linux-kernel - Re: [PATCH cgroup/for-6.0-fixes] cgroup: Fix threadgroup_rwsem <-> cpus_read

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Date:   Wed, 17 Aug 2022 06:20:08 +1000
From:   Imran Khan <imran.f.khan@...cle.com>
To:     Tejun Heo <tj@...nel.org>, Mukesh Ojha <quic_mojha@...cinc.com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        Xuewen Yan <xuewen.yan94@...il.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: [PATCH cgroup/for-6.0-fixes] cgroup: Fix threadgroup_rwsem <->
 cpus_read_lock() deadlock

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@...nel.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@...cle.com>

Thanks,
-- Imran