linux-kernel - Re: [PATCH v2] cgroup: Reorganize css_set

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Date:   Mon, 10 Oct 2022 10:22:34 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH v2] cgroup: Reorganize css_set_lock and kernfs path
 processing

On Wed, Oct 05, 2022 at 06:47:31AM -1000, Tejun Heo <tj@...nel.org> wrote:
> Hmm... isn't current's root cgrp guaranteed to be alive?

True on the default hierarchy. v1 hierarchies (singular ones with root
cgroup only) can be unmounted.

> How would cgroup_get_live() fail?

kill_sb is not synchronized via css_set_lock.

> Also, shouldn't cgroup_get() enough for path walking?

If ref count dropped to zero, release callback (css_release_work_fn)
would be queued, cgroup_get would increase the refcount but it won't
cancel this.

Note these were concerns with the first version of the patch that also touched
cgroup_show_path() (that processes v1 hierarchies too). With the
reduction I avoided this.

Strictly speaking, even css_set_lock is unnecessary around
current_cgns_cgroup_from_root() when called with cgrp_dfl_root as the
cset->cgrp_links is not traversed at all.

> If you really wanna do it this way, can you please add a detailed comment
> here why this is safe? But I'd prefer just doing a strightforward ref
> inc/dec around it.

I see the the extraction under css_set_lock without inc/dec turns out
confusing. Let me expand the idea above and avoid css_set_lock
completely (another message).

Michal