linux-kernel - Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu hotplug when using schedutil

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Message-ID: <Ys2duuPDNq/q+oOz@slm.duckdns.org>
Date:   Tue, 12 Jul 2022 06:13:46 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Xuewen Yan <xuewen.yan@...soc.com>, rafael@...nel.org,
        viresh.kumar@...aro.org, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org,
        ke.wang@...soc.com, xuewyan@...mail.com, linux-pm@...r.kernel.org,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu
 hotplug when using schedutil

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