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Date:   Mon, 14 Mar 2022 12:19:41 +0100
From:   Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
To:     Zhang Qiao <zhangqiao22@huawei.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Zhao Gongyi <zhaogongyi@huawei.com>,
        Waiman Long <longman@redhat.com>, Tejun Heo <tj@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [PATCH 4.19 01/34] cgroup/cpuset: Fix a race between
 cpuset_attach() and cpu hotplug
Message-ID: <20220314111940.GC1035@blackbody.suse.cz>
References: <20220228172207.090703467@linuxfoundation.org>
 <20220228172208.566431934@linuxfoundation.org>
 <20220308151232.GA21752@blackbody.suse.cz>
 <Yi73dKB10LBTGb+S@kroah.com>
 <aa25447a-f6ff-2ff2-72e9-3bbab1d430e9@huawei.com>
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Hello.

In my opinion there are two approaches:
a) drop this backport (given other races present),
b) swap the locks compatible with v4.19 as this patch proposes.

On Mon, Mar 14, 2022 at 05:11:50PM +0800, Zhang Qiao <zhangqiao22@huawei.com> wrote:
> +       /*
> +        * It should hold cpus lock because a cpu offline event can
> +        * cause set_cpus_allowed_ptr() failed.
> +        */
> +       cpus_read_lock();

Maybe just a nit, the old kernels before commit c5c63b9a6a2e ("cgroup:
Replace deprecated CPU-hotplug functions.") v5.15-rc1~159^2~5
would be more consistent with get_online_cpus() here (but they're
equivalent functionally so the locking order is correct).

Michal