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Date:   Mon, 11 Jul 2022 17:11:30 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Tejun Heo <tj@kernel.org>
Cc:     Qais Yousef <qais.yousef@arm.com>,
        Xuewen Yan <xuewen.yan@unisoc.com>, rafael@kernel.org,
        viresh.kumar@linaro.org, mingo@redhat.com, peterz@infradead.org,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, 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 Mon, 11 Jul 2022 10:58:28 -1000
Tejun Heo <tj@kernel.org> wrote:

> I don't think lockdep would be able to track CPU1 -> CPU2 dependency here
> unfortunately.
> 
> > AFAIU:
> > 
> > 
> > CPU0                                     CPU1                                   CPU2
> > 
> > // attach task to a different
> > // cpuset cgroup via sysfs
> > __acquire(cgroup_threadgroup_rwsem)
> > 
> >                                          // pring up CPU2 online
> >                                          __acquire(cpu_hotplug_lock)
> >                                          // wait for CPU2 to come online

Should there be some annotation here that tells lockdep that CPU1 is now
blocked on CPU2?

Then this case would be caught by lockdep.

-- Steve


> >                                                                                 // bringup cpu online
> >                                                                                 // call cpufreq_online() which tries to create sugov kthread
> > __acquire(cpu_hotplug_lock)                                                     copy_process()
> >                                                                                    cgroup_can_fork()
> >                                                                                       cgroup_css_set_fork()
> >                                                                                       __acquire(cgroup_threadgroup_rwsem)
> > // blocks forever                        // blocks forever                            // blocks forever
> >