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

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Date:   Mon, 15 Aug 2022 11:05:56 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Mukesh Ojha <quic_mojha@...cinc.com>
Cc:     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

+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).

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