linux-kernel - Re: [PATCH 2/2] cgroup: Use separate work structs on css release path

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Date:   Wed, 1 Jun 2022 17:00:44 -0700
From:   Tadeusz Struk <tadeusz.struk@...aro.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Michal Koutný <mkoutny@...e.com>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Bui Quang Minh <minhquangbui99@...il.com>
Subject: Re: [PATCH 2/2] cgroup: Use separate work structs on css release path

On 6/1/22 16:43, Tejun Heo wrote:
> On Wed, Jun 01, 2022 at 04:37:17PM -0700, Tadeusz Struk wrote:
>> Yes, but as far as I can see the percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn)
>> doesn't change the value of the refcnt, it just causes the css_killed_ref_fn() to be called
> 
> Yeah, the base ref is special for percpu_ref.
> 
>> on it. Only css_get() & css_put() modify the refcnt value.
>> And for the "free the thing" the css_killed_work_fn() does that.
>> It calls offline_css(css) and css_put(css) for the whole css hierarchy.
> 
> Yeah, the freeing path depends on the css_put(css) invoking css_release()
> which schedules the work item which actually frees. Am I misunderstanding
> something here?

What I'm trying to say is that it's not really a ref imbalance problem.
I think once the kill_css() has been called on a css, and it is enqueued to be
killed, we shouldn't call css_put(css) on it anymore outside of the "killed call
flow path". It will all be handled by the css_killed_ref_fn() function.

The fact the css_release() is called (via cgroup_kn_unlock()) just after
kill_css() causes the css->destroy_work to be enqueued twice on the same WQ
(cgroup_destroy_wq), just with different function. This results in the
BUG: corrupted list in insert_work issue.

-- 
Thanks,
Tadeusz