()

Hello,

On Thu, Nov 14, 2019 at 08:33:40PM +0100, Michal Hocko wrote:
> > 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.

It's the same difference as between "opening a file and deleting it"
and "deleting a file and opening it".  We shoud allow the former while
not allowing the latter.

> > 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? 

Yeah, we should audit and convert the uses and update the doc.

Thanks.

-- 
tejun