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Sender: Tejun Heo <htejun@gmail.com>
Date:   Tue, 12 Jul 2022 06:13:46 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Xuewen Yan <xuewen.yan@unisoc.com>, rafael@kernel.org,
        viresh.kumar@linaro.org, mingo@redhat.com, peterz@infradead.org,
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        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, linux-kernel@vger.kernel.org,
        ke.wang@unisoc.com, xuewyan@foxmail.com, linux-pm@vger.kernel.org,
        Waiman Long <longman@redhat.com>
Subject: Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu
 hotplug when using schedutil
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On Tue, Jul 12, 2022 at 01:57:02PM +0100, Qais Yousef wrote:
> Is there a lot of subsystems beside cpuset that needs the cpus_read_lock()?
> A quick grep tells me it's the only one.
> 
> Can't we instead use cpus_read_trylock() in cpuset_can_attach() so that we
> either hold the lock successfully then before we go ahead and call
> cpuset_attach(), or bail out and cancel the whole attach operation which should
> unlock the threadgroup_rwsem() lock?

But now we're failing user-initiated operations randomly. I have a hard time
seeing that as an acceptable solution. The only thing we can do, I think, is
establishing a locking order between the two locks by either nesting
threadgroup_rwsem under cpus_read_lock or disallowing thread creation during
hotplug operations.

Thanks.

-- 
tejun