linux-kernel - Re: [PATCH 3/3] cgroup/cpuset: Allow only one active attach operation per cpuset

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Date:   Mon, 3 Apr 2023 18:47:36 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Christian Brauner <brauner@...nel.org>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        gscrivan@...hat.com
Subject: Re: [PATCH 3/3] cgroup/cpuset: Allow only one active attach
 operation per cpuset

On Fri, Mar 31, 2023 at 10:50:45AM -0400, Waiman Long <longman@...hat.com> wrote:
> The current cpuset code uses the global cpuset_attach_old_cs variable
> to store the old cpuset value between consecutive cpuset_can_attach()
> and cpuset_attach() calls. Since a caller of cpuset_can_attach() may
> not need to hold the global cgroup_threadgroup_rwsem, parallel cpuset
> attach operations are possible.

Do I understand correctly this consequence of the cpuset_attach_task()
on the clone path?
In that particular case (with CLONE_INTO_CGROUP) cgroup_mutex is taken,
so the access the the old_cs variable should still be synchronized with
regular migrations that are also under cgroup_mutex.

> When there are concurrent cpuset attach operations in progress,
> cpuset_attach() may fetch the wrong value from cpuset_attach_old_cs
> causing incorrect result.  To avoid this problem while still allowing
> certain level of parallelism, drop cpuset_attach_old_cs and use a
> per-cpuset attach_old_cs value. Also restrict to at most one active
> attach operation per cpuset to avoid corrupting the value of the
> per-cpuset attach_old_cs value.

Secondly, semantically wouldn't a `void *ss_priv[CGROUP_SUBSYS_COUNT]`
in struct cgroup_taskset make it simpler wrt the exclusivity guarantees?

Thirdly, if my initial assumptino is right -- I'd suggest ordering this
before the patch `cgroup/cpuset: Make cpuset_fork() handle
CLONE_INTO_CGROUP properly` to spare backporters possible troubles if
this is would be a fixup to that.

Thanks,
Michal

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