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Date:   Wed, 17 Aug 2022 06:20:08 +1000
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Subject: Re: [PATCH cgroup/for-6.0-fixes] cgroup: Fix threadgroup_rwsem <->
 cpus_read_lock() deadlock
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To:     Tejun Heo <tj@kernel.org>, Mukesh Ojha <quic_mojha@quicinc.com>
Cc:     =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        Xuewen Yan <xuewen.yan94@gmail.com>, lizefan.x@bytedance.com,
        hannes@cmpxchg.org, tglx@linutronix.de, steven.price@arm.com,
        peterz@infradead.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, Zhao Gongyi <zhaogongyi@huawei.com>,
        Zhang Qiao <zhangqiao22@huawei.com>
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From:   Imran Khan <imran.f.khan@oracle.com>
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Hello Tejun,

On 16/8/22 9:27 am, Tejun Heo wrote:
> Bringing up a CPU may involve creating new tasks which requires read-locking
> threadgroup_rwsem, so threadgroup_rwsem nests inside cpus_read_lock().
> However, cpuset's ->attach(), which may be called with thredagroup_rwsem
> write-locked, also wants to disable CPU hotplug and acquires
> cpus_read_lock(), leading to a deadlock.
> 
> Fix it by guaranteeing that ->attach() is always called with CPU hotplug
> disabled and removing cpus_read_lock() call from cpuset_attach().
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
> Hello, sorry about the delay.
> 
> So, the previous patch + the revert isn't quite correct because we sometimes
> elide both cpus_read_lock() and threadgroup_rwsem together and
> cpuset_attach() woudl end up running without CPU hotplug enabled. Can you
> please test whether this patch fixes the problem?
> 

This fixes the issue seen in my setup. As my setup is 5.4 based I used
cgroup_attach_lock/unlock(true) in the backport version of your patch.

Feel free to add my

Reviewed-and-tested-by: Imran Khan <imran.f.khan@oracle.com>

Thanks,
-- Imran