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Date:   Wed, 1 Jun 2022 17:40:51 -0700
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To:     Tejun Heo <tj@kernel.org>
Cc:     =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Bui Quang Minh <minhquangbui99@gmail.com>
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From:   Tadeusz Struk <tadeusz.struk@linaro.org>
Subject: Re: [PATCH 2/2] cgroup: Use separate work structs on css release path
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On 6/1/22 17:29, Tejun Heo wrote:
> On Wed, Jun 01, 2022 at 05:26:34PM -0700, Tadeusz Struk wrote:
>> Ok the problem is that
>>
>> 1. kill_css() triggers css_killed_ref_fn(), which enqueues &css->destroy_work on cgroup_destroy_wq
>> 2. Last put_css() calls css_release(), which enqueues &css->destroy_work on cgroup_destroy_wq
>>
>> We have two instances of the same work struct enqueued on the same WQ (cgroup_destroy_wq),
>> which causes "BUG: corrupted list in insert_work"
> 
> #2 shouldn't be happening before kill_ref_fn() is done with the css. If what
> you're saying is happening, what's broken is the fact that the refcnt is
> reaching 0 prematurely.

css_killed_ref_fn() will be called regardless of the value of refcnt (via percpu_ref_kill_and_confirm())
and it will only enqueue the css_killed_work_fn() to be called later.
Then css_put()->css_release() will be called before the css_killed_work_fn() will even
get a chance to run, and it will also *only* enqueue css_release_work_fn() to be called later.
The problem happens on the second enqueue. So there need to be something in place that
will make sure that css_killed_work_fn() is done before css_release() can enqueue
the second job. Does it sound right?
  
>> So I think the easiest way to solve this would be to have two separate work_structs,
>> one for the killed_ref path and css_release path as in:
> 
> If you do that, you'd just be racing the free path against the kill path and
> the css might get freed while the kill path is still accessing it.
> 
> Thanks.
> 


-- 
Thanks,
Tadeusz