linux-kernel - Re: Query regarding deadlock involving cgroup_threadgroup_rwsem and cpu_hotplug

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Date:   Mon, 15 Aug 2022 17:25:52 +0800
From:   Xuewen Yan <xuewen.yan94@...il.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Mukesh Ojha <quic_mojha@...cinc.com>, Tejun Heo <tj@...nel.org>,
        Imran Khan <imran.f.khan@...cle.com>, lizefan.x@...edance.com,
        hannes@...xchg.org, tglx@...utronix.de, steven.price@....com,
        peterz@...radead.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, Zhao Gongyi <zhaogongyi@...wei.com>,
        Zhang Qiao <zhangqiao22@...wei.com>
Subject: Re: Query regarding deadlock involving cgroup_threadgroup_rwsem and cpu_hotplug_lock

Hi Michal

On Mon, Aug 15, 2022 at 5:06 PM Michal Koutný <mkoutny@...e.com> wrote:
>
> +Cc: Zhao Gongyi <zhaogongyi@...wei.com>, Zhang Qiao <zhangqiao22@...wei.com>
>
> On Fri, Aug 12, 2022 at 03:57:00PM +0530, Mukesh Ojha <quic_mojha@...cinc.com> wrote:
> > The original patch of yours [1]  and the revert of [2] is fixing the issue
> > and it is also confirmed here [3].
> > Can we get proper fix merge on your tree?
> >
> > [1] https://lore.kernel.org/lkml/YuGbYCfAG81mZBnN@slm.duckdns.org/
> >
> > [2]
> > https://lore.kernel.org/all/20220121101210.84926-1-zhangqiao22@huawei.com/
>
> The revert + Tejun's patch looks fine wrt the problem of the reverted
> patch (just moves cpus_read_lock to upper callers).

Your means is that the problem should be fixed by [1]+[2]'s revert ?
I just tested the case which reverted the [2]. Need I test with [1] and [2]?

Thanks!

>
> I'd just suggest a comment that'd explicitly document also the lock
> order that we stick to, IIUC, it should be:
>
>         cpu_hotplug_lock // cpus_read_lock
>         cgroup_threadgroup_rwsem
>         cpuset_rwsem
>
> Michal
>
> >
> > [3] https://lore.kernel.org/lkml/CAB8ipk-72V-bYRfL-VcSRSyXTeQqkBVj+1d5MHSVV5CTar9a0Q@mail.gmail.com/
> >
> > -Mukesh