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Date:   Mon, 3 Sep 2018 10:54:23 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Kevin Shanahan <kevin@shanahan.id.au>,
        Siegfried Metz <frame@mailbox.org>,
        linux-kernel@vger.kernel.org, rafael.j.wysocki@intel.com,
        len.brown@intel.com, rjw@rjwysocki.net, diego.viola@gmail.com,
        rui.zhang@intel.com, viktor_jaegerskuepper@freenet.de
Subject: Re: REGRESSION: boot stalls on several old dual core Intel CPUs
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On Mon, Sep 03, 2018 at 09:38:15AM +0200, Thomas Gleixner wrote:
> On Mon, 3 Sep 2018, Peter Zijlstra wrote:
> > On Sat, Sep 01, 2018 at 11:51:26AM +0930, Kevin Shanahan wrote:
> > > commit 01548f4d3e8e94caf323a4f664eb347fd34a34ab
> > > Author: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > > Date:   Tue Aug 18 17:09:42 2009 +0200
> > > 
> > >     clocksource: Avoid clocksource watchdog circular locking dependency
> > > 
> > >     stop_machine from a multithreaded workqueue is not allowed because
> > >     of a circular locking dependency between cpu_down and the workqueue
> > >     execution. Use a kernel thread to do the clocksource downgrade.
> > 
> > I cannot find stop_machine usage there; either it went away or I need to
> > like wake up.
> 
> timekeeping_notify() which is involved in switching clock source uses stomp
> machine.

ARGH... OK, lemme see if I can come up with something other than
endlessly spawning that kthread.

A special purpose kthread_worker would make more sense than that.