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

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Message-ID: <YpgEY/lJbLidLOhc@slm.duckdns.org>
Date:   Wed, 1 Jun 2022 14:29:23 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Tadeusz Struk <tadeusz.struk@...aro.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 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.

> 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.

-- 
tejun